How much water does a roof collect from 1 inch of rain?

Each square foot of flat footprint gets 0.6234 gallons from one inch of rain (144 cubic inches ÷ 231 cubic inches per gallon), so 1,000 sq ft gets 623 gallons. At 85% efficiency a 1,000 sq ft footprint collects 530 gallons.

Do I measure the sloped roof or the flat footprint?

The flat footprint, the roof as seen from above, including the overhang. Rain falls vertically, so what counts is the ground area the roof covers. The sloped area (the one used to buy shingles) is larger on a pitched roof and would overstate what you collect, by about 12% on a 6/12 roof. This is a common mistake.

What collection efficiency should I use?

The Texas Manual on Rainwater Harvesting says most installers assume 75 to 90%. This calculator defaults to 85%, a common planning figure. Smooth metal roofs with tight gutters are at the high end; rough roofs, long dry spells (which let dust soak up the first rain) and gutters that overflow in downpours are at the low end. Published coefficients for specific roof materials vary between sources, so treat the figure as a range.

How many rain barrels do I need?

Enough to hold the storm you want to catch. 3 barrels of 55 gallons hold the 159 gallons from 1 inch on 300 sq ft; half an inch needs about half as many. A whole 1,500 sq ft roof under 1 inch of rain collects 795 gallons, which is 15 barrels, so most people catch one downspout and let the rest drain. The overflow line shows what a barrel cannot hold.

What happens when the barrel is full?

It overflows. A barrel needs an overflow outlet piped away from the house foundation, and the overflow line shows how much does not fit: one 55 gallon barrel under the example storm overflows by 104 gallons.

What is a first flush diverter and how much should it divert?

It sends the first rain of a storm, which washes dust, bird droppings and debris off the roof, away from the barrel. The Texas Manual on Rainwater Harvesting suggests a minimum of 10 gallons per 1,000 sq ft of roof; the real amount depends on how dirty the roof is and the days since the last rain, and the manual notes there is no exact calculation. On 500 sq ft, 0.5 in of rain, the diverter holds back 5 gallons, so 127.5 gallons are collected instead of 132.5.

How do I calculate rainwater in litres?

1 mm of rain on 1 m² is exactly 1 litre. A 100 m² footprint gets 100 L per mm; at 85% efficiency 85 L per mm. For 25 mm that is 2,125 L. The calculator does the same sum in US units: pick mm for the rainfall and the m button for the roof, and the water shows in litres (1 US gallon is 3.785 L).

Can I use a month or a year of rain?

For the amount of water, yes: enter the monthly or annual rain and read “Water collected”. A 1,500 sq ft footprint with 30 in of rain a year collects about 23,844 gallons at 85%. The barrel count is then meaningless, as the barrels are emptied between rains and a year of water would need 434 barrels at once; size storage from the biggest storm you want to catch, or from how much you use between rains.

Is it legal to collect rainwater?

Rules on collecting rainwater vary by state and by country, and some places set limits on how it can be stored or used, or require approvals. Check local rules before you install. Stored rainwater is not safe to drink without treatment.

Rainwater Harvesting Calculator: How Much Rain Will Your Roof Catch?

A 20 × 15 ft piece of roof (300 sq ft of flat footprint, about what one downspout drains on a small house) collects 159 gallons from 1 inch of rain at 85% efficiency, so you need 3 barrels of 55 gallons to hold it all (159 ÷ 55 = 2.9, rounded up), and a single barrel overflows by 104 gallons. Gallons = footprint in sq ft × inches of rain × 0.6234 × efficiency. Measure the roof from above (its flat footprint), not along the slope: rain falls straight down.

Figures based on the sources listed belowUpdated 2026-10-04How we calculate

Roof footprint length (flat, from above)

Roof footprint width (flat, from above)

Spare room in the barrels

Just enough (0%)+20% for a bigger stormCustom

Rainfall depth

The rain to catch: one storm (barrels fill on a single storm), or a month or a year of rain for a yield estimate. Barrels only hold what fits at once, so a month or a year gives a total, not a barrel count to buy.

Collection efficiencyThe share of the rain that reaches the barrel: the rest is lost to wetting the roof, splash, evaporation and gutter overflow in heavy rain. Most installers assume 75 to 90%; 85% is a common planning figure.

Barrel sizeRain barrels are usually 50 or 55 US gallons. The label size may be more than you can use: the inlet and overflow sit below the rim.

Barrels you have

Used for the overflow: what is left when they are full. The barrels needed to hold it all are the headline either way.

First flush diverterA first flush diverter sends the first, dirtiest rain of a storm away from the barrel. 10 gal per 1,000 sq ft is the minimum the Texas Manual on Rainwater Harvesting suggests. It is taken off once, so use it with one storm, not a month or a year.

Price per barrel (optional)

Buy 3 barrels

300 sq ft + 0% waste = 300 sq ft

Water collected: 159 gal
Collected per 1 in (25.4 mm) of rain: 159 gal
Held back by the first flush diverter: 0 gal
Storage in the 1 barrel you have: 55 gal
Overflow with the barrels you have: 104 gal

How this number is calculated

300 sq ft (no waste added) ÷ 103.8 sq ft per barrel = 3 barrels

Shopping list

Rain barrel (55 gallon)
Rain barrel diverter kit
First flush diverter
Rain barrel stand
Rain barrel spigot and overflow fitting
Mosquito screen for a rain barrel

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Gallons per 1 inch of rain by roof footprint

Flat footprint (the roof seen from above, not the sloped area). Raw is 0.6234 gal per sq ft with nothing lost; the other columns apply 85% and 95% efficiency. Barrels are 55 gal, to hold 1 in of rain at 85%. Worked by this calculator at build time.
Roof footprint	Raw, 100%	Collected at 85%	Collected at 95%	55 gal barrels at 85%
500 sq ft	312 gal	265 gal	296 gal	5
1,000 sq ft	623 gal	530 gal	592 gal	10
1,500 sq ft	935 gal	795 gal	888 gal	15
2,000 sq ft	1,247 gal	1,060 gal	1,184 gal	20
2,500 sq ft	1,558 gal	1,325 gal	1,481 gal	25
3,000 sq ft	1,870 gal	1,590 gal	1,777 gal	29

55 gallon barrels to hold one storm

Barrels to hold the whole storm at 85% efficiency, no first flush, by the footprint draining to them and the rain in one storm. Most people catch one downspout’s share of the roof, not all of it.
Roof footprint	0.25 in	0.5 in	1 in	1.5 in	2 in
300 sq ft	1	2	3	5	6
500 sq ft	2	3	5	8	10
1,000 sq ft	3	5	10	15	20
1,500 sq ft	4	8	15	22	29
2,000 sq ft	5	10	20	29	39

Litres per millimetre of rain, metric

1 mm of rain on 1 m² is 1 L. At 85% efficiency it is 0.85 L per m² per mm; the last column is 1 inch (25.4 mm).
Roof footprint	Raw, per 1 mm	Collected at 85%, per 1 mm	Collected at 85%, per 25.4 mm
50 m²	50 L	42.5 L	1,080 L
100 m²	100 L	85 L	2,159 L
150 m²	150 L	127.5 L	3,239 L
200 m²	200 L	170 L	4,318 L
250 m²	250 L	212.5 L	5,398 L

Methodology & assumptions

Roof area is the flat footprint: length × width as seen from above, including the eave overhang that rain falls on, not the sloped area you would measure along the shingles. Rain falls vertically, so a steep roof and a flat one over the same footprint catch the same water; using the sloped area (the figure for shingles) overstates the catch by the pitch factor, about 12% at 6/12. Parts of the roof that drain somewhere else can be taken off with the “Add a part that drains elsewhere” button. One inch of rain on one square foot is 144 cubic inches of water (12 in × 12 in × 1 in), and a US gallon is exactly 231 cubic inches, so it is 144 ÷ 231 = 0.6234 US gallons; that is 623 gallons per inch on 1,000 sq ft. In metric, 1 mm of rain on 1 m² is 1 litre exactly, so litres = m² × mm; the two agree because 1 in is 25.4 mm, 1 sq ft is 0.09290304 m² and 1 US gallon is 3.785411784 L. Gallons collected = footprint × inches × 0.6234 × efficiency. The efficiency (also called the runoff or collection coefficient) is the share of the rain that ends up in the barrel: the rest wets the roof, splashes over the gutter in a downpour or evaporates. The Texas Manual on Rainwater Harvesting says most installers assume 75 to 90%, and 85% (the default here, which gives about 0.53 gal per sq ft per inch) is a common planning figure. Published coefficients for individual roofs vary between sources (smooth metal is usually quoted at 90 to 95%, asphalt shingles anywhere from about 70 to 90%), so the picker is a range, not a roof-type table. The first flush diverter, when on, holds back 10 gallons for every 1,000 sq ft of roof, the minimum the Texas Manual suggests; it is subtracted once from the rain you enter, so it suits a single storm, and the real amount that needs washing off depends on dust, debris and the days since the last rain. Barrels = gallons collected ÷ barrel size, rounded up. “Barrels you have” sets the overflow, the collected gallons that do not fit. The rainfall figure is whatever depth you enter: for one storm the barrel count is the number that holds it; for a month or a year it is a yield, and storage does not work like that because barrels are emptied between rains. Rainfall for your area is not built in: use a local figure for the storm size or the monthly or annual total from your weather service. Rain barrel size on the label is the full volume; you may get less because the inlet and overflow sit below the rim. Collecting rainwater is regulated differently in different states and countries, and some places have rules on barrel size, use and treatment: check local rules before you install. Stored rainwater is not drinking water without treatment.

Texas Water Development Board, The Texas Manual on Rainwater Harvesting (3rd ed.): 0.623 gal per sq ft per inch, catchment from the horizontal footprint, collection efficiency of 75 to 90%, first flush of at least 10 gal per 1,000 sq ft of roof (the figures were read from search-result excerpts; the PDF could not be opened here, so check them against the manual)
Texas Water Development Board, “Rainwater Volumes from Roof Runoff”: an 85% system efficiency is assumed for the collectible volume (search excerpt only; page not opened)
NIST, Guide for the Use of the International System of Units (SI), appendix B.8: the US liquid gallon is 231 cubic inches, 3.785411784 L
Roof runoff coefficients by material (metal 0.90 to 0.95, asphalt shingles 0.70 to 0.90) differ between secondary sources: a typical range, not a published standard
Typical 50 and 55 US gallon rain barrel sizes: common retail sizes, not a standard

How to calculate rainwater harvesting and rain barrels

Measure the roof from above. Take the length and width of the roof section that drains to the barrel as seen from above, overhang included. Use the flat footprint, not the sloped length of the roof.
Choose the rainfall. Enter the rain in one storm to size barrels, or a month or a year of rain to estimate the yield. Use inches or millimetres.
Pick the efficiency. Use 85% as a planning figure, higher for smooth metal with tight gutters and lower for rough roofs.
Work out the water. Gallons = footprint in sq ft × inches of rain × 0.6234 × efficiency. In metric, litres = m² × mm × efficiency.
Divide by the barrel size. Round up the water divided by 55 gallons (or your barrel size) to get the barrels, and compare with the barrels you have for the overflow.

Worked examples

Project	Result
20 × 15 ft of roof (one downspout’s share), 1 in storm, 85% efficiency, 55 gal barrels, one barrel owned	300 sq ft footprint → 159 gal collected, 3 barrels to hold it, 104 gal overflow from the one you have
Whole 50 × 30 ft roof footprint, 1 in storm, 85% efficiency, 55 gal barrels	1,500 sq ft footprint → 795 gal collected, 15 barrels to hold it
25 × 20 ft of roof, 0.5 in storm, 85% efficiency, with a first flush diverter (10 gal per 1,000 sq ft)	500 sq ft footprint → 127.5 gal collected after 5 gal of first flush, 3 barrels to hold it
Whole 50 × 30 ft roof footprint, 30 in of rain over a year, 85% efficiency (a yield, not storage)	1,500 sq ft footprint → 23,844 gal collected in the year, which would take 434 barrels to hold at once

Frequently asked questions

How much water does a roof collect from 1 inch of rain?: Each square foot of flat footprint gets 0.6234 gallons from one inch of rain (144 cubic inches ÷ 231 cubic inches per gallon), so 1,000 sq ft gets 623 gallons. At 85% efficiency a 1,000 sq ft footprint collects 530 gallons.
Do I measure the sloped roof or the flat footprint?: The flat footprint, the roof as seen from above, including the overhang. Rain falls vertically, so what counts is the ground area the roof covers. The sloped area (the one used to buy shingles) is larger on a pitched roof and would overstate what you collect, by about 12% on a 6/12 roof. This is a common mistake.
What collection efficiency should I use?: The Texas Manual on Rainwater Harvesting says most installers assume 75 to 90%. This calculator defaults to 85%, a common planning figure. Smooth metal roofs with tight gutters are at the high end; rough roofs, long dry spells (which let dust soak up the first rain) and gutters that overflow in downpours are at the low end. Published coefficients for specific roof materials vary between sources, so treat the figure as a range.
How many rain barrels do I need?: Enough to hold the storm you want to catch. 3 barrels of 55 gallons hold the 159 gallons from 1 inch on 300 sq ft; half an inch needs about half as many. A whole 1,500 sq ft roof under 1 inch of rain collects 795 gallons, which is 15 barrels, so most people catch one downspout and let the rest drain. The overflow line shows what a barrel cannot hold.
What happens when the barrel is full?: It overflows. A barrel needs an overflow outlet piped away from the house foundation, and the overflow line shows how much does not fit: one 55 gallon barrel under the example storm overflows by 104 gallons.
What is a first flush diverter and how much should it divert?: It sends the first rain of a storm, which washes dust, bird droppings and debris off the roof, away from the barrel. The Texas Manual on Rainwater Harvesting suggests a minimum of 10 gallons per 1,000 sq ft of roof; the real amount depends on how dirty the roof is and the days since the last rain, and the manual notes there is no exact calculation. On 500 sq ft, 0.5 in of rain, the diverter holds back 5 gallons, so 127.5 gallons are collected instead of 132.5.
How do I calculate rainwater in litres?: 1 mm of rain on 1 m² is exactly 1 litre. A 100 m² footprint gets 100 L per mm; at 85% efficiency 85 L per mm. For 25 mm that is 2,125 L. The calculator does the same sum in US units: pick mm for the rainfall and the m button for the roof, and the water shows in litres (1 US gallon is 3.785 L).
Can I use a month or a year of rain?: For the amount of water, yes: enter the monthly or annual rain and read “Water collected”. A 1,500 sq ft footprint with 30 in of rain a year collects about 23,844 gallons at 85%. The barrel count is then meaningless, as the barrels are emptied between rains and a year of water would need 434 barrels at once; size storage from the biggest storm you want to catch, or from how much you use between rains.
Is it legal to collect rainwater?: Rules on collecting rainwater vary by state and by country, and some places set limits on how it can be stored or used, or require approvals. Check local rules before you install. Stored rainwater is not safe to drink without treatment.

Handy tools

Unit Converter: Barrels and tanks are sold in gallons or litres, rainfall in inches or millimetres: convert between them (1 US gallon is 3.785 L).