Quick answer: a round tank holds π × (diameter ÷ 2)² × height; multiply cubic feet by 7.48 for gallons. If you are harvesting rain, the other half of the sum is roof area (ft²) × rainfall (in) × 0.623 = gallons — about 935 gallons per inch from a 1,500 ft² roof.
Rainwater catchment: how much your roof actually yields
Liters = roof footprint (m²) × rainfall (mm) × collection efficiency
Two things surprise people. The first is that you use the roof's footprint — its plan area — not the sloped surface area. Rain falls vertically, so a steep roof catches no more than a flat one covering the same ground. The second is that the 0.623 constant is exact, not a rule of thumb: one inch of rain on one square foot is 0.0833 ft³, and multiplied by 7.4805 gallons per cubic foot that is 0.623 gallons.
| Roof footprint | Per 1 in of rain | Per 1 in at 85% efficiency | Per 25 in annual |
|---|---|---|---|
| 500 ft² | 312 gal | 265 gal | 6,620 gal |
| 1,000 ft² | 623 gal | 530 gal | 13,240 gal |
| 1,500 ft² | 935 gal | 795 gal | 19,860 gal |
| 2,000 ft² | 1,246 gal | 1,059 gal | 26,480 gal |
| 2,500 ft² | 1,558 gal | 1,324 gal | 33,100 gal |
The 85% efficiency figure accounts for first-flush diversion, splash, wind loss and evaporation from a warm roof. Metal roofs run at the higher end; rough or absorbent surfaces at the lower end.
Sizing the tank
Storage should be sized against your longest dry spell, not your annual rainfall. Annual totals tell you whether the roof can keep up; the dry gap tells you how big the buffer has to be.
- Work out daily demand from the tank. Garden irrigation might be 30 gallons a day; a whole-house supply is far more.
- Find the longest dry period you want to ride out — often 30 to 60 days.
- Multiply. 50 gal/day × 40 days = 2,000 gallons of storage.
- Check the roof can refill it: 2,000 ÷ 795 gal per inch ≈ 2.5 inches of rain on a 1,500 ft² roof.
If step 4 needs more rain than you typically get in a wet spell, the tank is bigger than your roof can serve — and the money is better spent on catchment area or on reducing demand.
Common water tank sizes
| Type | Typical dimensions | Capacity | Filled weight |
|---|---|---|---|
| Rain barrel | 24 in ⌀ × 34 in | ≈ 66 gal | 550 lb |
| IBC tote | 48 × 40 × 46 in | 275 gal | 2,290 lb |
| Slimline poly | 30 × 72 in × 72 in tall | ≈ 600 gal | 5,000 lb |
| Round poly (small) | 5 ft ⌀ × 6 ft | ≈ 880 gal | 7,340 lb |
| Round poly (medium) | 7.5 ft ⌀ × 7 ft | ≈ 2,300 gal | 19,200 lb |
| Round poly (large) | 10 ft ⌀ × 8 ft | ≈ 4,700 gal | 39,200 lb |
Those weights are the reason tank bases matter. A 2,300-gallon tank puts nearly ten tons onto a circle about seven and a half feet across — that needs compacted sand or a concrete pad, dead level, extending past the tank's footprint. An unevenly supported poly tank can split at the base seam.
Common mistakes & pro tips
- Using the sloped roof area. Catchment is the plan footprint. A 30° pitch adds 15% to the surface but nothing to the rain caught.
- Sizing from annual rainfall. Annual totals hide the dry gap that actually empties a tank.
- Ignoring the dead volume. Usable water sits between the outlet and the overflow, typically 5 to 10% less than the rating.
- Outside dimensions on a poly tank. Walls can be 10 mm or more; on a small tank that is real capacity.
- Pro tip — first-flush diverters. Discarding the first 10 gallons or so of a storm takes most of the roof debris and bird droppings with it, and pays for itself in filter life.
- Pro tip — two tanks beat one. Splitting storage lets you take one offline for cleaning, and gives flexibility in siting around a building.
Frequently asked questions
How do I calculate water tank capacity?
For a round tank, capacity = π × (diameter ÷ 2)² × height, then multiply cubic feet by 7.48 for US gallons. A tank 5 ft across and 6 ft tall holds π × 6.25 × 6 = 117.8 ft³ ≈ 881 gallons. Use inside dimensions — poly tank walls are thick enough to matter.
How much rainwater can I collect from my roof?
Roughly 0.623 gallons per square foot of roof area per inch of rain, before losses. A 1,500 ft² roof yields about 935 gallons per inch. Real systems collect 75 to 90% of that after first-flush diversion, splash and overflow, so plan on roughly 800 gallons per inch from that roof.
What size rainwater tank do I need?
Size it to bridge your longest dry spell, not your average rainfall. Multiply daily use by the number of days you need to cover: a household using 50 gallons a day of tank water through a 40-day dry season needs about 2,000 gallons of storage. Check that your roof can actually refill it in a normal wet period.
How much does a full water tank weigh?
About 8.34 lb per gallon, so a 1,000-gallon tank holds roughly 8,340 lb — over four tons — plus the tank itself. That load needs a properly prepared base: compacted sand or a concrete pad, level and larger than the tank's footprint.
Why does my tank hold less than its rated capacity?
Rated capacity is usually measured to the brim, but the outlet sits above the floor and the overflow sits below the top. The water between those two points is what you can actually use, and it is commonly 5 to 10% less than the nameplate.