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Rainwater Harvesting Calculator: How Much Rain Your Roof Can Collect, Month by Month

Short answer

Multiply the roof footprint (sq ft) by rainfall (inches) by 0.623, then by about 0.8 for losses. A 1,000 sq ft roof in Raleigh can collect roughly 24,200 gallons a year — but its 2-year storm alone sends about 1,750 gallons, so a 55-gallon barrel overflows early in the storm. Enter your zip to see it month by month.

Fills in your average rainfall for each month (NOAA 1991–2020) and your 2-year storm.
Roof surface

The share of the rain that reaches storage after the first flush, splash and evaporation.

Storage
Planning to use it for watering?

Example · enter your zip above

Zip 27601 · Wake County, NC

Collectable a year
24,200 gal
Per inch of rain
498 gal
Storage fills in
0.11 in of rain
Overflows a year
24,200 gal
Collectable rain and overflow by monthBar chart of rainwater this roof can collect each month, in gallons, and how much overflows a 55-gallon storage: January 1,800 collected, 1,800 overflow; February 1,500 collected, 1,500 overflow; March 2,050 collected, 2,050 overflow; April 1,850 collected, 1,850 overflow; May 1,900 collected, 1,900 overflow; June 2,250 collected, 2,250 overflow; July 2,500 collected, 2,500 overflow; August 2,450 collected, 2,450 overflow; September 2,600 collected, 2,600 overflow; October 1,800 collected, 1,800 overflow; November 1,750 collected, 1,750 overflow; December 1,750 collected, 1,750 overflow.01.3k2.6kgallonsJFMAMJJASONDstorage 55 gal
Kept or used Overflows

Without drawing it down, 55 gallons of storage fills once and then everything overflows. Storage only keeps more rain if you use the water between storms. It overflows in every month.

In one hard storm

Your 2-year storm drops 3.48 inches in a day. This roof sends about 1,750 gallons to the storage; 55 gallons fills after the first 0.11 inch, and about 1,700 gallons leaves through the overflow.

Size a dry well for the overflow →Or a rain garden →

North Carolina: Expressly allowed by state law

Is it illegal to collect rainwater here? No. State law expressly allows it. North Carolina's full rules

Compare storage sizes

How each storage size performs with this roof and rainfall
StorageFills inOverflow, 2-year storm
55 gal0.11 in1,700 gal
110 gal0.22 in1,600 gal
275 gal0.55 in1,450 gal
550 gal1.1 in1,200 gal
1,000 gal2.01 in730 gal
2,500 gal5.02 in0 gal

Rainfall: NOAA NCEI 1991–2020 gridded normals (about a 5 km cell) at the zip's centre point. Storage can refill at most once per rain day (days with 0.1 in or more, from the nearest NOAA station). A planning estimate from monthly averages: real years run wetter and drier, and a monthly balance can't see the order of storms and watering inside a month, so for small barrels treat the watering figure as the optimistic end.

How to read your result

Collectable is what the roof can deliver: footprint × rain × 0.623 × the roof surface's efficiency (80% for shingle). It is a supply ceiling. What you actually keep is set by storage and use: rain fills the tank, watering empties it, and anything above capacity overflows. With no use, any barrel fills in its first storm and keeps nothing more all year.

A tank can only refill when it rains, so the calculator also caps each month's refills at the number of days with at least a tenth of an inch of rain at the nearest NOAA station. That is why a single barrel covers far less watering than its yearly fill count suggests.

Month by month matters because rain and watering don't line up. In Raleigh the wettest month is September; in much of the West most rain falls in winter, when gardens need least, which is why a zip's monthly pattern changes the answer more than its yearly total.

How big a rain barrel or tank?

Worked example: a 1,000 sq ft shingle roof in Raleigh watering 300 sq ft of beds with an inch a week from April to September — about 810 gallons a month.

Storage sizes compared for the worked example
StorageFills inWatering coveredRain used
55 gal0.11 in of rain45%9% of the year's
110 gal0.22 in of rain90%18% of the year's
275 gal0.55 in of rain100%20% of the year's
550 gal1.1 in of rain100%20% of the year's
1,000 gal2.01 in of rain100%20% of the year's
2,500 gal5.02 in of rain100%20% of the year's

NOAA 1991–2020 monthly averages for zip 27601; monthly balance, run until the year repeats. A planning estimate, not a guarantee: dry years cover less.

Past the point where watering is covered, a bigger tank mostly stores water you won't use. Below it, every extra barrel helps, and linking two or three barrels is a common way to add storage a step at a time.

Where the overflow goes

A barrel changes where roof water ends up, not how much there is. Once it is full, every gallon still has to go somewhere, and an overflow spilling at the base of the wall is the same problem as a downspout that ends there. Run the overflow at least 10 feet out, downhill, to a rain garden, a dry well or a buried line to daylight. The calculator gives the overflow from your zip's 2-year storm and passes it to the dry well calculator; the roof runoff calculator shows the same storm per downspout.

Where the overflow goes

Rain barrel overflow routed away from the houseCross-section of a house with a rain barrel. Roof water runs down the downspout, through a first-flush diverter that sends the dirtiest first water away, into a covered barrel on a raised stand. Once the barrel is full, an overflow pipe near its top carries the extra water at least 10 feet out and downhill from the house to a rain garden or dry well, instead of letting it spill at the foundation.firstflushbarreloverflowrain gardenor dry well10 ft or more, downhill
A barrel fills in the first fraction of an inch of a storm. Everything after that leaves through the overflow, so the overflow needs the same planning as a downspout.

No. Collecting rainwater from your roof is not illegal in any US state. 7 states expressly allow it by law, 4 (Colorado, Nevada, Utah and Virginia) allow it with limits, and the other 39 have no state law restricting it. Where limits exist, they cap how much you store, where you use it, or how drinking-water and plumbed systems are built. Enter a zip and the calculator shows your state's status; the rainwater collection laws by state page has the caps, permits, drinking-water rules and rebates, quoted from each state's own law. Not legal advice — HOA covenants and local codes can add their own rules.

The formula and its assumptions

Gallons = roof footprint (sq ft) × rain (in) × 0.623 × efficiency. 0.623 is the gallons in an inch of water over a square foot. Efficiency is the share that reaches storage after the first-flush diverter, splash-out, gutter overshoot and evaporation; the Texas Water Development Board notes most installers use 75 to 85%. We use 85% for metal, 80% for asphalt shingle, 75% for tile, wood or gravel. Monthly rain is the NOAA 1991–2020 normal for your zip, from our rainfall by zip code dataset; the storm is NOAA Atlas 14's 2-year, 24-hour depth for your county. Watering is an inch a week, about 4.3 weeks a month. Gutters also have to carry the water to the barrel — the gutter size calculator checks that.

What this calculator does not tell you

It works from averages; a dry year can fall well short of them, and a drought summer can leave a tank empty for weeks. It doesn't design a drinking-water system, a pump or a plumbed connection. It doesn't know your HOA rules or local plumbing code. And open water breeds mosquitoes: keep every barrel lidded and screened, and empty it before a hard freeze.

Frequently asked questions

How much rainwater can I collect from my roof?
Multiply the roof's footprint in square feet by the rain in inches and by 0.623, then by about 0.75 to 0.85 for losses. A 1,000 sq ft shingle roof in Raleigh, NC (48.5 inches a year) can deliver about 24,200 gallons a year. How much you keep depends on your storage and how fast you use it — the calculator works that out month by month from your zip.
How many gallons is 1 inch of rain on a roof?
0.623 gallons per square foot of roof footprint, so about 623 gallons on 1,000 sq ft before losses. After first-flush diversion, splash and evaporation, plan on roughly 467 to 530 gallons reaching storage.
What size rain barrel or tank do I need?
It depends on how much you'll use, not on how much falls. A 55-gallon barrel on a 1,000 sq ft roof fills after about 0.11 inch of rain, so most of every storm overflows. In Raleigh, watering 300 sq ft of beds through the growing season (about 810 gallons a month) needs around 110 gallons of storage to cover 90% of it; a single barrel covers about 45%.
Does roof pitch or material matter?
Pitch doesn't: rain falls vertically, so what counts is the footprint seen from above. Material does, a little. The Texas Water Development Board says most installers assume 75 to 85% of the rain reaches storage; smooth metal sits at the top of that range and rough or absorbent roofs at the bottom.
Is it legal to collect rainwater in my state?
No. Collecting rainwater from your roof is not illegal in any US state. 7 states expressly allow it by law, 4 (Colorado, Nevada, Utah and Virginia) allow it with limits, and the other 39 have no state law restricting it. Where limits exist, they cap how much you store, where you use it, or how drinking-water and plumbed systems are built. Colorado caps a home at two barrels holding 110 gallons combined, and Utah allows two 100-gallon containers without registering. The calculator shows your state's status when you enter a zip, and our rainwater collection laws page has every state's rule in its own words.
Can I drink harvested rainwater?
Not straight from a barrel. Drinking-water rainwater systems need treatment, and in several states a permit or a system built to plumbing-code standards. This calculator estimates supply for watering and other outdoor use only.

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