How to Calculate Tank Volume
The formula depends on your tank's cross-sectional shape. All formulas below produce volume in cubic inches — multiply by 0.004329 to convert to US gallons.
| Cylindrical tank | π × r² × h × 0.00433 |
| Rectangular tank | L × W × H × 0.00433 |
| Oval tank | π × (L/2) × (W/2) × D × 0.00433 |
| Cubic inches → gallons | ÷ 231 (exact conversion) |
| Gallons → liters | × 3.785 |
Common Tank Dimension Reference
Use this table to verify your calculation or find a standard tank near your target volume.
| Tank Size | Typical Dimensions (vertical) | Typical Diameter × Height | Guide |
|---|---|---|---|
| 50 Gallon | ~24" dia × 38" H | 24 × 38 | 50 Gal Guide |
| 100 Gallon | ~30" dia × 46" H | 30 × 46 | 100 Gal Guide |
| 150 Gallon | ~35" dia × 47" H | 35 × 47 | 150 Gal Guide |
| 200 Gallon | ~36" dia × 56" H | 36 × 56 | 200 Gal Guide |
| 300 Gallon | ~46" dia × 52" H | 46 × 52 | 300 Gal Guide |
| 500 Gallon | ~48" dia × 72" H | 48 × 72 | 500 Gal Guide |
| 1,000 Gallon | ~64" dia × 75" H | 64 × 75 | 1,000 Gal Guide |
| 2,500 Gallon | ~90" dia × 95" H | 90 × 95 | 2,500 Gal Guide |
| 5,000 Gallon | ~102" dia × 135" H | 102 × 135 | 5,000 Gal Guide |
| Dimensions are approximate averages for Norwesco vertical poly tanks. Actual dimensions vary by manufacturer and model. Always confirm with the manufacturer spec sheet. | |||
Worked Examples
Example 1: Cylindrical Tank, 48" diameter × 60" tall
Radius = 24 inches. Volume = π × 24² × 60 = 3.14159 × 576 × 60 = 108,573 cubic inches. ÷ 231 = 469.6 gallons.
Water weight at full: 469.6 × 8.345 = 3,917 lbs (1.96 US tons).
Example 2: Rectangular Tank, 36" × 24" × 30"
Volume = 36 × 24 × 30 = 25,920 cubic inches. ÷ 231 = 112.2 gallons.
Water weight at full: 112.2 × 8.345 = 936 lbs.
Example 3: RV Fresh Water Tank (oval), 48" × 36" × 14" depth
Volume = π × 24 × 18 × 14 = 3.14159 × 24 × 18 × 14 = 19,085 cubic inches. ÷ 231 = 82.6 gallons.
Why Tank Capacity and Labeled Capacity May Differ
The nominal capacity printed on a tank (e.g., "500 gallons") is the maximum design capacity — filling to the very top of the inlet. In practice, tanks are used at 80–90% capacity to allow for thermal expansion, sloshing during transport, and to keep the outlet fitting submerged. A "500 gallon" tank in field use holds about 400–450 gallons of usable water.
For transport tanks on trailers, maximum safe fill is typically 90% — water at full capacity creates stability and sloshing issues. Always factor in a 10% buffer when calculating how much water you can safely transport.
Frequently Asked Questions
Most poly water tanks have slightly tapered walls, domed tops, and other non-geometric features. For a close estimate, use the largest diameter and the fill height as your cylinder dimensions — this will slightly overestimate the actual capacity, which is fine for most planning purposes. For exact capacity, refer to the manufacturer's spec sheet, which will have the tested nominal capacity.
One cubic foot holds 7.481 US gallons. So a tank measuring 2' × 3' × 4' (24 cubic feet) holds 24 × 7.481 = 179.5 gallons. This conversion is useful when reading tank specs that list volume in cubic feet.
For a vertical cylindrical tank, partial volume = π × r² × fill_height × 0.00433. If a 48"-diameter tank has water 30" deep: π × 24² × 30 × 0.00433 = 234.8 gallons. For horizontal tanks with partial fill, the calculation is more complex — use the full volume and multiply by the percentage of the tank that is filled by height.