Why Is Rainfall Measured in mm?

Rainfall is measured in millimeters (mm) because rainfall represents the depth of water that would cover a flat horizontal surface if the water were distributed evenly.

The most useful relationship is:

1 mm of rainfall = 1 liter of water per square meter (1 L/m²)

This makes rainfall easy to compare between locations of different sizes. Whether the rain falls on one square meter, one hectare or an entire watershed, the rainfall depth can still be expressed using the same millimeter value.

What Does 1 mm of Rainfall Mean?

One millimeter of rainfall means that the precipitation would form a water layer 1 mm deep over a horizontal surface.

Because one square meter has an area of 1,000,000 mm², a water layer 1 mm deep has a volume of 1,000,000 mm³, which is equal to 1 liter.

Why Use Millimeters Instead of Liters?

Liters describe a volume of water, but the total volume depends on the size of the area.

For example, 10 mm of rainfall represents:

• 10 liters over 1 m²

• 1,000 liters over 100 m²

• 100,000 liters over 1 hectare

The rainfall event is the same in each case: 10 mm. Using depth therefore provides a standardized measurement that does not depend on the area being discussed.

Why Are Millimeters Convenient for Rainfall Measurement?

1. Rainfall Is Naturally Expressed as Depth

A rain gauge collects precipitation over a known opening area. The amount collected can be converted directly into an equivalent water depth.

2. Small Rainfall Changes Are Easy to Express

Rainfall events often involve relatively small depths, so millimeters provide a convenient level of resolution without requiring decimals in meters.

3. Rainfall Can Be Compared Between Locations

A measurement of 25 mm means the same rainfall depth whether it is recorded at a small farm, a city weather station or a hydrological monitoring site.

4. Water Volume Is Easy to Calculate

Because 1 mm equals 1 L/m², rainfall depth can be converted easily into total water volume when the area is known.

5. Millimeters Are Widely Used in Meteorology and Hydrology

Millimeters provide a standardized metric unit for recording and comparing precipitation data across meteorological, agricultural and hydrological applications.

How to Calculate Water Volume from Rainfall in mm

If the rainfall depth and surface area are known, the total water volume can be estimated using:

Water Volume (liters) = Rainfall (mm) × Area (m²)

For example:

20 mm rainfall × 500 m² = 10,000 liters of water

For one hectare:

20 mm × 10,000 m² = 200,000 liters = 200 m³

This is useful in agriculture, reservoirs, irrigation planning and water-resource calculations. However, the calculated rainfall volume is not the same as the amount of water that remains available to plants because runoff, infiltration, evaporation and drainage also affect the final water balance.

Rainfall in mm vs Inches

Some countries and weather services also report rainfall in inches.

The conversion is:

1 inch = 25.4 mm

How Does a Rain Gauge Measure Rainfall in mm?

A rain gauge has a known collection area. Rain entering this area is collected or detected and converted into an equivalent rainfall depth.

Different instruments use different measurement methods:

• Manual gauges collect water for direct reading.

• Tipping bucket gauges count calibrated volumes of water.

• Weighing gauges measure accumulated precipitation mass.

• Piezoelectric sensors estimate rainfall from raindrop impacts.

For more detail, see How Does a Rain Gauge Work?

Example: 0.2 mm Tipping Bucket Rain Gauge

The NiuBoL NBL-W-RS tipping bucket rain gauge is available with a 0.2 mm rainfall resolution. This means each calibrated bucket tip represents 0.2 mm of rainfall.

The tipping events can be recorded through pulse output or processed through an RS485 Modbus transmitter, depending on the rain gauge configuration.

Why Rainfall in mm Is Useful for Agriculture

For agriculture, rainfall depth provides a convenient indication of natural water input.

For example, 15 mm of rain theoretically corresponds to 15 liters of water per square meter. However, not all of this water necessarily remains in the crop root zone.

Actual crop water availability also depends on:

• Soil texture

• Infiltration rate

• Runoff

• Drainage

• Evaporation

• Root depth

• Existing soil moisture

For irrigation management, rainfall data is therefore more useful when combined with soil-moisture measurements and crop information.

Why Rainfall in mm Is Useful for Hydrology

In hydrology, rainfall depth provides a standardized input for analysing catchments, river flow, reservoirs and flood conditions.

When rainfall depth is combined with catchment area, engineers can estimate the total precipitation volume falling over that region.

Actual river runoff will normally be lower than the total rainfall volume because some water infiltrates into the soil, evaporates, is stored on the surface or is retained by vegetation.

Common Misunderstandings About Rainfall in mm

Rainfall mm Is Not the Size of Raindrops

A rainfall value of 10 mm does not mean individual raindrops are 10 mm in diameter. It refers to accumulated water depth.

Rainfall mm Is Not Water Remaining on the Ground

A rainfall measurement represents precipitation received, not the actual depth of standing water after infiltration, runoff and drainage.

10 mm Rain Does Not Mean 10 Liters Everywhere

It means 10 liters per square meter. Total volume depends on the surface area.

Resolution and Accuracy Are Different

A rain gauge with 0.1 mm resolution is not automatically more accurate than a 0.2 mm instrument. Resolution describes the smallest reported increment, while accuracy describes closeness to the reference rainfall amount.

Rainfall Measurement FAQ

Q1. Why is rainfall measured in mm?

A1. Because rainfall is expressed as an equivalent water depth over a horizontal surface. Millimeters provide a convenient and standardized unit for this depth.

Q2. What does 1 mm of rainfall mean?

A2. It means the rainfall would form a water layer 1 mm deep. This is equivalent to 1 liter of water per square meter.

Q3. How much water is 10 mm of rain?

A3. Ten millimeters of rainfall equals 10 liters per square meter, or approximately 100 cubic meters over one hectare.

Q4. How much is 1 inch of rain in millimeters?

A4. One inch of rainfall equals 25.4 mm.

Q5. Is rainfall measured as depth or volume?

A5. Rainfall is normally reported as depth. If the area is known, that depth can then be converted into a total water volume.

Q6. Does 20 mm of rainfall mean 20 mm of standing water?

A6. No. It represents precipitation depth. The actual standing water depends on infiltration, runoff, drainage and evaporation.

Q7. What instrument measures rainfall in mm?

A7. A rain gauge is used to measure rainfall. Common types include manual, tipping bucket, weighing and electronic rainfall sensors.

Q8. What does 0.2 mm rain gauge resolution mean?

A8. It means the rain gauge reports rainfall in increments of 0.2 mm. For a 0.2 mm tipping bucket gauge, each completed tip represents 0.2 mm of rain.

Q9. Why is rainfall measured in mm instead of cm?

A9. Millimeters provide a convenient level of detail for typical rainfall amounts. Using centimeters would require more decimal values for smaller rain events.

Q10. Can rainfall in mm be used to calculate irrigation water?

A10. Yes, rainfall depth can be converted into water volume, but irrigation decisions should also consider soil moisture, runoff, infiltration, drainage, crop demand and evapotranspiration.

Tipping Bucket Rain Gauge Datasheets

1. NBL-W-ARS Tipping Bucket Rain Gauge Instruction Manual

NBL-W-ARS-Tipping-bucket-rain-gauge-instruction-manual.pdf

2. NBL-W-RS Rain Sensor Instruction Manual

NBL-W-RS-Rain-sensors-instruction-manual-V4.0.pdf

3. NBL-W-DRS Double Tipping Bucket Rain Sensor Manual

NBL-W-DRS-Double-Tipping-Bucket-Rain-Sensor-Instruction-Manual.pdf

Automatic Rainfall Monitoring with NiuBoL

NiuBoL provides tipping bucket rain gauges and automatic rainfall sensors for weather stations, agriculture, hydrology, flood monitoring and environmental IoT applications.

For product selection, provide the required rainfall resolution, output signal, quantity, cable length, installation environment and whether a data logger, 4G communication, solar power or private-server integration is required.

View NiuBoL Tipping Bucket Rain Gauge →