Tank Volume Calculator
Enter internal dimensions to get capacity in L/gal, the weight of the water when full, and the barrel equivalent — before freeboard and outlets take their share.
Last updated: 2026-09-28
How the calculation works
- Pure geometry on internal dimensions — wall thickness excluded.
- Weight assumes water (≈1 kg/L); other fluids scale by specific gravity.
- Barrel conversion uses the US oil barrel (159 L) for fuel-storage cross-reference.
Formula
cylinder: V = π/4·d²·h box: V = L·W·H
| Symbol | Meaning | Unit |
|---|---|---|
V | Internal capacity | L |
Worked example
Interpreting the result
Rated capacity is brim volume; usable capacity is what sits between the outlet and the overflow — often 70–85% of rated. When sizing storage (rainwater, buffer tanks, dosing), work backwards from the usable requirement and verify the floor or pad can carry the full weight plus the empty tank. Cylindrical tanks resist pressure better; rectangular ones ship and install more efficiently.
Assumptions
- Internal measurements at operating condition.
- Water density for the weight row.
Limitations
- Horizontal cylinders, cones and dished ends need different geometry.
- Usable volume (outlet height, freeboard, suction limits) is not deducted.
Frequently asked questions
How many gallons in a 1000 litre tank?
264 US gallons. In oil barrels: 6.3. The calculator shows all three units from the dimensions directly.
Why is usable capacity less than rated?
Outlets sit above the floor (sediment), pumps need suction head, and overflows require freeboard — plan on 70–85% of the geometric volume.