What Is the Rainfall Calculator?
This rainfall calculator works out the volume of rain that falls on a given catchment area from the depth of rain that has fallen. Flip it around and it does the reverse: it tells you how much land or roof you need to collect a target volume of water. Along the way you get the weight of that water, its value at your utility's water rate, and a few everyday comparisons so the numbers actually mean something.
It supports both the American (US) system — feet, square feet, acres, inches of rain, US gallons — and the metric system — meters, square meters, hectares, millimeters of rain, liters and cubic meters.
Why Does It Rain? What Does the Water Cycle Look Like?
Does the water cycle ring any bells? Here is a refresher on its five stages.
- Evaporation. Water at the surface absorbs heat from the sun and turns into water vapor.
- Condensation. When that vapor reaches high, cold altitudes, it turns back into tiny ice crystals or water droplets that cluster together and form fog near the ground or clouds across the atmosphere.
- Precipitation. The moment we have been waiting for. When the wind or temperature changes, droplets merge into bigger droplets. Eventually the air cannot hold any more water and it precipitates — as rain, or as snow if the temperature is below 0 °C (32 °F).
- Runoff. Precipitation that does not soak in runs over the Earth's surface toward streams, rivers and lakes — and, if you have gutters, into your tank.
- Infiltration. The rest of the water moves down into the soil, seeping deeper to recharge groundwater.
How to Calculate Rainfall Volume
Calculating rainfall volume sounds like a headache, but it is genuinely simple. All you need is:
- the size of the catchment area — the roof, yard or plot of land the rain lands on; and
- the depth of rain that fell on it, which is exactly what a rain gauge measures.
Multiply those two values together and you have the volume of rainfall:
Volume = Catchment area × Rainfall depth
The metric system makes this beautifully easy, because 1 mm of rain falling on 1 m² is exactly 1 liter of water. So area in square meters multiplied by rain in millimeters gives you liters directly, with no conversion at all.
Example: your backyard covers 50 m² and 5 cm (50 mm) of rain fell yesterday. Volume = 50 m² × 50 mm = 2,500 liters — that is 2.5 m³, or about 660 US gallons, or two and a half metric tons of water sitting on your lawn.
American example: a 40 ft × 30 ft roof (1,200 ft²) catches 1 inch of rain. Volume = 1,200 ft² × (1/12) ft = 100 ft³ ≈ 748 US gallons.
Runoff Coefficient — Why You Never Collect 100%
Not every drop that lands makes it into your barrel. Some soaks into the surface, some evaporates before it drains, and some splashes over the edge of the gutter. The fraction that actually reaches your tank is called the runoff coefficient.
- Metal roof — about 0.95. The smoothest, least absorbent surface there is.
- Tile roof — about 0.90.
- Asphalt shingle roof — about 0.85.
- Concrete or paved surface — about 0.80.
- Gravel roof — about 0.70. The gravel holds water back.
- Green roof — about 0.30. Most of the rain is deliberately retained by the plants and growing medium.
- Lawn or bare soil — about 0.20. Nearly all of it infiltrates, which is usually the point.
Leave the setting on all the rain that fell (1.00) when you want the total volume of precipitation rather than the harvestable amount.
How to Calculate the Catchment Area You Need
Run the formula backwards when you know the volume you want:
Area = Volume ÷ (Rainfall depth × Runoff coefficient)
This is the calculation to do before you buy a tank. If you want 5,000 liters from a season that reliably delivers 200 mm of rain onto a tile roof (0.90), you need 5,000 ÷ (200 × 0.90) ≈ 28 m² of roof.
Why Calculate Rainfall Volume?
- Rainwater harvesting. Size a tank, a cistern or a rain barrel to what your roof will actually deliver.
- Garden and irrigation planning. Find out whether last night's shower watered your beds or whether you still need to.
- Drainage and runoff. Estimate how much water your gutters, swales and French drains have to move during a storm.
- Ponds, lakes and reservoirs. Work out how much water a year of rainfall contributes to a body of water.
- Structural load. Water is heavy — 1 liter weighs 1 kilogram, and a gallon weighs about 8.34 pounds. Flat roofs and tarpaulins need to carry it.
- Farming. Convert a rainfall total into the volume delivered across a field in acres or hectares.
How to Use the Rainfall Calculator
- Pick your measurement system: American (US) or metric.
- Choose what you want to find: the rainfall volume, or the catchment area needed for a target volume.
- Enter the catchment either as length × width or as a total area in ft², yd², acres, m² or hectares. For a pitched roof, use the ground footprint — a slope catches the same rain as its shadow.
- Enter the rainfall depth in inches, millimeters or centimeters.
- Select the catchment surface to apply its runoff coefficient, or leave it at 1.00 for the total rain that fell.
- Optionally add your water price and pick a currency — USD, RUB (₽), EUR, GBP, CAD, AUD, JPY, CNY, BRL, MXN, INR or KRW — to see what the collected water is worth.
- Press Calculate.
FAQs
How much water is 1 inch of rain?
One inch of rain delivers about 0.623 US gallons per square foot, so roughly 623 gallons per 1,000 ft². Over a full acre it comes to about 27,150 US gallons — a little over 100 cubic meters.
How much water is 1 mm of rain?
Exactly 1 liter per square meter. Over a hectare, 1 mm of rain is 10,000 liters, or 10 m³.
Should I use the roof's sloped area or its footprint?
Use the footprint — the area the roof covers when seen from above. Rain falls vertically, so a pitched roof intercepts exactly as much rain as its flat shadow on the ground, even though its surface is larger.
How much does rainwater weigh?
Water weighs about 1 kilogram per liter, or 8.34 pounds per US gallon. A cubic meter is one metric ton. The calculator shows the weight in kilograms, metric tons, pounds and US short tons — worth checking before you put a full tank on a balcony or a flat roof.
Does the calculator account for gutters overflowing or first-flush diversion?
No. The runoff coefficient covers ordinary surface losses. If you use a first-flush diverter, subtract its capacity from the result, and remember that a downpour heavier than your gutters can carry will spill regardless of the arithmetic.
What is a CCF on my water bill?
CCF means one hundred cubic feet — about 748 US gallons or 2,832 liters. Many US utilities bill in CCF, so it is one of the price units you can choose when valuing the water you collect.
Can I use this for snow?
Only after converting the snow to its liquid equivalent — snow depth is many times the depth of the water it holds. Melted snow behaves like rain in this formula once you have that liquid figure.