Last updated June 16, 2026 by our expert review team
Tank Volume Formulas
To find a tank's volume in gallons, calculate its volume in cubic feet and multiply by 7.48 — for example, a vertical cylinder 2 ft across and 4 ft tall holds π × 1² × 4 = 12.6 ft³, or about 94 gallons.
The shape sets the formula; partial fill on a horizontal cylinder uses the circular-segment area, not a simple percentage.
| Shape | Volume formula |
|---|---|
| Vertical cylinder | π × r² × height |
| Horizontal cylinder | π × r² × length |
| Rectangular | length × width × height |
| Gallons | cubic feet × 7.4805 |
r is radius (half the diameter). Multiply cubic feet by 7.4805 for US gallons or 28.317 for liters.
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How the Tank Volume Calculator Works
The calculator computes the tank's volume from its shape and dimensions in cubic feet, then converts to gallons and liters. Diameter is halved to a radius internally.
Partial fill is the tricky part. Vertical and rectangular tanks fill proportionally, but a horizontal cylinder does not — its cross-section narrows toward the top and bottom, so the calculator uses the circular-segment area at the liquid depth.
Formulas
Cylinder = π × r² × (height or length)
Rectangular = length × width × height
Gallons = cubic feet × 7.4805
Horizontal partial = [ r²·acos((r−h)/r) − (r−h)·√(2rh−h²) ] × length
Quick Reference
- 1 ft³
- 7.48 gal
- 1 ft³
- 28.32 L
- 1 US gal
- 3.785 L
- Horiz. ¼ depth
- ≈ 20% full
- Horiz. ½ depth
- = 50% full
- Horiz. ¾ depth
- ≈ 80% full
Why the gauge lies
A horizontal tank isn't half-full at half-depth
In a horizontal cylinder the middle is widest, so volume fills faster than depth in the middle and slower at the ends. Depth % only equals volume % for vertical and rectangular tanks.
a linear gauge would read 10% full
a linear gauge would read 25% full
matches depth
a linear gauge would read 75% full
a linear gauge would read 90% full
How to Measure Your Tank
No spec sheet? Measure it. For a round tank you usually can't reach across, wrap a tape around the outside and divide the circumference by π (3.1416) to get the diameter.
- 1Measure the diameter. On a round tank, wrap a tape around the outside for the circumference, then divide by π (3.1416). Or measure straight across the widest point.
- 2Measure the length or height. Vertical cylinder: floor to top seam. Horizontal cylinder: end to end. Rectangular: all three sides.
- 3Read the liquid level. For a partial reading, dip a clean stick to the wet line and measure the depth from the bottom.
- 4Subtract freeboard. Tanks are never filled to the brim — leave headspace for expansion, fittings, and the fill neck.
Common Tank Sizes
- 275 gal
Home heating oil
Oval (obround)
- 275 gal
IBC tote
Rectangular
- 500 gal
Propane (residential)
Horizontal cyl.
- 300 gal
Stock tank
Vertical cyl.
- 1,000 gal
Septic tank
Rectangular
Tank Volume Examples
Horizontal Fuel Tank
Horizontal, 2 ft dia × 6 ftUse a stick reading as the liquid level to gauge how much fuel or water is left right now.
Galvanized Stock Tank
Vertical, 6 ft dia × 2 ftA round stock tank doubles as a stock-tank pool. Round up for freeboard so it doesn't overflow.
Rectangular Aquarium
4 ft × 18 in × 2 ftGlass aquariums are sized by gallons. Leave an inch of freeboard below the rim.
Avoid these
Tank Volume Mistakes
Entering radius as diameter
The calculator wants diameter (full width) and halves it. Radius in the diameter field quarters the volume.
Treating horizontal partial fill as linear
A horizontal cylinder at quarter-depth holds less than a quarter. Use the segment math, not a percentage.
Mixing feet and inches
Convert every dimension to the same unit first, or use the unit toggle consistently.
Ignoring freeboard and fittings
Geometric capacity is the maximum. Leave headspace for expansion, baffles, and the fill neck.
Tank Volume Calculator FAQs
How do I calculate the volume of a tank in gallons?
How many gallons is my horizontal cylinder tank?
How do I measure a tank's diameter if I can't reach across it?
How do I find a partially filled tank's volume?
Why is a horizontal tank that's half-depth not half-full by volume except at the middle?
What is the difference between radius and diameter when entering a tank?
How do I convert tank volume to liters?
Does this work for capsule or oval tanks?
Sizing a pool instead? Use the pool volume calculator. For plain volume, see the cubic feet calculator or cubic yard calculator.
Important Disclaimer
These estimates are for planning purposes only. Actual costs vary by location, material availability, and project complexity. Always get at least 3 local quotes. This calculator does not replace professional advice.