Rain Barrel Size Calculator
How many gallons your roof yields in a storm, what size barrel that justifies, and where the overflow goes, which is the part that floods basements.
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Results
Working
- catchment = 40 x 30 x 25% = 300 sq ft
- gallons per inch = 300 sq ft x 0.6234 x 0.85 capture = 159 gal
- per storm = 159 x 1 in = 159 gal
- per wet month = 159 x 4 in = 635.9 gal
- overflow = 159 - 55 barrel = 104 gal
- full weight = 55 gal x 8.34 lb = 458.7 lb
Quick answer
One inch of rain on one square foot of roof yields 0.623 gallons, and real capture is 80 to 90 per cent of that. A 1,200 sq ft roof footprint therefore gives about 640 gallons per inch of rain, which is why a single 55 gallon barrel overflows in almost every storm and why the overflow route matters more than the barrel.
gallons = roof footprint sq ft x rainfall in x 0.623 x capture efficiency
Worked example
A 40 by 30 ft house, one of four downpipes, asphalt shingle, in a 1 in storm.
| Per typical storm | 159 gal — At 1 in of rain |
|---|---|
| Per wet month | 635.9 gal — At 4 in |
| Per inch of rain | 159 gal — The number to reuse for any storm size |
| Catchment area | 300 sq ft — 25% of a 1200 sq ft footprint |
| Size that suits | 65 gal — Holds a storm without standing full for weeks |
| Chosen barrel fills to | 100% — 104 gal overflows |
| It lasts | 9.6 days — At 40 gal a week |
| Weight when full | 458.7 lb — Stand it on something that will take it |
Working
- catchment = 40 x 30 x 25% = 300 sq ft
- gallons per inch = 300 sq ft x 0.6234 x 0.85 capture = 159 gal
- per storm = 159 x 1 in = 159 gal
- per wet month = 159 x 4 in = 635.9 gal
- overflow = 159 - 55 barrel = 104 gal
- full weight = 55 gal x 8.34 lb = 458.7 lb
This is the usual surprise: a quarter of an ordinary roof throws off more than two and a half barrels in a single inch of rain. The barrel is not really storage for the storm, it is a buffer for the dry days after it, which is why sizing against weekly use gives a better answer than sizing against rainfall.
Rainwater yield by roof size
Gallons captured from the whole roof at 85 per cent efficiency. Divide by the number of downpipes for one barrel's share.
| Roof footprint | 0.25 in | 0.5 in | 1 in | 2 in |
|---|---|---|---|---|
| 400 sq ft | 53 gal | 106 gal | 212 gal | 423.9 gal |
| 800 sq ft | 106 gal | 212 gal | 423.9 gal | 847.8 gal |
| 1200 sq ft | 159 gal | 317.9 gal | 635.9 gal | 1271.7 gal |
| 1600 sq ft | 212 gal | 423.9 gal | 847.8 gal | 1695.6 gal |
| 2400 sq ft | 317.9 gal | 635.9 gal | 1271.7 gal | 2543.5 gal |
Footprint is the plan area. A steeper pitch has more surface but catches no more rain, because rain falls vertically.
At a glance
| What it does | Turns roof area and rainfall into gallons captured, and sizes a barrel against real use |
|---|---|
| Inputs needed | Roof footprint, share per downpipe, roof surface, storm and monthly rainfall, weekly use |
| Main output | Gallons per storm and the barrel size that suits |
| Formula | gallons = footprint x rainfall in x 0.623 x efficiency |
| Category | Garden & Outdoor |
| Cost | Free, no sign-up, no download |
How to use the Rain Barrel Size Calculator
- Measure the roof footprint from outside wall to outside wall. Ignore the pitch, since rain falls vertically.
- Divide by downpipes: a rectangular house with four normally sends about a quarter of the roof to each.
- Enter a typical storm for your area rather than a record one, then a wet month for the seasonal figure.
- Enter what you actually draw off per week. Sizing against use gives a better answer than sizing against rainfall.
- Plan the overflow before buying. It has to leave the wall the way the downpipe did.
Frequently asked questions
How many gallons of water can I collect from my roof?
About 0.623 gallons per square foot of roof footprint per inch of rain, before losses. Real capture is 80 to 90 per cent of that on a hard roof, so a 1,200 sq ft roof gives roughly 640 gallons in an inch of rain.
What size rain barrel do I need?
Size it against how much you draw off between storms rather than how much falls in one. A barrel big enough to hold a whole storm usually stands full for weeks, which stagnates. Two smaller barrels linked in series beat one large one for most gardens.
Does roof pitch change how much water I collect?
No. Rain falls vertically, so what matters is the plan footprint. A steep roof has more surface area but presents the same area to the sky as a flat one of the same footprint.
How much does a full rain barrel weigh?
Water weighs 8.34 lb per gallon, so a 55 gallon drum is about 460 lb full, and a 275 gallon tote is over 2,300 lb. That is well beyond what a timber deck or a slab on sand will hold level, so a firm, level base is not optional.
Where should the overflow go?
Wherever the downpipe used to discharge, and no closer to the building. Once the barrel is full every subsequent gallon comes out of the overflow, so an unplanned overflow simply relocates the whole roof's runoff to the foundation.
Is rain barrel water safe to drink?
No. It carries whatever is on the roof, including bird droppings and material shed by the roof covering. Use it for irrigation and washing, and avoid wetting edible leaves close to harvest.
Do rain barrels breed mosquitoes?
Only if the inlet is open. A fine mesh screen over the inlet and a sealed lid stops them entirely, and also keeps out the leaf litter that otherwise blocks the tap.
Yields are estimates. Capture efficiency varies with roof surface, gutter condition, wind during the storm and how much first-flush is diverted. Check local rules before installing: a few jurisdictions restrict rainwater harvesting, and many require the overflow to be routed as the original downpipe was.
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