Tank capacity
How to Find Capacity of a Cylinder
Full or partial capacity for an upright cylindrical tank — read it in gallons or litres and see how much is left to fill.
A tank calculation is a capacity question, and this how to find capacity of a cylinder treats it that way. Enter the tank's inside diameter and the liquid height, and it returns the volume held — in gallons or litres — plus how much room is left at the top.
A storage tank raises a slightly different question from a plain solid: you rarely want the total volume, you want how much is in it now and how much more it will take. That means working from the liquid height, not just the tank height, and keeping an eye on the headspace at the top.
How the calculation works
- Measure the tank's inside diameter and halve it for the radius.
- Measure the current liquid height inside the tank.
- The tool computes V = π r² h for the liquid volume.
- Compare it with the full capacity to see how full the tank is.
Getting an accurate measurement
Measure the inside diameter and the liquid height for a true capacity. Keep every measurement in the same system, and let the tool convert between units for you.
The formula behind it
The volume of a cylinder is:
V = π r² hA worked example
Worked example. Take a cylinder with a diameter of 10 cm (so a radius of 5 cm) and a height of 25 cm. The base area is π × 5² = 78.54 cm², and multiplying by the height gives a volume of about 1.96 litres (1,963 cm³ · 0.52 US gallons).
| Unit | Equal to |
|---|---|
| 1 US gallon | 231 in³ · 3.785 L |
| 1 Imperial gallon | 277.42 in³ · 4.546 L |
| 1 cubic foot | 7.481 US gal · 28.32 L |
| 1 litre | 0.264 US gal · 0.220 Imp gal |
| 1 US gallon | 128 US fl oz |
The figure is the geometric capacity. Usable capacity is a little lower once you allow for fittings and the headspace you leave at the top, so treat it as a close upper bound.
Conversions use standard factors: 1 US gallon = 231 in³, 1 Imperial gallon = 4.54609 L, 1 litre = 1000 cm³.
Once the result is in
The figure above is usually the start, not the finish. These are the questions that tend to come next, answered here so you do not have to run another search:
Is this the usable capacity?
It is the geometric capacity from your inside dimensions. Real usable volume is a little lower once you allow for the outlet height, fittings and the headspace left at the top.
What about a tank lying on its side?
Upright tanks fill evenly; horizontal ones do not. If your tank is on its side, switch to the horizontal-tank mode so a dip depth is converted correctly rather than treated as a simple fraction.
How full is the tank right now?
Enter the current liquid height and the calculator shows the volume held, the full capacity and the percentage full, so you can decide at a glance whether a delivery is needed.
How much will it take to fill?
The result lists the empty space remaining in your chosen unit, and that is exactly how much you need to order to bring the tank up to the height you entered as full.
Where people use this
A few of the jobs it gets used for:
- Reading how full a vertical storage tank is
- Ordering the right delivery to top a tank up
- Monitoring a chemical or water tank on site
- Planning irrigation storage on a farm
Common mistakes to avoid
The usual overestimate comes from measuring the outside and ignoring headspace. For a live reading use the current liquid height, not the tank's full height, and remember the geometric figure is a maximum — fittings and the gap at the top mean the usable amount is a little less.
One result, every unit
Showing every unit at once is about speed and safety: you see the answer in litres, gallons and cubic feet side by side, so there is no chance of applying the wrong conversion factor. Change the output unit at the top to make any of them the headline figure, switch the precision to exact if you want the value in terms of pi, and use copy-result to paste a tidy summary wherever you need it.
What to work out next
Once you have your answer you may also want to work in gallons horizontal tank litres & liters. These related calculators pick up where this one leaves off:
To sum up, the whole job is two measurements and one formula. Enter them above, pick your unit, and you have an accurate figure plus every equivalent unit beside it. Keep a copy with the copy-result button, save the shareable link if someone else needs the same numbers, and use the history panel to compare a few options without starting from scratch.
Good to know
Is the volume of a cylinder squared or cubed?
Does this work for a tank or a pipe?
How do I calculate the volume of a cylindrical tank?
How do I find how full my tank is?
Should I measure inside or outside?
What is the formula for the volume of a cylinder?
V = π r² h — the area of the circular base (π r²) multiplied by the height (h). If you have the diameter instead of the radius, halve it first: r = d ÷ 2.