Horizontal tank
Cylinder on Its Side
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 cylinder on its side handles the geometry properly, giving you the liquid volume at any depth along with a fill gauge.
A tank on its side is the classic trap in volume work, because it fills as a curved slice rather than an even stack. That is why a dip-stick reading does not map to a simple fraction — and why converting depth to volume needs the segment maths this tool runs for you.
Using the calculator
- Measure the inside diameter and the length of the tank.
- Measure the liquid depth straight down from the surface to the bottom.
- The tool finds the filled circular-segment area from that depth.
- It multiplies the segment area by the length for the liquid volume.
- Read the fill percentage and remaining space.
Getting an accurate measurement
Take the depth straight down from the liquid surface to the bottom of the tank. Keep every measurement in the same system, and let the tool convert between units for you.
How the volume is worked out
The volume of a cylinder is:
V = π r² hA worked example
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.
Conversions use standard factors: 1 US gallon = 231 in³, 1 Imperial gallon = 4.54609 L, 1 litre = 1000 cm³.
Follow-on questions, answered
Most jobs need one or two more things once the volume lands. The common follow-ups, with the numbers you need, are below:
How do I convert a dip reading to volume?
Measure the depth from the liquid surface to the bottom of the tank and enter it as the liquid depth; the tool converts that dip into a volume using the circular-segment area.
Why isn't half the depth half the volume?
Because a round tank is widest in the middle. At the halfway depth you are at exactly 50%, but above and below that the relationship curves, which is why a chart or this tool is needed.
What is the full capacity?
The result shows both the liquid volume at your depth and the full pi r squared times length capacity, so you can see exactly how much room is left.
Does it handle dished ends?
This assumes flat ends. Tanks with curved heads hold a little more, so treat the flat-end figure as a close, slightly conservative estimate.
Common uses
A few of the jobs it gets used for:
- Dipping a horizontal fuel tank and converting depth to volume
- Gauging a side-lying water tank between deliveries
- Estimating remaining product in a cylindrical vessel
What to watch out for
The big misconception is treating depth as a simple fraction of volume — half-deep is not half-full except at the exact middle. Also make sure you dip to the true bottom of the tank, and remember this assumes flat ends; dished heads add a little that a flat-end figure leaves out.
Getting the number in the unit you need
One number, many units. Whatever you enter, the result lists the same volume as litres, US gallons, Imperial gallons, millilitres, cubic centimetres, cubic inches, cubic feet, cubic yards and cubic metres at the same time. You almost never have to convert by hand: read the figure in the unit your job uses and copy any of the others straight from the breakdown. If you need a shareable version, the copy-link button preserves your exact inputs so a colleague opens the page with the same calculation already filled in.
Keep going
Once you have your answer you may also want to work in tank capacity gallons hollow tube & pipe. These related calculators pick up where this one leaves off:
In short: measure the width and the height, keep them in the same unit, and the tool turns them into a volume you can read in litres, gallons, cubic feet or anything else, with the working shown so you can trust it. If a result ever looks off, re-enter the measurement you are least sure of, or switch to the diameter input and let the tool halve it for you, which catches the most common error before it matters.