Tank Fill-Level Calculator
For a horizontal cylindrical tank, enter its diameter and length, then slide the fill height to see the current volume and how it compares to other fill levels.
Fill-level chart
Capacity by fill height
How it works
The liquid's cross-section at a given fill height is a circular segment, not a simple percentage of the circle:
Segment area = ½r²(θ − sin θ), where θ = 2·cos⁻¹((r − h) / r)
Here r is the tank radius and h is the fill height measured from the bottom. The segment area × tank length ÷ 231 gives the fill volume in US gallons.
Worked example
A 24 in diameter × 60 in long tank filled to 12 in (exactly half): the segment area equals half the circle's area (π × 12² ÷ 2 ≈ 226 in²), giving 226 × 60 ÷ 231 ≈ 58.7 gallons — exactly half of the tank's ~117.5 gallon total capacity, as expected at the midpoint.
Assumptions
- Tank ends are flat (not domed/elliptical); domed ends add extra volume not modeled here.
- Fill height is measured vertically from the lowest inside point of the tank.
- Waste space such as sump depth or an outlet above the floor is not subtracted automatically.
Reference
Circular-segment geometry: Wikipedia — Circular Segment.
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Frequently asked questions
How do I calculate the volume of a horizontal cylindrical tank at partial fill?
Find the circular-segment area at the fill height, then multiply by the tank length. It is not a simple linear percentage of total volume.
Does fill percentage mean 50% height is 50% volume?
Only at exactly the halfway point. Other fill heights are not a straight-line percentage because of the circular cross-section.
How do I measure fill height on a real tank?
Measure the vertical distance from the bottom inside of the tank to the liquid surface, using a dipstick or sight gauge.