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Horizontal tank calculator

A tank lying on its side needs segment maths, not a simple π r² h. Enter the liquid depth and get the true volume and fill percentage.

Horizontal tanks are the classic gotcha — half the depth is not half the volume. This horizontal tank calculator handles the geometry properly, giving you the liquid volume at any depth along with a fill gauge.

How the calculation works

  1. Measure the inside diameter and the length of the tank.
  2. Measure the liquid depth straight down from the surface to the bottom.
  3. The tool finds the filled circular-segment area from that depth.
  4. It multiplies the segment area by the length for the liquid volume.
  5. Read the fill percentage and remaining space.

Worked example. Take a cylinder with a diameter of 15 cm (so a radius of 7.5 cm) and a height of 30 cm. The base area is π × 7.5² = 176.71 cm², and multiplying by the height gives a volume of about 5.30 litres (5,301 cm³ · 1.40 US gallons).

This assumes flat ends and a level tank. Dished (curved) heads add a small amount of extra volume, so the flat-end result is slightly conservative.

Every horizontal tank page

Each page below targets a specific question and opens with the calculator ready to go:

Good to know

Why can't I use π r² h for a horizontal tank?
Because a tank on its side fills as a circular segment, not a simple stack. Half the depth is not half the volume. The calculator uses the correct segment formula.
How do I measure the liquid depth?
Measure straight down from the top of the liquid to the bottom of the tank (a dip stick works well). Enter that depth and the inside diameter.
Does this include dished ends?
This mode assumes flat ends. Tanks with dished (curved) heads hold a little more; treat the flat-end figure as a close, slightly conservative estimate.

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