Free rain gauge calculator. Enter gauge diameter and rainfall depth to find the volume of rainwater your gauge collected in millilitres.
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Enter your rain gauge diameter in centimetres and the rainfall depth in millimetres. This rain gauge volume calculator first finds the gauge opening area from the diameter, then multiplies that area by the rainfall depth to give the collected volume in millilitres. The formula is volume in ml equals pi times radius squared times rainfall depth, with units converted so the answer comes out in millilitres. Use the comparison table below to check your result against other common gauge sizes.
How do you measure rainfall with a rain gauge? A rain gauge is an open cylinder that catches rain over a known opening area, and the depth of water inside it equals the rainfall depth, because the same rain falls evenly over the opening. That is why rainfall is reported in millimetres or inches of depth, not in litres: depth is the same whatever the gauge size, while volume depends on the opening area. Read the gauge at eye level, on level ground, away from trees and walls that block rain or drip extra water into it.
Rainfall depth to volume formula for a circular gauge is simple geometry. Gauge opening area equals pi times radius squared, where radius is half the diameter, and collected volume equals area times rainfall depth. In millimetres, volume in cubic millimetres equals pi times radius in millimetres squared times depth in millimetres, and 1,000 cubic millimetres equals 1 millilitre. So volume in ml equals pi times radius squared times depth divided by 1,000. This calculator does that conversion for you when you enter diameter in centimetres and depth in millimetres.
For a gauge diameter of 10 centimetres and rainfall of 12 millimetres, the radius is 5 centimetres, the opening area is about 78.5 square centimetres, and the volume is 94.2 millilitres. For a gauge diameter of 20 centimetres and rainfall of 10 millimetres, the radius is 10 centimetres and the volume is 314.2 millilitres, because doubling the diameter quadruples the area. That quadrupling is the key check: a wider gauge collects much more water for the same rainfall depth, even though the depth reading is identical.
Rain gauge diameter to volume in ml at 10 millimetres of rain: an 80 millimetre gauge collects about 50.3 ml, a 100 millimetre gauge collects about 78.5 ml, a 127 millimetre gauge (about 5 inches, a common standard size) collects about 126.7 ml, and a 200 millimetre gauge collects about 314.2 ml. At 25 millimetres of rain, multiply each figure by 2.5: about 125.7 ml, 196.3 ml, 316.7 ml and 785.4 ml respectively. Use this table to spot a reading error: if your gauge volume is far from the table for its size, check the diameter and depth units again.
How do I convert mm of rain to ml? Multiply the gauge opening area in square millimetres by the depth in millimetres, then divide by 1,000. How many ml in a rain gauge after a storm depends on both numbers, not on depth alone, which is why a depth only answer is incomplete for a gauge. Two shortcuts help for larger areas: 1 mm rain equals how many litres per square metre? Exactly 1 litre, because 1 millimetre over 1 square metre is 1 litre. And 1 inch rain gallons per square foot is about 0.623 gallons, the shortcut used by roof harvesting calculators.
A catchment area calculator is a different tool from a rain gauge volume calculator, and the difference matters. A gauge uses its small opening area, while a roof or catchment uses a large area, often with a runoff coefficient for losses to absorption, evaporation and first flush diversion. The roof formula is volume equals rainfall times catchment area times runoff coefficient, with unit conversion, and a typical coefficient is 0.8 to 0.9 for a roof. Do not use a gauge volume as a roof volume: the same 10 millimetres over a 100 square metre roof is 1,000 litres before losses, while over a 100 millimetre gauge it is only 78.5 millilitres.
Gauge readings go wrong in predictable ways: the gauge is tilted, so the opening area is effectively smaller; the gauge overflows in heavy rain, so volume is lost; water evaporates before reading, so volume is low; or units are mixed, such as diameter in inches with depth in millimetres. Empty the gauge after each reading, keep it level and clean, and read it promptly after rain. For very heavy rain, use a larger gauge or a measuring cylinder with a known diameter, and record both diameter and depth with every volume you report.
How much water does a rain gauge collect is useful beyond curiosity: gardeners compare gauge volume with watering needs, rainwater harvesting planners check local rainfall before sizing a barrel, and stormwater planners use depth and area to estimate runoff. A rain collection calculator for a roof starts from the same depth figure, then scales it by catchment area and efficiency. Keep a simple log of date, depth and volume, and your local rainfall pattern becomes clear within a few months.
Multiply the collecting area by the rainfall depth, with unit conversion. For a gauge, volume in ml equals pi times radius squared times depth, divided by 1,000 when radius and depth are in millimetres. For a roof, volume equals rainfall times catchment area times a runoff coefficient.
You need the gauge area as well as the depth: volume in ml equals gauge area in square millimetres times depth in millimetres divided by 1,000. For example, a 100 millimetre gauge at 10 millimetres of rain collects about 78.5 ml.
It depends on gauge size and rainfall depth. At 10 millimetres of rain, an 80 millimetre gauge holds about 50.3 ml, a 100 millimetre gauge about 78.5 ml, and a 200 millimetre gauge about 314.2 ml.
A catchment area is the surface that collects rain, such as a roof, measured in square metres or square feet. A rain gauge opening is a tiny catchment, while a roof is a large one, so the same rainfall depth gives very different volumes.
Yes, for roofs and ground catchments. Smooth roofs lose less water than rough or absorbent surfaces, so calculators use a runoff coefficient, often 0.8 to 0.9 for a roof, to estimate collected water after losses. A gauge has no runoff loss in the same sense, because it catches rain directly.
Place an open cylinder on level ground in an open spot, read the water depth after rain, and record it in millimetres or inches. Depth is the rainfall figure; multiply by the opening area to get volume in millilitres.
Use the depth figure from your gauge as the rainfall input, then scale it by roof area for harvesting: volume in litres equals rainfall in millimetres times roof area in square metres times a runoff coefficient. Do not use the gauge volume itself as the roof volume.
It depends on roof area, rainfall rate and barrel size. As a guide, 500 square feet of roof at 1 inch of rain per hour, with a 0.8 runoff coefficient, collects about 249 gallons per hour, so a 50 gallon barrel fills in about 12 minutes at that rate.
Use the horizontal footprint area, not the sloped surface area, because rainfall depth is measured vertically. Using the sloped area overstates the collected volume.