About this calculator
Capacity of a water, fuel or septic tank, plus how much is in it at a measured level, which matters for horizontal tanks where the level isn’t proportional.
How it’s calculated
Horizontal cylinder part-full: segment area = r² × acos((r − h) ÷ r) − (r − h) × √(2rh − h²), multiplied by length.
Worked example
With these inputs:
- Tank
- Horizontal cylinder
- Diameter
- 4 ft
- Length / height
- 8 ft
- Liquid level (optional)
- 0 in
the result is:
- Tank capacity
- 2,846.7 L
- Total capacity
- 2,846.7 L
Tips from the trade
- Use inside dimensions. Wall thickness matters on small tanks.
- For rain barrels, 1" of rain on 1,000 sq ft of roof gives about 620 gallons.
Results are estimates for planning and pricing. Check quantities against your drawings, the manufacturer’s data and the building code adopted in your province before ordering.