diff --git a/homeassistant/components/accuweather/const.py b/homeassistant/components/accuweather/const.py
index e572feafcf8..5696a35ea2f 100644
--- a/homeassistant/components/accuweather/const.py
+++ b/homeassistant/components/accuweather/const.py
@@ -5,6 +5,7 @@ from homeassistant.const import (
     LENGTH_FEET,
     LENGTH_INCHES,
     LENGTH_METERS,
+    LENGTH_MILLIMETERS,
     PERCENTAGE,
     SPEED_KILOMETERS_PER_HOUR,
     SPEED_MILES_PER_HOUR,
@@ -24,7 +25,6 @@ ATTR_UNIT_METRIC = "Metric"
 CONCENTRATION_PARTS_PER_CUBIC_METER = f"p/{VOLUME_CUBIC_METERS}"
 COORDINATOR = "coordinator"
 DOMAIN = "accuweather"
-LENGTH_MILIMETERS = "mm"
 MANUFACTURER = "AccuWeather, Inc."
 NAME = "AccuWeather"
 UNDO_UPDATE_LISTENER = "undo_update_listener"
@@ -238,7 +238,7 @@ SENSOR_TYPES = {
         ATTR_DEVICE_CLASS: None,
         ATTR_ICON: "mdi:weather-rainy",
         ATTR_LABEL: "Precipitation",
-        ATTR_UNIT_METRIC: LENGTH_MILIMETERS,
+        ATTR_UNIT_METRIC: LENGTH_MILLIMETERS,
         ATTR_UNIT_IMPERIAL: LENGTH_INCHES,
     },
     "PressureTendency": {
diff --git a/homeassistant/components/bom/sensor.py b/homeassistant/components/bom/sensor.py
index 848be51cc8f..12f43430829 100644
--- a/homeassistant/components/bom/sensor.py
+++ b/homeassistant/components/bom/sensor.py
@@ -21,6 +21,7 @@ from homeassistant.const import (
     CONF_NAME,
     LENGTH_KILOMETERS,
     LENGTH_METERS,
+    LENGTH_MILLIMETERS,
     PERCENTAGE,
     PRESSURE_MBAR,
     SPEED_KILOMETERS_PER_HOUR,
@@ -71,7 +72,7 @@ SENSOR_TYPES = {
     "press_qnh": ["Pressure qnh", "qnh"],
     "press_msl": ["Pressure msl", "msl"],
     "press_tend": ["Pressure Tend", None],
-    "rain_trace": ["Rain Today", "mm"],
+    "rain_trace": ["Rain Today", LENGTH_MILLIMETERS],
     "rel_hum": ["Relative Humidity", PERCENTAGE],
     "sea_state": ["Sea State", None],
     "swell_dir_worded": ["Swell Direction", None],
diff --git a/homeassistant/components/buienradar/sensor.py b/homeassistant/components/buienradar/sensor.py
index d1d1f0cf632..b1e41122dc0 100644
--- a/homeassistant/components/buienradar/sensor.py
+++ b/homeassistant/components/buienradar/sensor.py
@@ -30,6 +30,7 @@ from homeassistant.const import (
     DEGREE,
     IRRADIATION_WATTS_PER_SQUARE_METER,
     LENGTH_KILOMETERS,
+    LENGTH_MILLIMETERS,
     PERCENTAGE,
     PRESSURE_HPA,
     SPEED_KILOMETERS_PER_HOUR,
@@ -82,22 +83,26 @@ SENSOR_TYPES = {
     "pressure": ["Pressure", PRESSURE_HPA, "mdi:gauge"],
     "visibility": ["Visibility", LENGTH_KILOMETERS, None],
     "windgust": ["Wind gust", SPEED_KILOMETERS_PER_HOUR, "mdi:weather-windy"],
-    "precipitation": ["Precipitation", f"mm/{TIME_HOURS}", "mdi:weather-pouring"],
+    "precipitation": [
+        "Precipitation",
+        f"{LENGTH_MILLIMETERS}/{TIME_HOURS}",
+        "mdi:weather-pouring",
+    ],
     "irradiance": ["Irradiance", IRRADIATION_WATTS_PER_SQUARE_METER, "mdi:sunglasses"],
     "precipitation_forecast_average": [
         "Precipitation forecast average",
-        f"mm/{TIME_HOURS}",
+        f"{LENGTH_MILLIMETERS}/{TIME_HOURS}",
         "mdi:weather-pouring",
     ],
     "precipitation_forecast_total": [
         "Precipitation forecast total",
-        "mm",
+        LENGTH_MILLIMETERS,
         "mdi:weather-pouring",
     ],
     # new in json api (>1.0.0):
-    "rainlast24hour": ["Rain last 24h", "mm", "mdi:weather-pouring"],
+    "rainlast24hour": ["Rain last 24h", LENGTH_MILLIMETERS, "mdi:weather-pouring"],
     # new in json api (>1.0.0):
-    "rainlasthour": ["Rain last hour", "mm", "mdi:weather-pouring"],
+    "rainlasthour": ["Rain last hour", LENGTH_MILLIMETERS, "mdi:weather-pouring"],
     "temperature_1d": ["Temperature 1d", TEMP_CELSIUS, "mdi:thermometer"],
     "temperature_2d": ["Temperature 2d", TEMP_CELSIUS, "mdi:thermometer"],
     "temperature_3d": ["Temperature 3d", TEMP_CELSIUS, "mdi:thermometer"],
@@ -108,23 +113,23 @@ SENSOR_TYPES = {
     "mintemp_3d": ["Minimum temperature 3d", TEMP_CELSIUS, "mdi:thermometer"],
     "mintemp_4d": ["Minimum temperature 4d", TEMP_CELSIUS, "mdi:thermometer"],
     "mintemp_5d": ["Minimum temperature 5d", TEMP_CELSIUS, "mdi:thermometer"],
-    "rain_1d": ["Rain 1d", "mm", "mdi:weather-pouring"],
-    "rain_2d": ["Rain 2d", "mm", "mdi:weather-pouring"],
-    "rain_3d": ["Rain 3d", "mm", "mdi:weather-pouring"],
-    "rain_4d": ["Rain 4d", "mm", "mdi:weather-pouring"],
-    "rain_5d": ["Rain 5d", "mm", "mdi:weather-pouring"],
+    "rain_1d": ["Rain 1d", LENGTH_MILLIMETERS, "mdi:weather-pouring"],
+    "rain_2d": ["Rain 2d", LENGTH_MILLIMETERS, "mdi:weather-pouring"],
+    "rain_3d": ["Rain 3d", LENGTH_MILLIMETERS, "mdi:weather-pouring"],
+    "rain_4d": ["Rain 4d", LENGTH_MILLIMETERS, "mdi:weather-pouring"],
+    "rain_5d": ["Rain 5d", LENGTH_MILLIMETERS, "mdi:weather-pouring"],
     # new in json api (>1.0.0):
-    "minrain_1d": ["Minimum rain 1d", "mm", "mdi:weather-pouring"],
-    "minrain_2d": ["Minimum rain 2d", "mm", "mdi:weather-pouring"],
-    "minrain_3d": ["Minimum rain 3d", "mm", "mdi:weather-pouring"],
-    "minrain_4d": ["Minimum rain 4d", "mm", "mdi:weather-pouring"],
-    "minrain_5d": ["Minimum rain 5d", "mm", "mdi:weather-pouring"],
+    "minrain_1d": ["Minimum rain 1d", LENGTH_MILLIMETERS, "mdi:weather-pouring"],
+    "minrain_2d": ["Minimum rain 2d", LENGTH_MILLIMETERS, "mdi:weather-pouring"],
+    "minrain_3d": ["Minimum rain 3d", LENGTH_MILLIMETERS, "mdi:weather-pouring"],
+    "minrain_4d": ["Minimum rain 4d", LENGTH_MILLIMETERS, "mdi:weather-pouring"],
+    "minrain_5d": ["Minimum rain 5d", LENGTH_MILLIMETERS, "mdi:weather-pouring"],
     # new in json api (>1.0.0):
-    "maxrain_1d": ["Maximum rain 1d", "mm", "mdi:weather-pouring"],
-    "maxrain_2d": ["Maximum rain 2d", "mm", "mdi:weather-pouring"],
-    "maxrain_3d": ["Maximum rain 3d", "mm", "mdi:weather-pouring"],
-    "maxrain_4d": ["Maximum rain 4d", "mm", "mdi:weather-pouring"],
-    "maxrain_5d": ["Maximum rain 5d", "mm", "mdi:weather-pouring"],
+    "maxrain_1d": ["Maximum rain 1d", LENGTH_MILLIMETERS, "mdi:weather-pouring"],
+    "maxrain_2d": ["Maximum rain 2d", LENGTH_MILLIMETERS, "mdi:weather-pouring"],
+    "maxrain_3d": ["Maximum rain 3d", LENGTH_MILLIMETERS, "mdi:weather-pouring"],
+    "maxrain_4d": ["Maximum rain 4d", LENGTH_MILLIMETERS, "mdi:weather-pouring"],
+    "maxrain_5d": ["Maximum rain 5d", LENGTH_MILLIMETERS, "mdi:weather-pouring"],
     "rainchance_1d": ["Rainchance 1d", PERCENTAGE, "mdi:weather-pouring"],
     "rainchance_2d": ["Rainchance 2d", PERCENTAGE, "mdi:weather-pouring"],
     "rainchance_3d": ["Rainchance 3d", PERCENTAGE, "mdi:weather-pouring"],
diff --git a/homeassistant/components/darksky/sensor.py b/homeassistant/components/darksky/sensor.py
index 48ceffbac50..49b5c6c69f6 100644
--- a/homeassistant/components/darksky/sensor.py
+++ b/homeassistant/components/darksky/sensor.py
@@ -18,6 +18,7 @@ from homeassistant.const import (
     DEGREE,
     LENGTH_CENTIMETERS,
     LENGTH_KILOMETERS,
+    LENGTH_MILLIMETERS,
     PERCENTAGE,
     PRESSURE_MBAR,
     SPEED_KILOMETERS_PER_HOUR,
@@ -110,11 +111,11 @@ SENSOR_TYPES = {
     ],
     "precip_intensity": [
         "Precip Intensity",
-        f"mm/{TIME_HOURS}",
+        f"{LENGTH_MILLIMETERS}/{TIME_HOURS}",
         "in",
-        f"mm/{TIME_HOURS}",
-        f"mm/{TIME_HOURS}",
-        f"mm/{TIME_HOURS}",
+        f"{LENGTH_MILLIMETERS}/{TIME_HOURS}",
+        f"{LENGTH_MILLIMETERS}/{TIME_HOURS}",
+        f"{LENGTH_MILLIMETERS}/{TIME_HOURS}",
         "mdi:weather-rainy",
         ["currently", "minutely", "hourly", "daily"],
     ],
@@ -330,11 +331,11 @@ SENSOR_TYPES = {
     ],
     "precip_intensity_max": [
         "Daily Max Precip Intensity",
-        f"mm/{TIME_HOURS}",
+        f"{LENGTH_MILLIMETERS}/{TIME_HOURS}",
         "in",
-        f"mm/{TIME_HOURS}",
-        f"mm/{TIME_HOURS}",
-        f"mm/{TIME_HOURS}",
+        f"{LENGTH_MILLIMETERS}/{TIME_HOURS}",
+        f"{LENGTH_MILLIMETERS}/{TIME_HOURS}",
+        f"{LENGTH_MILLIMETERS}/{TIME_HOURS}",
         "mdi:thermometer",
         ["daily"],
     ],
diff --git a/homeassistant/components/homematic/sensor.py b/homeassistant/components/homematic/sensor.py
index 121b3af5d81..09ceb7c1da2 100644
--- a/homeassistant/components/homematic/sensor.py
+++ b/homeassistant/components/homematic/sensor.py
@@ -9,6 +9,7 @@ from homeassistant.const import (
     DEVICE_CLASS_TEMPERATURE,
     ENERGY_WATT_HOUR,
     FREQUENCY_HERTZ,
+    LENGTH_MILLIMETERS,
     PERCENTAGE,
     POWER_WATT,
     PRESSURE_HPA,
@@ -55,7 +56,7 @@ HM_UNIT_HA_CAST = {
     "AVERAGE_ILLUMINATION": "lx",
     "LOWEST_ILLUMINATION": "lx",
     "HIGHEST_ILLUMINATION": "lx",
-    "RAIN_COUNTER": "mm",
+    "RAIN_COUNTER": LENGTH_MILLIMETERS,
     "WIND_SPEED": SPEED_KILOMETERS_PER_HOUR,
     "WIND_DIRECTION": DEGREE,
     "WIND_DIRECTION_RANGE": DEGREE,
diff --git a/homeassistant/components/homematicip_cloud/sensor.py b/homeassistant/components/homematicip_cloud/sensor.py
index 86881f565ce..f307fb9274e 100644
--- a/homeassistant/components/homematicip_cloud/sensor.py
+++ b/homeassistant/components/homematicip_cloud/sensor.py
@@ -30,6 +30,7 @@ from homeassistant.const import (
     DEVICE_CLASS_ILLUMINANCE,
     DEVICE_CLASS_POWER,
     DEVICE_CLASS_TEMPERATURE,
+    LENGTH_MILLIMETERS,
     PERCENTAGE,
     POWER_WATT,
     SPEED_KILOMETERS_PER_HOUR,
@@ -367,7 +368,7 @@ class HomematicipTodayRainSensor(HomematicipGenericEntity):
     @property
     def unit_of_measurement(self) -> str:
         """Return the unit this state is expressed in."""
-        return "mm"
+        return LENGTH_MILLIMETERS
 
 
 class HomematicipPassageDetectorDeltaCounter(HomematicipGenericEntity):
diff --git a/homeassistant/components/isy994/const.py b/homeassistant/components/isy994/const.py
index 7724420f391..7bef122f7d3 100644
--- a/homeassistant/components/isy994/const.py
+++ b/homeassistant/components/isy994/const.py
@@ -57,6 +57,7 @@ from homeassistant.const import (
     LENGTH_KILOMETERS,
     LENGTH_METERS,
     LENGTH_MILES,
+    LENGTH_MILLIMETERS,
     MASS_KILOGRAMS,
     MASS_POUNDS,
     PERCENTAGE,
@@ -361,7 +362,7 @@ UOM_FRIENDLY_NAME = {
     "43": "mV",
     "44": TIME_MINUTES,
     "45": TIME_MINUTES,
-    "46": f"mm/{TIME_HOURS}",
+    "46": f"{LENGTH_MILLIMETERS}/{TIME_HOURS}",
     "47": TIME_MONTHS,
     "48": SPEED_MILES_PER_HOUR,
     "49": SPEED_METERS_PER_SECOND,
@@ -388,7 +389,7 @@ UOM_FRIENDLY_NAME = {
     "75": "weekday",
     "76": DEGREE,
     "77": TIME_YEARS,
-    "82": "mm",
+    "82": LENGTH_MILLIMETERS,
     "83": LENGTH_KILOMETERS,
     "85": "Ω",
     "86": "kΩ",
@@ -404,7 +405,7 @@ UOM_FRIENDLY_NAME = {
     "103": CURRENCY_DOLLAR,
     "104": CURRENCY_CENT,
     "105": LENGTH_INCHES,
-    "106": f"mm/{TIME_DAYS}",
+    "106": f"{LENGTH_MILLIMETERS}/{TIME_DAYS}",
     "107": "",  # raw 1-byte unsigned value
     "108": "",  # raw 2-byte unsigned value
     "109": "",  # raw 3-byte unsigned value
diff --git a/homeassistant/components/meteo_france/const.py b/homeassistant/components/meteo_france/const.py
index 8b4d3a33501..59524ed1a80 100644
--- a/homeassistant/components/meteo_france/const.py
+++ b/homeassistant/components/meteo_france/const.py
@@ -4,6 +4,7 @@ from homeassistant.const import (
     DEVICE_CLASS_PRESSURE,
     DEVICE_CLASS_TEMPERATURE,
     DEVICE_CLASS_TIMESTAMP,
+    LENGTH_MILLIMETERS,
     PERCENTAGE,
     PRESSURE_HPA,
     SPEED_KILOMETERS_PER_HOUR,
@@ -108,7 +109,7 @@ SENSOR_TYPES = {
     },
     "precipitation": {
         ENTITY_NAME: "Daily precipitation",
-        ENTITY_UNIT: "mm",
+        ENTITY_UNIT: LENGTH_MILLIMETERS,
         ENTITY_ICON: "mdi:cup-water",
         ENTITY_DEVICE_CLASS: None,
         ENTITY_ENABLE: True,
diff --git a/homeassistant/components/netatmo/sensor.py b/homeassistant/components/netatmo/sensor.py
index 2368d54efdf..af41e01c7df 100644
--- a/homeassistant/components/netatmo/sensor.py
+++ b/homeassistant/components/netatmo/sensor.py
@@ -12,6 +12,7 @@ from homeassistant.const import (
     DEVICE_CLASS_PRESSURE,
     DEVICE_CLASS_SIGNAL_STRENGTH,
     DEVICE_CLASS_TEMPERATURE,
+    LENGTH_MILLIMETERS,
     PERCENTAGE,
     PRESSURE_MBAR,
     SPEED_KILOMETERS_PER_HOUR,
@@ -52,9 +53,9 @@ SENSOR_TYPES = {
     "pressure": ["Pressure", PRESSURE_MBAR, None, DEVICE_CLASS_PRESSURE],
     "noise": ["Noise", "dB", "mdi:volume-high", None],
     "humidity": ["Humidity", PERCENTAGE, None, DEVICE_CLASS_HUMIDITY],
-    "rain": ["Rain", "mm", "mdi:weather-rainy", None],
-    "sum_rain_1": ["Rain last hour", "mm", "mdi:weather-rainy", None],
-    "sum_rain_24": ["Rain last 24h", "mm", "mdi:weather-rainy", None],
+    "rain": ["Rain", LENGTH_MILLIMETERS, "mdi:weather-rainy", None],
+    "sum_rain_1": ["Rain last hour", LENGTH_MILLIMETERS, "mdi:weather-rainy", None],
+    "sum_rain_24": ["Rain last 24h", LENGTH_MILLIMETERS, "mdi:weather-rainy", None],
     "battery_vp": ["Battery", "", "mdi:battery", None],
     "battery_lvl": ["Battery Level", "", "mdi:battery", None],
     "battery_percent": ["Battery Percent", PERCENTAGE, None, DEVICE_CLASS_BATTERY],
diff --git a/homeassistant/components/openweathermap/const.py b/homeassistant/components/openweathermap/const.py
index bc7a428f366..d2ea7f34d50 100644
--- a/homeassistant/components/openweathermap/const.py
+++ b/homeassistant/components/openweathermap/const.py
@@ -14,6 +14,7 @@ from homeassistant.const import (
     DEVICE_CLASS_PRESSURE,
     DEVICE_CLASS_TEMPERATURE,
     DEVICE_CLASS_TIMESTAMP,
+    LENGTH_MILLIMETERS,
     PERCENTAGE,
     PRESSURE_PA,
     SPEED_METERS_PER_SECOND,
@@ -112,8 +113,8 @@ WEATHER_SENSOR_TYPES = {
         SENSOR_DEVICE_CLASS: DEVICE_CLASS_PRESSURE,
     },
     ATTR_API_CLOUDS: {SENSOR_NAME: "Cloud coverage", SENSOR_UNIT: PERCENTAGE},
-    ATTR_API_RAIN: {SENSOR_NAME: "Rain", SENSOR_UNIT: "mm"},
-    ATTR_API_SNOW: {SENSOR_NAME: "Snow", SENSOR_UNIT: "mm"},
+    ATTR_API_RAIN: {SENSOR_NAME: "Rain", SENSOR_UNIT: LENGTH_MILLIMETERS},
+    ATTR_API_SNOW: {SENSOR_NAME: "Snow", SENSOR_UNIT: LENGTH_MILLIMETERS},
     ATTR_API_CONDITION: {SENSOR_NAME: "Condition"},
     ATTR_API_WEATHER_CODE: {SENSOR_NAME: "Weather Code"},
 }
diff --git a/homeassistant/components/tellduslive/sensor.py b/homeassistant/components/tellduslive/sensor.py
index c8f27a9412a..7b785a808e8 100644
--- a/homeassistant/components/tellduslive/sensor.py
+++ b/homeassistant/components/tellduslive/sensor.py
@@ -6,6 +6,7 @@ from homeassistant.const import (
     DEVICE_CLASS_HUMIDITY,
     DEVICE_CLASS_ILLUMINANCE,
     DEVICE_CLASS_TEMPERATURE,
+    LENGTH_MILLIMETERS,
     PERCENTAGE,
     POWER_WATT,
     SPEED_METERS_PER_SECOND,
@@ -40,8 +41,13 @@ SENSOR_TYPES = {
         DEVICE_CLASS_TEMPERATURE,
     ],
     SENSOR_TYPE_HUMIDITY: ["Humidity", PERCENTAGE, None, DEVICE_CLASS_HUMIDITY],
-    SENSOR_TYPE_RAINRATE: ["Rain rate", f"mm/{TIME_HOURS}", "mdi:water", None],
-    SENSOR_TYPE_RAINTOTAL: ["Rain total", "mm", "mdi:water", None],
+    SENSOR_TYPE_RAINRATE: [
+        "Rain rate",
+        f"{LENGTH_MILLIMETERS}/{TIME_HOURS}",
+        "mdi:water",
+        None,
+    ],
+    SENSOR_TYPE_RAINTOTAL: ["Rain total", LENGTH_MILLIMETERS, "mdi:water", None],
     SENSOR_TYPE_WINDDIRECTION: ["Wind direction", "", "", None],
     SENSOR_TYPE_WINDAVERAGE: ["Wind average", SPEED_METERS_PER_SECOND, "", None],
     SENSOR_TYPE_WINDGUST: ["Wind gust", SPEED_METERS_PER_SECOND, "", None],
diff --git a/homeassistant/components/tof/sensor.py b/homeassistant/components/tof/sensor.py
index 58f50f4899e..d9ad178cab2 100644
--- a/homeassistant/components/tof/sensor.py
+++ b/homeassistant/components/tof/sensor.py
@@ -9,14 +9,12 @@ import voluptuous as vol
 
 from homeassistant.components import rpi_gpio
 from homeassistant.components.sensor import PLATFORM_SCHEMA
-from homeassistant.const import CONF_NAME
+from homeassistant.const import CONF_NAME, LENGTH_MILLIMETERS
 import homeassistant.helpers.config_validation as cv
 from homeassistant.helpers.entity import Entity
 
 _LOGGER = logging.getLogger(__name__)
 
-LENGTH_MILLIMETERS = "mm"
-
 CONF_I2C_ADDRESS = "i2c_address"
 CONF_I2C_BUS = "i2c_bus"
 CONF_XSHUT = "xshut"
diff --git a/homeassistant/components/trafikverket_weatherstation/sensor.py b/homeassistant/components/trafikverket_weatherstation/sensor.py
index 958adfd6915..bb1bad67f82 100644
--- a/homeassistant/components/trafikverket_weatherstation/sensor.py
+++ b/homeassistant/components/trafikverket_weatherstation/sensor.py
@@ -17,6 +17,7 @@ from homeassistant.const import (
     DEGREE,
     DEVICE_CLASS_HUMIDITY,
     DEVICE_CLASS_TEMPERATURE,
+    LENGTH_MILLIMETERS,
     PERCENTAGE,
     SPEED_METERS_PER_SECOND,
     TEMP_CELSIUS,
@@ -97,7 +98,7 @@ SENSOR_TYPES = {
     ],
     "precipitation_amount": [
         "Precipitation amount",
-        "mm",
+        LENGTH_MILLIMETERS,
         "precipitation_amount",
         "mdi:cup-water",
         None,
diff --git a/homeassistant/components/wunderground/sensor.py b/homeassistant/components/wunderground/sensor.py
index 6699c2f7e1d..e1bd79b7ea0 100644
--- a/homeassistant/components/wunderground/sensor.py
+++ b/homeassistant/components/wunderground/sensor.py
@@ -23,6 +23,7 @@ from homeassistant.const import (
     LENGTH_INCHES,
     LENGTH_KILOMETERS,
     LENGTH_MILES,
+    LENGTH_MILLIMETERS,
     PERCENTAGE,
     PRESSURE_INHG,
     SPEED_KILOMETERS_PER_HOUR,
@@ -392,7 +393,7 @@ SENSOR_TYPES = {
         "Precipitation 1hr", "precip_1hr_in", "mdi:umbrella", LENGTH_INCHES
     ),
     "precip_1hr_metric": WUCurrentConditionsSensorConfig(
-        "Precipitation 1hr", "precip_1hr_metric", "mdi:umbrella", "mm"
+        "Precipitation 1hr", "precip_1hr_metric", "mdi:umbrella", LENGTH_MILLIMETERS
     ),
     "precip_1hr_string": WUCurrentConditionsSensorConfig(
         "Precipitation 1hr", "precip_1hr_string", "mdi:umbrella"
@@ -401,7 +402,7 @@ SENSOR_TYPES = {
         "Precipitation Today", "precip_today_in", "mdi:umbrella", LENGTH_INCHES
     ),
     "precip_today_metric": WUCurrentConditionsSensorConfig(
-        "Precipitation Today", "precip_today_metric", "mdi:umbrella", "mm"
+        "Precipitation Today", "precip_today_metric", "mdi:umbrella", LENGTH_MILLIMETERS
     ),
     "precip_today_string": WUCurrentConditionsSensorConfig(
         "Precipitation Today", "precip_today_string", "mdi:umbrella"
@@ -879,16 +880,36 @@ SENSOR_TYPES = {
         "mdi:weather-windy",
     ),
     "precip_1d_mm": WUDailySimpleForecastSensorConfig(
-        "Precipitation Intensity Today", 0, "qpf_allday", "mm", "mm", "mdi:umbrella"
+        "Precipitation Intensity Today",
+        0,
+        "qpf_allday",
+        LENGTH_MILLIMETERS,
+        LENGTH_MILLIMETERS,
+        "mdi:umbrella",
     ),
     "precip_2d_mm": WUDailySimpleForecastSensorConfig(
-        "Precipitation Intensity Tomorrow", 1, "qpf_allday", "mm", "mm", "mdi:umbrella"
+        "Precipitation Intensity Tomorrow",
+        1,
+        "qpf_allday",
+        LENGTH_MILLIMETERS,
+        LENGTH_MILLIMETERS,
+        "mdi:umbrella",
     ),
     "precip_3d_mm": WUDailySimpleForecastSensorConfig(
-        "Precipitation Intensity in 3 Days", 2, "qpf_allday", "mm", "mm", "mdi:umbrella"
+        "Precipitation Intensity in 3 Days",
+        2,
+        "qpf_allday",
+        LENGTH_MILLIMETERS,
+        LENGTH_MILLIMETERS,
+        "mdi:umbrella",
     ),
     "precip_4d_mm": WUDailySimpleForecastSensorConfig(
-        "Precipitation Intensity in 4 Days", 3, "qpf_allday", "mm", "mm", "mdi:umbrella"
+        "Precipitation Intensity in 4 Days",
+        3,
+        "qpf_allday",
+        LENGTH_MILLIMETERS,
+        LENGTH_MILLIMETERS,
+        "mdi:umbrella",
     ),
     "precip_1d_in": WUDailySimpleForecastSensorConfig(
         "Precipitation Intensity Today",
diff --git a/homeassistant/const.py b/homeassistant/const.py
index f3bb26d3ef0..e1738cf6425 100644
--- a/homeassistant/const.py
+++ b/homeassistant/const.py
@@ -403,6 +403,7 @@ TIME_MONTHS = "m"
 TIME_YEARS = "y"
 
 # Length units
+LENGTH_MILLIMETERS: str = "mm"
 LENGTH_CENTIMETERS: str = "cm"
 LENGTH_METERS: str = "m"
 LENGTH_KILOMETERS: str = "km"
diff --git a/tests/components/accuweather/test_sensor.py b/tests/components/accuweather/test_sensor.py
index b94d17066c8..4ce34dfebe9 100644
--- a/tests/components/accuweather/test_sensor.py
+++ b/tests/components/accuweather/test_sensor.py
@@ -6,7 +6,6 @@ from homeassistant.components.accuweather.const import (
     ATTRIBUTION,
     CONCENTRATION_PARTS_PER_CUBIC_METER,
     DOMAIN,
-    LENGTH_MILIMETERS,
 )
 from homeassistant.components.sensor import DOMAIN as SENSOR_DOMAIN
 from homeassistant.const import (
@@ -17,6 +16,7 @@ from homeassistant.const import (
     ATTR_UNIT_OF_MEASUREMENT,
     DEVICE_CLASS_TEMPERATURE,
     LENGTH_METERS,
+    LENGTH_MILLIMETERS,
     PERCENTAGE,
     SPEED_KILOMETERS_PER_HOUR,
     STATE_UNAVAILABLE,
@@ -52,7 +52,7 @@ async def test_sensor_without_forecast(hass):
     assert state
     assert state.state == "0.0"
     assert state.attributes.get(ATTR_ATTRIBUTION) == ATTRIBUTION
-    assert state.attributes.get(ATTR_UNIT_OF_MEASUREMENT) == LENGTH_MILIMETERS
+    assert state.attributes.get(ATTR_UNIT_OF_MEASUREMENT) == LENGTH_MILLIMETERS
     assert state.attributes.get(ATTR_ICON) == "mdi:weather-rainy"
     assert state.attributes.get("type") is None
 
diff --git a/tests/components/homematicip_cloud/test_sensor.py b/tests/components/homematicip_cloud/test_sensor.py
index 55d1e32bf2b..de9fc276795 100644
--- a/tests/components/homematicip_cloud/test_sensor.py
+++ b/tests/components/homematicip_cloud/test_sensor.py
@@ -24,6 +24,7 @@ from homeassistant.components.homematicip_cloud.sensor import (
 from homeassistant.components.sensor import DOMAIN as SENSOR_DOMAIN
 from homeassistant.const import (
     ATTR_UNIT_OF_MEASUREMENT,
+    LENGTH_MILLIMETERS,
     PERCENTAGE,
     POWER_WATT,
     SPEED_KILOMETERS_PER_HOUR,
@@ -337,7 +338,7 @@ async def test_hmip_today_rain_sensor(hass, default_mock_hap_factory):
     )
 
     assert ha_state.state == "3.9"
-    assert ha_state.attributes[ATTR_UNIT_OF_MEASUREMENT] == "mm"
+    assert ha_state.attributes[ATTR_UNIT_OF_MEASUREMENT] == LENGTH_MILLIMETERS
     await async_manipulate_test_data(hass, hmip_device, "todayRainCounter", 14.2)
     ha_state = hass.states.get(entity_id)
     assert ha_state.state == "14.2"