Rainfall Calculator
Turn a rain gauge reading into litres of water: total rainfall volume on your catchment, how much is actually harvestable, and how much a tank of a given size will spill.
🌍 What is a Rainfall Calculator?
A rainfall calculator converts a depth measurement, the millimetres shown by a rain gauge or quoted in a forecast, into an actual volume of water landing on a surface of known size. The conversion rests on one clean identity: one millimetre of rain over one square metre is one litre. Everything else in the calculation is bookkeeping around that fact, namely how much of that water runs off cleanly into a gutter, and how much a storage tank can actually hold before it spills.
People reach for this calculation in a lot of practical settings. A householder sizing a water butt wants to know whether a typical 10 mm shower will fill it or barely wet the bottom. A grower planning drip irrigation wants to know how many litres a polytunnel roof yields over a monsoon month. A civil drainage designer needs the volume arriving at a gully during a design storm so the pipe can be sized. A field ecologist converting a season's rain gauge record into water delivered per hectare is doing exactly the same arithmetic at a much larger area.
Two misconceptions cause most of the errors. The first is using the sloping surface area of a pitched roof instead of its horizontal footprint. Rain falls vertically, so a steeply pitched roof captures precisely as much water as a flat roof covering the same ground plan, and using the slope area inflates the answer. The second is treating the runoff coefficient as an optimistic guess rather than a measured property of the surface. Smooth metal roofing genuinely delivers around 0.9 of what lands on it, but a gravel yard delivers about half, and dry permeable soil can absorb nearly all of a light shower before any runoff begins at all.
This calculator handles all four quantities at once. It reports the total volume of rain that fell on the catchment, the harvestable runoff after the coefficient is applied, how much of that a tank of your chosen size can hold, and how much spills over the top. The chart shows stored volume rising with rainfall depth and then flattening the moment the tank fills, which makes the tank-sizing trade-off visible at a glance, and the scenario table runs the same catchment through everything from a 1 mm drizzle to a 100 mm cloudburst.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 — Suburban Roof, Ordinary Rainy Day
25 mm of rain on a 100 m² tiled roof, C = 0.85, 5000 L tank
Example 2 — Monsoon Cloudburst Overwhelms the Tank
120 mm on a 150 m² metal roof, C = 0.9, 10000 L tank
Example 3 — Garden Shed, Light Shower
6 mm on a 20 m² shed roof, C = 0.8, 1000 L butt
❓ Frequently Asked Questions
🔗 Related Calculators
How many litres is 1 mm of rain?
One millimetre of rainfall over one square metre of ground is exactly one litre of water, because 1 mm depth times 1 m² area equals 0.001 m³, and one cubic metre holds 1000 litres. That identity is why the whole rainfall volume calculation reduces to litres = area in m² times depth in mm.
What is the formula for rainfall volume?
Volume in litres = catchment area (m²) times rainfall depth (mm). To get harvestable water, multiply by the runoff coefficient C, which accounts for splash, evaporation, wetting of the surface and leakage: harvestable litres = A times d times C. Divide litres by 1000 for cubic metres.
What is a runoff coefficient?
The runoff coefficient is the fraction of rain landing on a surface that actually reaches the gutter or drain. It is close to 0.9 for smooth metal roofing, around 0.8 for concrete, near 0.5 for gravel, and as low as 0.1 for dry permeable soil that soaks up most of the shower before any runoff starts.
Should I use the sloped roof area or the footprint?
Use the horizontal footprint, the shadow the roof casts at midday. Rain falls vertically, so a 45 degree pitched roof intercepts exactly the same rainfall as a flat roof covering the same ground plan. Using the sloping surface area would overstate your yield by the secant of the pitch angle.
How much rain does it take to fill a 5000 litre tank?
Divide the tank capacity by the product of area and runoff coefficient. For a 100 m² roof at C = 0.85, each millimetre yields 85 litres, so 5000 divided by 85 is about 58.8 mm of rain. Anything beyond that in a single storm overflows unless the tank was partly empty to start with.
Does this calculator include first-flush diversion?
Not as a separate input. The simplest way to include it is to subtract the first-flush depth from your rainfall figure before entering it, typically 0.4 to 1 mm per event, or to lower the runoff coefficient slightly to absorb the loss across a season.
How much does the collected rain weigh?
Fresh water has a density very close to 1000 kg per cubic metre, so every 1000 litres weighs about one tonne. A 2500 litre catch is roughly 2.5 tonnes, which is why tank bases, roof-mounted butts and gutter brackets have to be sized structurally, not just by volume.
Why is rainfall measured in millimetres instead of litres?
Depth is area independent. A rain gauge in one field and a satellite estimate over a whole basin can both report 25 mm and mean the same thing, whereas a volume figure would be meaningless without also stating the area it fell on. Depth also converts directly to litres per square metre.
What counts as heavy rain?
Meteorological services usually call anything under 2.5 mm per hour light, 2.5 to 7.6 mm per hour moderate, and above 7.6 mm per hour heavy. Daily totals are graded differently: 25 mm in a day is a decent soaking, 65 mm is heavy, and above 115 mm most agencies issue a flood warning.
Can I use this for a garden or a field instead of a roof?
Yes, but drop the runoff coefficient sharply. A lawn on dry soil might only shed 0.1 of the rain that lands on it, and a well-mulched bed close to zero, because the point of those surfaces is to hold water in place. Use high coefficients only for sealed surfaces like roofs and paving.