Rainwater Collection Calculator
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A square foot of roof yields about 0.62 gallons per inch of rain before losses. That single figure is most of what rainwater planning is, and people size tanks by eye and are usually out by a factor.
Footprint, not surface area. A steeper roof does not catch more rain.
One storm, a month, or a year. Your county figure, not a national one.
Installers work on 75 to 85 percent once a diverter is in line.
What that roof yields
Enter a roof footprint and a rainfall figure.
First flush diverter for that roof
Enter a roof footprint.
This is planning arithmetic, not a system design. It cannot see your gutter sizing, your overflow route, your freeze risk, or what your local rules allow. A diverter that freezes solid and cracks is a common failure and no calculator predicts it.
None of this makes the water drinkable. A first flush diverter removes the worst of the debris, not the pathogens. The Texas manual is explicit that rainwater intended for domestic potable use must be treated with appropriate filtration and disinfection, and the Texas Water Development Board notes there are no federal or state water quality standards for harvested rainwater.
Why there are two answers here
Collection volume and diverter size come from the same measurement, so asking for your roof twice across two pages would be worse rather than tidier. Measure the footprint once and both numbers fall out.
The theoretical figure is shown next to the realistic one on purpose. The gap between them is not rounding, it is the water you will never see, and knowing its size is what stops a tank being ordered on the optimistic number.
What this does not tell you
It is planning arithmetic. It cannot see your gutter sizing, your overflow route, your freeze risk, or what your local rules allow. Some jurisdictions regulate harvesting; check yours before buying a tank.
And it does not make water drinkable. No arrangement of diverters and filters described anywhere on this site turns roof runoff into drinking water. Read the first flush diverter guide for what a diverter does and does not do, including why they freeze and crack.
Common questions
How much rainwater can I collect from my roof?
The Texas Water Development Board puts it at approximately 0.62 gallons per square foot of roof area per inch of rainfall, before losses. Multiply your roof footprint by the rainfall in inches by 0.62. Then take 75 to 85 percent of that, because splash, wind, wetting the roof surface and gutter overshoot all take a share.
Do I use the roof footprint or the actual sloped surface area?
Footprint. Rain falls vertically, so what matters is the area the roof covers when seen from above, not the surface area of the slope. A steeply pitched roof does not catch more rain than a flat one of the same footprint.
How big should a first flush diverter be?
The Texas Manual on Rainwater Harvesting gives a rule of thumb of a minimum of 10 gallons for every 1,000 square feet of collection surface, and elsewhere cites a range of one to two gallons per 100 square feet. Those are the same scale. The manual is openly honest that opinions on the right figure vary.
Does a first flush diverter make rainwater safe to drink?
No. It removes the worst of the debris, not the pathogens. The Texas manual is explicit that rainwater intended for domestic potable use must be treated using appropriate filtration and disinfection. The Texas Water Development Board also notes there are currently no federal or state water quality standards for harvested rainwater.
Why does this calculator not know my local rainfall?
Because rainfall is local and you can look yours up more accurately than any national average we could print. A single figure baked into a calculator would look precise and be wrong for almost everybody using it.
Supplies you will buy again
What the arithmetic turns into, once you know the numbers.
Downpipe first flush diverter kit
The standpipe design the guide describes: it fills with the dirtiest first minutes of runoff, then lets the rest pass to the tank.
Holds a column of water between storms, so it is a poor choice where winters cross freezing. Size it on roof footprint, per downspout, and check the drain orifice.
Check price on Amazon
Downspout leaf filter with steep mesh screen
Pre-filtration that holds no standing water, which is why it keeps working in a freezing climate. Screens leaves and coarse debris before they can block anything downstream.
A screen is not a diverter. It removes debris, not the fine and biological load in the first flush, and neither one makes rainwater safe to drink.
Check price on Amazon
Next
First Flush Diverters
Sizing from the Texas manual, why they crack, and the line they do not cross.
Rain Barrels
Where the water goes once you know how much of it there is.
The Homestead Planning Kit
Worksheet 05 is this calculation on paper, for the roof you are standing under.
Generator Sizing
The other calculation people guess at, and the one that costs most when guessed high.