Tank Volume Calculator
Cistern and tank gallons — not a swimming-pool average-depth takeoff. Click any i for detail.
Tank
Shape
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Shape
Rectangle uses L × W × H. Cylinder uses diameter × height. Oval is an ellipse footprint × height. |
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Length (ft)
i
Length
Long axis of a rectangle or oval tank. |
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Width (ft)
i
Width
Short axis of a rectangle or oval. |
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Diameter (ft)
i
Diameter
Inside diameter of a vertical cylinder. |
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Height (ft)
i
Height
Straight-wall height / liquid depth of the tank. |
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Fill (%)
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Fill
100 is brim-full. Use less if you never fill to the rim. |
How to use this calculator
- Pick the tank shape.
- Enter inside dimensions in feet.
- Set fill percent if the tank is not full.
- Read gallons and cubic feet.
Results explained
Gallons ≈ cubic feet × 7.48052, then × fill%. A pool with a deep end uses the pool volume page.
Quick reference: tank gallons
U.S. gallons from geometry — not a metered delivery ticket.
| Item | Detail |
|---|---|
| 1 ft³ | ≈ 7.48 gal |
| Rectangle | L × W × H |
| Vertical cylinder | π r² h |
| Swimming pool | Pool volume calculator |
Horizontal cylinders and dished heads are not modeled.
How the estimate is built
gal = ft³ × 7.48052 × fill%. Cylinder ft³ = π (D/2)² H.
Example scenario
8 × 4 × 4 ft rectangle, 100% full → 128 ft³ → about 958 gallons.
FAQ
Pool instead?
Use the pool volume calculator — average depth matters.
Inches?
Convert to feet (÷ 12) first.
Septic tank size?
Bedroom bands are the septic tank size page.