Rainwater Calculator: A Worked Guide

A rainwater calculator turns roof area, annual rainfall, roof surface and efficiency into three answers: yearly collection, collection from one storm, and weeks of garden watering covered. This guide works the verified example through in full, then repeats the same formula for other roof sizes and rainfall levels so you can read your own roof in the tables.

What the rainwater calculator gives you

The calculator asks for roof footprint, roof surface type, annual rainfall, efficiency, and garden weekly water use. Roof footprint means the ground area under the roof, not the sloped surface area of the roof covering.

From those inputs it gives three outputs: water you could collect in a year, water from a single storm, and how many weeks of garden watering the yearly total covers. Yearly collection shows the scale of the resource. Per storm collection shows how much water arrives at once, which shapes tank size.

The calculator does not give a cost figure, a specific tank size in litres, a first flush volume, a pump or pressure figure, or a per crop water use figure. It does not give a monthly breakdown, and it does not size gutters or downpipes. For tank size, use the per storm figure and the weeks figure together, as shown later. One millimetre of rain on one square metre is one litre, and one inch on 1000 square feet is about 620 gallons.

The full formula, step by step

Collectable water for the year equals roof area multiplied by rainfall multiplied by runoff coefficient multiplied by efficiency. Roof area is in square metres, rainfall is in millimetres per year, and the result is in litres per year.

Per storm collection equals roof area multiplied by 25 millimetres multiplied by runoff coefficient multiplied by efficiency. The storm depth is fixed at 25 millimetres, so every per storm figure in this guide uses 25 millimetres.

Weeks covered equals yearly total divided by weekly use, in litres per week, stated as whole weeks.

Runoff coefficient is the share of rain that runs off the roof, after water that splashes, evaporates, or soaks into the roof material. Efficiency accounts for gutter and first flush losses, stated on the page as about 90 percent. First flush is the first, dirtiest water from the roof, which a first flush diverter discards.

The page does not state any runoff coefficient value. The only coefficient used here is 0.95 for a metal roof, derived from the worked example arithmetic, not a page stated figure.

The verified worked example, step by step

The verified example uses an 80 square metre metal roof in an 800 millimetre rainfall area, at 90 percent efficiency. It collects about 54700 litres a year, and about 1710 litres from a single 25 millimetre storm, covering 109 weeks of watering at 500 litres a week, if you could store it.

Yearly calculation: 80 multiplied by 800 equals 64000, the raw volume before any loss. Apply the derived metal coefficient of 0.95, and 64000 multiplied by 0.95 equals 60800. Apply efficiency of 90 percent, and 60800 multiplied by 0.9 equals 54720, stated on the page as about 54700 litres.

Per storm calculation: 80 multiplied by 25 equals 2000. Apply 0.95, and 2000 multiplied by 0.95 equals 1900. Apply 0.9, and 1900 multiplied by 0.9 equals 1710 litres, exactly as the page states.

Weeks calculation: 54700 divided by 500 equals 109.4, stated as 109 whole weeks. The derivation of 0.95 is 54700 divided by 80 times 800 times 0.9, which equals 0.9497, rounded to 0.95. The page never prints 0.95, but the arithmetic implies it for the metal roof in the example.

Yearly collection by roof size

The table below repeats the same yearly formula for other roof sizes, at 800 millimetres of rainfall, the derived metal coefficient of 0.95, and 90 percent efficiency. Per storm collection uses the fixed 25 millimetre storm. Weeks use 500 litres per week. Read across for your roof size, and read the weeks column as whole weeks.

Roof area	Yearly collection	Per storm collection	Weeks at 500 L per week
40 square metres	27360 L	855 L	54
60 square metres	41040 L	1283 L	82
80 square metres	54720 L	1710 L	109
100 square metres	68400 L	2138 L	136
120 square metres	82080 L	2565 L	164
150 square metres	102600 L	3206 L	205

The formula is linear in roof area when rainfall and coefficients stay fixed. A 40 square metre roof collects half the verified example, and doubling the roof doubles the yearly total. Weeks covered also scale with roof area: at 500 litres per week, a 60 square metre roof covers 82 whole weeks, while a 150 square metre roof covers 205 whole weeks. Those counts assume you could store the full yearly total.

Yearly collection by rainfall level

The table below holds roof size fixed at 80 square metres and varies annual rainfall, using the same derived metal coefficient of 0.95 and 90 percent efficiency. Per storm collection does not change across the rows, because the storm is fixed at 25 millimetres. Weeks use 500 litres per week.

Annual rainfall	Yearly collection	Per storm collection	Weeks at 500 L per week
400 mm	27360 L	1710 L	54
600 mm	41040 L	1710 L	82
800 mm	54720 L	1710 L	109
1000 mm	68400 L	1710 L	136
1200 mm	82080 L	1710 L	164

Rainfall is the other linear input. At 400 millimetres a year, the roof collects 27360 litres, exactly half the 800 millimetre total. At 1200 millimetres, it collects 82080 litres, one and a half times the 800 millimetre total. The fixed per storm column deserves a plain note: annual rainfall changes the number of storms and the total volume, but not the volume from one 25 millimetre storm on a given roof. One such storm on this roof collects 1710 litres whether the annual total is 400 or 1200 millimetres. Use your local annual rainfall figure, not a national average.

How many weeks of watering your roof can cover

Weeks covered turns a large yearly litre figure into a time span you can picture. The table below uses the page stated yearly total of about 54700 litres, divided by five weekly use levels.

Weekly garden use	Weeks covered from about 54700 L
250 L per week	218
400 L per week	136
500 L per week	109
750 L per week	72
1000 L per week	54

At 250 litres per week, the total covers 218 whole weeks. At 1000 litres per week, the same total covers 54 whole weeks. Weekly use matters as much as roof size, because the yearly total is fixed while the divisor changes.

If you do not know your weekly use, the garden watering schedule calculator can help you plan a watering routine. Keep the two tools separate: the watering schedule tool plans when and how much to water, while this rainwater calculator measures how long the collected water lasts at that rate.

What one 25 mm storm means for tank size

The calculator does not name a tank size in litres. It suggests a tank large enough for a few weeks of garden use, or large enough to capture one typical storm.

For the verified example, one 25 millimetre storm collects 1710 litres. A tank that can hold that storm captures the full storm instead of overflowing part way through it.

Compare the two figures for your own roof. If you use 500 litres a week, two weeks of use is 1000 litres, below the 1710 litre storm figure, so the storm figure sets the larger requirement. If you use 1000 litres a week, two weeks of use is 2000 litres, above the storm figure, so the weeks figure sets the larger requirement.

The drip irrigation pump calculator can help with pump sizing for a drip system, while the rain barrel calculator gives a barrel based view of the same storm water.

Imperial figures, kept separate

Imperial figures here are rounded equivalents of the metric figures, not exact conversions, and they are kept separate from the metric examples above. Do not mix a metric roof area with an imperial rainfall figure in the same calculation. The page states that one inch of rain on 1000 square feet is about 620 gallons, or 0.62 gallons per square foot per inch, minus losses. The table below applies that factor to one inch of rain, before runoff and efficiency losses.

Roof area in square feet	Water from one inch of rain
500 square feet	310 gallons
1000 square feet	620 gallons
1500 square feet	930 gallons
2000 square feet	1240 gallons

For a rounded equivalent of the verified example, 80 square metres is about 861 square feet, 800 millimetres is about 31.5 inches, and 54700 litres is about 14450 gallons. The page gives no imperial worked example, so this guide does not invent one.

Common mistakes

First, using roof area as the sloped surface area instead of the footprint. The calculator asks for footprint, the ground area under the roof.

Second, skipping efficiency and treating raw rainfall volume as collectable water. Raw volume for the verified example is 64000 litres, but collectable water is about 54700 litres after runoff coefficient and efficiency.

Third, treating the fixed 25 millimetre storm as a variable. The calculator per storm figure always uses 25 millimetres, so replacing it with a local storm depth gives a result that is not the calculator output.

Fourth, reading weeks covered as a storage promise. Weeks covered assumes you could store the water. It does not mean the tank you have today will hold 109 weeks of water.

Fifth, borrowing the derived 0.95 coefficient for a roof that is not metal. The page gives no coefficient for any other roof type.

Sixth, mixing metric and imperial inputs in one calculation.

How this guide differs from other rainwater guides

The site already has a live guide called How much rainwater can my roof collect, and Batch 23 gave a rain barrel view in Rainwater Collected Per Storm and Per Year. This guide does not repeat those angles. It centres the outputs that belong to this calculator alone: runoff coefficient, efficiency, and weeks of watering covered. The rain barrel view answers how many barrels one storm fills, using a 0.623 gallons factor. This calculator answers how much water the roof collects in a year and per storm after runoff coefficient and efficiency, and how many weeks that yearly total covers.

Frequently asked questions

Q: How much water can an 80 square metre metal roof collect in a year?
A: About 54700 litres a year in an 800 millimetre rainfall area at 90 percent efficiency, using the derived metal coefficient of 0.95.

Q: How much water does one 25 millimetre storm collect from that roof?
A: About 1710 litres, from 80 multiplied by 25 multiplied by 0.95 multiplied by 0.9.

Q: How many weeks of watering does that yearly total cover?
A: 109 weeks at 500 litres per week, from 54700 divided by 500, which equals 109.4.

Q: What is the runoff coefficient for a metal roof in this guide?
A: 0.95, derived from the worked example arithmetic, not a page stated figure. The derivation is 54700 divided by 80 times 800 times 0.9, which equals 0.9497, rounded to 0.95.

Q: What does 90 percent efficiency mean?
A: About 90 percent of the water that runs off the roof reaches the tank, after gutter and first flush losses.

Q: Can I use the per storm figure to choose a tank?
A: Yes, as a starting point. One 25 millimetre storm collects 1710 litres, so a tank that holds at least that volume captures the full storm.