The one coefficient that matters
A cubic foot holds 7.48 gallons. An inch of rain on one square foot is one twelfth of a cubic foot, so it is 7.48 divided by 12, which is 0.623 gallons. Everything on this page is that number multiplied by an area and a rainfall depth. A thousand square feet of roof catching an inch of rain is 623 gallons gross, which is a great deal more water than the rain barrel most people start with, and it is why anyone who installs a single 55 gallon barrel watches it overflow in the first real storm.
What comes off that gross figure is real but modest. Some water wets the roof and evaporates before it runs, some overshoots the gutter in heavy rain, some splashes at the corners, and gutters that are not clean lose more. Collection efficiency is commonly taken somewhere between 75 and 90 percent, and it is an input here because it depends on your gutters and your roof rather than on any published constant.
Footprint, not slope — the error that inflates every estimate
Rain falls vertically. A roof intercepts the rain that would otherwise land on the outline it casts on the ground, and pitching the roof steeper does not make it catch more. The area to use is the building footprint including the eave overhang, exactly the number you would use for a foundation drawing, and never the sloped surface area you would use to order shingles.
The size of the mistake is worth knowing because it appears in a lot of published examples. A 6/12 roof has about 12 percent more surface than footprint. A 9/12 roof has 25 percent more. A 12/12 roof has just over 41 percent more. Estimating catchment off the shingle figure quietly inflates the answer by that much, which is enough to make a tank size come out wrong.
Partial catchment is the other adjustment. A hip roof with four downspouts sends roughly a quarter of the water to each, so plumbing one downspout to a tank catches a quarter of the roof. The share field handles that, and getting it wrong is a more common error than the slope one.
First flush, and why it repeats
The first water off a roof in a storm carries the accumulated dust, pollen, leaf fragments, insect debris and bird and rodent droppings that settled since the last rain. A first-flush diverter sets that volume aside before flow reaches the tank. Volumes commonly quoted run around half a gallon to two gallons per hundred square feet of roof, with the higher end for roofs under trees, in dusty conditions, or after a long dry spell.
The detail people miss is that the diversion happens once per rain event, not once per inch. Ten separate quarter-inch showers divert ten first flushes and collect very little; one two-and-a-half inch storm diverts once and fills the tank. That is why the number of events is a field here, and why an annual figure calculated as one event overstates what a real year delivers.
Sizing a tank against demand rather than against supply
Two questions get confused. How much can this roof yield is a supply question, answered by area, rainfall and efficiency. How big should the tank be is a demand question, answered by how much you draw per day and how long the dry gap between useful rains runs. A roof that yields far more than you use annually still needs a tank sized for the gap, because water not stored when it falls is water down the overflow.
The dry-gap figure is local and seasonal, and it is not the average interval between rainy days. It is the length of the dry spell you are unwilling to run out during, which for irrigation might be three weeks and for a system with no backup is longer. Both demand and the storage target are inputs because neither is knowable from a page.
Weight, siting and the parts this does not cover
Water weighs about 8.34 pounds per gallon. A 1,000 gallon tank holds more than four tons and a 2,500 gallon cistern holds over ten, concentrated on a small footprint. The base is a structural question, not a landscaping one, and a tank on inadequate footing is a hazard as well as a maintenance problem. Elevated tanks compound it considerably.
Not covered here: overflow routing away from the foundation, screening and sealing against mosquitoes, freeze protection, pump selection, and anything to do with treatment. Roof water is not drinking water. It carries whatever settled on the roof and whatever the roofing material sheds, some materials are unsuitable for potable collection outright, and making it safe to drink is a design and testing question for a qualified water professional rather than a procedure to take from a calculator.
The legal position varies more than almost anything else on this site. Some states promote harvesting and offer incentives, some regulate tank size or permitted end uses, and in a few the practice interacts with prior appropriation water rights in ways that are genuinely restrictive. Plumbing a tank into a building brings in cross-connection rules and the local building department. Ask your state water resources or environmental agency and your local building official first.
For the gutters and downspouts feeding it, the gutter and downspout sizing calculator covers the intake side. If the water is going onto a garden, the irrigation zone calculator turns gallons into run times, and the hoop house sizing calculator is worth pairing with it since a hoop house is a catchment surface as well as a growing space.
Questions people ask
How many gallons come off a roof per inch of rain?
About 0.623 gallons per square foot of roof footprint per inch, before losses. That works out to roughly 623 gallons per 1,000 square feet per inch gross, or somewhere near 470 to 560 gallons once a realistic collection efficiency of 75 to 90 percent is applied and a first flush is diverted. The coefficient comes straight from unit conversion: a cubic foot is 7.48 gallons and an inch is a twelfth of a foot, so 7.48 divided by 12 gives 0.623. You will occasionally see 0.6 used as a round figure, which is close enough for a rain barrel and about 4 percent low for a cistern.
Do I use the roof surface area or the footprint?
The footprint, meaning the outline the building casts on the ground including the eave overhang. Rain falls vertically, so the roof intercepts exactly the rain that would have landed on that outline, and pitching the roof steeper catches no more water. This is one of the most common errors in published rainwater examples because people reuse the sloped area they calculated for shingles. The overstatement is not trivial: a 6/12 roof has about 12 percent more sloped surface than footprint, a 9/12 roof 25 percent, and a 12/12 roof over 41 percent. If only some downspouts feed the tank, reduce the footprint by that share as well.
How big should the first-flush diverter be?
Commonly quoted figures run around half a gallon to two gallons per hundred square feet of roof, with the higher end appropriate for roofs under trees, in dusty locations, or after a long dry spell. The purpose is to set aside the water that carries the accumulated dust, pollen, debris and animal droppings from the roof surface before flow reaches the tank. What matters as much as the volume is that it diverts once per rain event and then has to drain and reset before the next one. A season of frequent light showers diverts many first flushes and collects little, which is why the number of events is worth entering honestly rather than treating a monthly rainfall total as a single storm.
Can I drink rainwater collected from my roof?
Not as collected. Roof runoff carries dust, pollen, leaf and insect debris, bird and rodent droppings, and whatever the roofing material and any moss or algae treatment shed into it, and some roofing materials are unsuitable for potable collection at all. Making roof water safe to drink is a system design question involving material selection, exclusion, filtration, disinfection and ongoing testing, and it belongs with a qualified water professional and your local health department rather than with a procedure copied from anywhere. This page sizes the catch and the tank and deliberately gives no treatment guidance. For irrigation and other non-potable uses, standard practice still involves screening, keeping the tank sealed against mosquitoes and light, and routing the overflow away from the foundation.
Is rainwater harvesting legal where I live?
It depends on the state and often on the locality, and this is an area where assumptions are unreliable. Some states actively encourage collection and offer rebates or tax incentives. Others place limits on storage volume or on permitted end uses. In a few western states, collection interacts with prior appropriation water rights, on the reasoning that runoff would otherwise reach a watercourse where someone else holds a right to it, and the rules there have changed several times in recent decades. Separately from water law, plumbing a tank into a building raises cross-connection and backflow requirements, and a permanent cistern may need a permit. Contact your state water resources or environmental agency and your local building department before installing, rather than after.