Rainwater Collection Calculator

How to use the rainwater collection calculator

1. Pick your city. Each of the 75 has NOAA’s normal rain for every month, at its main weather station. Not listed? Pick Not listed and type your rain a year.
2. Type the roof’s length and width seen from above, overhangs included. The pitch does not matter: rain falls on the roof’s footprint.
3. Say how much of the roof drains to your barrels or tank: all of it, one side, or one downspout of four.
4. Type a rain to catch and what one barrel holds. You get the gallons a month and a year, the barrels or tank that rain fills, the first-flush diverter, and your state’s limit where it has one. The green button opens the gutter size calculator with the same city and roof.

How much rain a roof catches

An inch of rain on a square foot of roof is 144 cubic inches of water, and a gallon is 231 cubic inches, so each square foot gives 0.623 gallons an inch. The Texas Water Development Board’s rainwater manual rounds it: “In theory, approximately 0.62 gallons per square foot of collection surface per inch of rainfall can be collected.” Not all of it reaches the barrel: “Most installers assume an efficiency of 75 percent to 90 percent.” The calculator takes 85%, the manual’s own worked example, and shows the 75–90% beside it.

So the sum is: the rain in inches × the roof’s square feet × 0.623 × 0.85. On 1,000 sq ft, an inch of rain is 620 gallons, and about 530 of it is caught.

How many rain barrels, or how big a tank

A rain barrel fills fast. The manual’s barrel is “the 50- to 75-gallon drum”, and on a 1,500 sq ft roof one 55-gallon barrel is full after 0.07″ of rain. A 1″ rain on that roof is 790 gallons, so catching all of it takes 15 barrels or an 800-gallon tank. Once they are full, “rainwater can be lost as overflow.” Most homes put one or two barrels under a downspout for the garden and let the rest run over; a tank is sized to the rain you want to keep.

The first-flush diverter

The first water off a roof carries its dust, leaves and bird droppings. A first-flush diverter fills with it and sends the clean water on. The Texas manual gives two rules: “divert a minimum of 10 gallons for every 1,000 square feet of collection surface”, or “one to two gallons of first-flush diversion for each 100 square feet of collection area.” For 1,500 sq ft that is 15 gallons at least, 15–30 gallons on the second rule.

Where a state limits it

Colorado and Utah set a number in their laws. Colorado: “You can fill and refill two rain barrels with a combined storage capacity up to 110 gallons throughout the year”, for “Outdoor uses, such as lawn and garden irrigation, on the property where the water was collected.” Utah lets you keep two containers of up to 100 gallons each without registering, and “The total allowed storage capacity with registration is no more than 2,500 gallons.” Registering is free. Elsewhere, ask your town or homeowners’ association before a big tank goes in.

Worked examples

• Phoenix, 2,000 sq ft (50 × 40 ft): 7.2″ of rain a year gives 7,700 gallons. June’s normal is 0.02″, just 21 gallons, so a tank there fills in winter and in the summer storms, then waits.
• Denver, one side of a 50 × 30 ft roof (750 sq ft): 5,800 gallons a year. Colorado’s 110 gallons fill with 0.28″ of rain, so the barrels refill after most storms.
• Seattle, one downspout of four on a 60 × 40 ft roof (600 sq ft): 12,500 gallons a year, 2,000 of them in November and only 190 in July. A 1″ rain is 320 gallons: 6 barrels or a 350-gallon tank.
• In metric, 10 × 8 m (80 m²) with 1,000 mm a year: each millimetre on a square metre is one litre, so 80,000 litres fall and about 68,000 litres are caught.

Common mistakes

• Measuring along the slope. “Remember to calculate the horizontal equivalent of the ‘roof footprint’ when calculating your catchment area.” A steep roof catches no more than a flat one.
• Counting the whole roof for one barrel. A barrel under one downspout gets only the roof that drains to it.
• Drinking it. “Due to leaching of toxins, composite shingles are not appropriate for potable systems, but can be used to collect water for irrigation.” The EPA adds that roof water “can pick up pollutants such as bacteria from birds and other animals”.
• An open barrel. Colorado’s rule is a good one anywhere: “Rain barrels must have sealable lids to prevent insects or other pests from using the stored water.”

Questions

How much rainwater can I collect from my roof?

0.623 gallons for each square foot of roof (seen from above) and each inch of rain, before losses; about 85% of it reaches the barrel. On 1,000 sq ft, an inch of rain is 620 gallons and about 530 caught.

How do I calculate rainwater harvesting?

Rain in inches × the roof’s square feet × 0.623 × 0.85. In metric, millimetres of rain × square metres = litres, × 0.85.

How many rain barrels do I need?

To catch a whole 1″ rain off 1,000 sq ft, about 10 barrels of 55 gallons (530 gallons). Most people use one or two per downspout and let the rest run over.

How big a rainwater tank do I need?

Size it to the rain you want to keep: a 1″ rain on 1,500 sq ft needs an 800-gallon tank. Type your roof and the rain above; the tank rounds up to the next 50 gallons.

Is it legal to collect rainwater?

Colorado allows two barrels, 110 gallons in all, for outdoor use. Utah allows two containers of 100 gallons without registering, and up to 2,500 gallons registered, free. Ask your town or homeowners’ association elsewhere.

What size first-flush diverter do I need?

At least 10 gallons for every 1,000 sq ft of roof, or one to two gallons for each 100 sq ft (the Texas rainwater manual).

Read next

• Rainwater collection system ideas
• DIY ideas for a small garden
• Home gardening ideas
• Cottagecore garden ideas
• Mailbox flower bed ideas
• Front of home landscaping ideas

Water pooling where the barrels run over? The free french drain cost calculator prices a drain to carry it off. The gutter size calculator sizes the gutters and downspouts that feed the barrels, and the 2D planner draws the house and garden to scale.

Sources, read October 19, 2026: NOAA’s U.S. Climate Normals, 1991–2020 (each city’s station file); The Texas Manual on Rainwater Harvesting, Third Edition (2005); EPA, Soak Up the Rain: Rain Barrels; Colorado Division of Water Resources; Utah Division of Water Rights.