How much water can I collect from my roof?

Catchment area times rainfall depth times your runoff factor, less whatever you divert as a first flush. A millimetre of rain on a square metre is one litre exactly, by the definition of the litre, so nothing is rounded on the way in. On the example this page opens with, 100 square metres under 10 millimetres of rain is 1,000 litres gross, a runoff factor of 0.80 leaves 800 litres, and a 20 litre first flush leaves 780 litres, which the page also prints as 206 US gallons. Every one of those steps is on the screen at the same time, which is the point: you can check the one you doubt instead of trusting a single figure.

What runoff factor should I put in?

That is yours to decide, and the field is there so the number stays visible rather than baked into the answer. The factor is the share of the rain that actually reaches the tank: some of it wets the surface and evaporates, some overshoots the gutter in a downpour, and some is lost at the joints, so a roof never delivers the full depth that fell on it. It depends on the surface, the pitch and the gutters, so the 0.80 this page opens with is part of an example to type over rather than a figure it stands behind, and whatever you enter is printed beside the answer and in the assumptions under it. Put 1 in and you get the plain area times depth figure, which is the theoretical maximum rather than an estimate.

How many gallons is an inch of rain on 1,000 square feet?

623 US gallons before any losses. An inch of rain on one square foot is 0.6234 US gallons, worked from the exact definitions rather than a rounded table: a square foot is 0.09290304 square metres, an inch is 0.0254 metres, and a US liquid gallon is 3.785411784 litres. Switch the units under the fields to US units and the page works in square feet, inches and gallons throughout, with the litre figure in the fourth cell. Apply your runoff factor and take off the first flush and you have what reaches the tank.

What is a first flush, and what does this page do with it?

It is the volume you send to waste at the start of a rainfall instead of into the tank, because the first water off a roof carries what settled there since the last rain. This page treats it as a plain volume you set: it is subtracted once per rainfall, after the runoff factor has been applied, and it is printed on its own line so you can see what it cost you. If the diversion is bigger than the catch, the answer is 0 and never a negative number. On the same 100 square metre roof, 0.2 millimetres of rain is 20 litres gross and 16 litres after the factor, so a 20 litre diverter takes all of it.

Can I work out a whole month instead of one rainfall?

Yes. Switch the rainfall from one rainfall to a total, type the month total and say how many separate rainfalls it covers, and the first flush is diverted once for each of them rather than once for the month. On the example roof, 60 millimetres spread over 6 rainfalls is 6,000 litres gross, 4,800 after a 0.80 factor, 120 litres diverted in six goes, and 4,680 litres usable. Up to 366 rainfalls can be counted in one total, which is a full year of them.

Can this page tell me whether the water is drinkable?

No, and it does not try. It multiplies the figures you supply and prints the result: it knows nothing about your roof material, your gutters, what has settled on them, or the rules where you live. Treatment, testing and the local regulations are a different question from how many litres arrive, and this page answers only the second one. It also says nothing about what a roof, a gutter bracket or a full tank can carry, which is work for whoever is specifying the installation.

Rainwater Harvesting Calculator

Type the roof area, the depth of rain, the runoff factor your surface gives and the volume you divert as a first flush, and read what is left. The example the page opens on is 100 square metres under 10 millimetres of rain: that is 1,000 litres gross, 800 litres after a runoff factor of 0.80, and 780 litres once a 20 litre first flush has gone, which is 206 US gallons. All four figures stay on the screen at once, with the working beside them, because a single litre number you cannot take apart is the thing every other harvesting page hands you. The runoff factor is a field rather than a constant hidden in the code, since it depends on your roof and it scales the answer exactly as directly as the area and the depth do. One rainfall or a total across several, metric or US units, and the recoverable figure is never a negative number: if the first flush is bigger than the catch, the answer is 0.

Roof 100 m2Rain 10 mmRunoff 0.8First flush 20 L

roof area       100 m2
rainfall         10 mm   =  0.010 m

gross           1,000 L   (100 x 0.010 = 1 m3)
runoff 0.80       800 L
first flush        20 L
              -------
usable            780 L   =  206 US gal

Gross

1,000 L

After runoff

800 L

Usable

780 L

In gallons

206

100 m2 of roof at 10 mm of rain, runoff factor 0.80, less a 20 L first flush: 780 L usable, which is 206 US gallons.

1 mm of rain on 1 m2 is 1 litre, so 100 m2 x 10 mm is 1,000 L before any losses.
Runoff factor 0.80, entered by you: the share of that water the roof and gutters actually deliver.
First flush 20 L, diverted once before anything is collected.
Roof area is the flat footprint the rain falls on, not the sloped surface, and the area is yours to measure.

Your catchment

The runoff factor is the step that separates this from multiplying an area by a depth. Rain that lands on a roof is never all collected: some wets the surface, some runs past the gutter in a downpour and some evaporates, so the factor is the share that actually arrives. It is a field here because it depends on your roof, and it stays beside the answer rather than hiding inside it. A factor of 1 gives the plain area times depth figure. Roof area means the flat footprint the rain falls on, not the sloped surface. Depths up to 100,000 mm and roofs up to 1,000,000 m2 are taken, and a total can cover up to 366 rainfalls, one first flush each. Switching between metric and US units rewrites what you already typed instead of clearing it. The figures are kept in this browser so the page opens where you left it, and Start over at the top of the page forgets them.

100 m2, 10 mm, factor 0.80one rainfall
800 L before the first flush
Same roof, 2 mm of rain20 L first flush
140 L usable
First flush bigger than the catch0.2 mm of rain
0 L, never a negative number

Accuracy. Exact arithmetic: catchment area times rainfall depth times your runoff factor, less the first-flush volume you set, and floored at zero. The runoff factor is yours to enter because it depends on your roof surface, and it is shown beside the answer. This sizes a tank; it says nothing about whether the water is drinkable and nothing about what the roof or tank can carry. The gross catch, the figure after the factor and the figure after the diversion are three separate cells above, so nothing is folded into one number you cannot take apart.

Common questions

How much water can I collect from my roof?: Catchment area times rainfall depth times your runoff factor, less whatever you divert as a first flush. A millimetre of rain on a square metre is one litre exactly, by the definition of the litre, so nothing is rounded on the way in. On the example this page opens with, 100 square metres under 10 millimetres of rain is 1,000 litres gross, a runoff factor of 0.80 leaves 800 litres, and a 20 litre first flush leaves 780 litres, which the page also prints as 206 US gallons. Every one of those steps is on the screen at the same time, which is the point: you can check the one you doubt instead of trusting a single figure.
What runoff factor should I put in?: That is yours to decide, and the field is there so the number stays visible rather than baked into the answer. The factor is the share of the rain that actually reaches the tank: some of it wets the surface and evaporates, some overshoots the gutter in a downpour, and some is lost at the joints, so a roof never delivers the full depth that fell on it. It depends on the surface, the pitch and the gutters, so the 0.80 this page opens with is part of an example to type over rather than a figure it stands behind, and whatever you enter is printed beside the answer and in the assumptions under it. Put 1 in and you get the plain area times depth figure, which is the theoretical maximum rather than an estimate.
How many gallons is an inch of rain on 1,000 square feet?: 623 US gallons before any losses. An inch of rain on one square foot is 0.6234 US gallons, worked from the exact definitions rather than a rounded table: a square foot is 0.09290304 square metres, an inch is 0.0254 metres, and a US liquid gallon is 3.785411784 litres. Switch the units under the fields to US units and the page works in square feet, inches and gallons throughout, with the litre figure in the fourth cell. Apply your runoff factor and take off the first flush and you have what reaches the tank.
What is a first flush, and what does this page do with it?: It is the volume you send to waste at the start of a rainfall instead of into the tank, because the first water off a roof carries what settled there since the last rain. This page treats it as a plain volume you set: it is subtracted once per rainfall, after the runoff factor has been applied, and it is printed on its own line so you can see what it cost you. If the diversion is bigger than the catch, the answer is 0 and never a negative number. On the same 100 square metre roof, 0.2 millimetres of rain is 20 litres gross and 16 litres after the factor, so a 20 litre diverter takes all of it.
Can I work out a whole month instead of one rainfall?: Yes. Switch the rainfall from one rainfall to a total, type the month total and say how many separate rainfalls it covers, and the first flush is diverted once for each of them rather than once for the month. On the example roof, 60 millimetres spread over 6 rainfalls is 6,000 litres gross, 4,800 after a 0.80 factor, 120 litres diverted in six goes, and 4,680 litres usable. Up to 366 rainfalls can be counted in one total, which is a full year of them.
Can this page tell me whether the water is drinkable?: No, and it does not try. It multiplies the figures you supply and prints the result: it knows nothing about your roof material, your gutters, what has settled on them, or the rules where you live. Treatment, testing and the local regulations are a different question from how many litres arrive, and this page answers only the second one. It also says nothing about what a roof, a gutter bracket or a full tank can carry, which is work for whoever is specifying the installation.
Is anything uploaded, and does it cost anything?: No. The arithmetic runs in your browser, there is no account and nothing is sent anywhere. Your figures, the units and whether you are working on one rainfall or a total are kept in your own browser so the page opens where you left it, and Start over at the top of the page forgets them, as does clearing your browser data. Nothing here is gated and nothing is measured for you: the roof area, the rainfall and the runoff factor are all yours to enter.

Exact arithmetic: catchment area times rainfall depth times your runoff factor, less the first-flush volume you set, and floored at zero. The runoff factor is yours to enter because it depends on your roof surface, and it is shown beside the answer. This sizes a tank; it says nothing about whether the water is drinkable and nothing about what the roof or tank can carry.