Size your rain barrel system end to end: harvest from roof area and rainfall, monthly capture, how many barrels to link, first-flush diverter sizing, gutter and downspout compatibility, garden watering supply, and payback — all in your browser.
What this sizes: rain barrels and linked-barrel systems for outdoor, non-potable use (garden watering, lawn, washing). It is not a design tool for indoor potable rainwater systems or engineered cisterns — those need filtration, disinfection, and local code review.
Units:
1 · Barrel size from roof area
Enter roof length × width, or a total roof area directly (use the flat footprint, not the sloped surface).
2 · Monthly rainfall capture estimator
Enter average monthly rainfall — defaults are editable. Finds your annual harvest potential.
3 · How many barrels?
Total storage need ÷ barrel size, rounded up. Common sizes: 50, 55, 65 gal.
4 · First-flush diverter & overflow sizing
First-flush rule of thumb: divert at least 10 gallons per 1,000 sq ft of roof (Texas Water Development Board).
5 · Gutter & downspout compatibility check
Checks one downspout's share of the roof against its opening, the gutter size, and (optionally) gutter-run spacing.
6 · Garden watering supply planner
Default 0.5 gal/sq ft/week is a peak-season rate for garden beds (Stevens County Conservation District guidance).
7 · Cost & payback estimator
Defaults are editable — use your local combined water + sewer rate if billed for both.
Rain barrel sizing — FAQs
How many gallons of water does 1 inch of rain produce?
About 623 gallons per 1,000 sq ft of roof: 1 sq ft × 1 inch = 144 cubic inches, and 1 US gallon = 231 cubic inches, so gallons = roof area × rainfall × 0.623. At 90% collection efficiency (typical asphalt shingle roof) that is about 561 gallons of harvestable water.
What size rain barrel do I need for my house?
It depends on how much of a storm you want to catch. Example: a 1,200 sq ft roof in a 1-inch storm yields about 748 gallons gross (673 gallons at 90% efficiency) — catching all of it would take thirteen 55-gallon barrels. Most homes instead put one 55-gallon barrel per downspout and let the overflow feed the garden.
How many rain barrels do I need?
Divide your storage need by the barrel size and round up. Example: 561 gallons of need ÷ 55-gallon barrels = 11 barrels (605 gallons of capacity, 44 gallons spare). Link them at the same height with an overflow kit so they fill and drain as one tank.
What size first-flush diverter do I need?
Divert at least 10 gallons per 1,000 sq ft of roof (Texas Water Development Board guidance). For a 1,200 sq ft roof that is a 12-gallon diverter — about 18.4 feet of 4-inch pipe or 32.7 feet of 3-inch pipe as a standpipe chamber. Drain the chamber after each rain so it is ready for the next storm.
Can you connect multiple rain barrels together?
Yes — daisy-chain them. Set the barrels at the same height, connect near the top with a linking/overflow kit, and they behave as one larger tank: they fill together and drain together. Feed the first barrel from the downspout and run the last barrel's overflow to a garden bed or lawn, at least 6 feet from the foundation.
What size gutters and downspouts do I need for a rain barrel?
Rule of thumb: 1 square inch of downspout opening per 100 sq ft of roof in a 1-inch-per-hour rain (scale up for heavier storms). A 1,000 sq ft roof at 1 in/hr needs 10 sq in — two 2×3 downspouts (6 sq in each) or one 3×4 (12 sq in). A 5-inch K-style gutter handles up to about 5,500 sq ft; go 6-inch above that.
How long will a rain barrel water my garden?
A 200 sq ft bed needs about 100 gallons per week at the peak-season rate of 0.5 gallons per sq ft — so a full 55-gallon barrel lasts about half a week (roughly 4 days). Four weeks of autonomy for that bed takes 400 gallons of storage. The barrel refills every rain, so size for your longest dry stretch.
How much money does a rain barrel save?
Example: a $150 DIY system that offsets 3,000 gallons a year at $8.50 per 1,000 gallons saves $25.50/year — about a 5.9-year payback and $105 net over 10 years. Reported savings are often $35–$100/year, so typical payback is 2–3 years for a simple barrel setup.
Is rain barrel water safe for vegetable gardens?
Yes for watering — rain barrel water is fine for vegetables, flowers, and lawns. Do not drink it without treatment: install a first-flush diverter, screens on all openings, and a tight lid to keep out mosquitoes and debris. Avoid collecting from roofs with lead flashing or fresh tar coatings.
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