Free Tank Volume and Fill calculator
Find a tank's full capacity and the volume of liquid at any fill level — in cubic feet, US gallons, and liters. Pick a shape, enter the dimensions, and add a fill height to see how many gallons are in the tank right now, with the horizontal-cylinder partial-fill math handled correctly — updated live, as you type.
On this page14 sections
| Measure | Cubic feet | US gallons | Litres |
|---|---|---|---|
| Full capacity | 502.7 | 3,760 | 14,234 |
| At fill level | 98.3 | 735 | 2,783 |
Volumes use 1 ft³ = 7.48052 US gallons = 28.3168 L. A horizontal cylinder's filled volume uses the circular-segment formula, so depth and gallons are not proportional. Capsule tanks report full capacity only.
Real tanks have wall thickness and dished ends. How accurate is this?
Results are estimates. Consult a professional.
How the tank volume calculator works
A tank holds liquid, so its capacity is pure volume — the three-dimensional space inside the shell. The calculator takes the shape and the dimensions, works out the full volume in cubic feet, and converts that to the units you actually order in: US gallons and litres. Give it a fill height as well and it returns the volume of liquid sitting in the tank right now, not just the tank's total size.
What the two numbers mean
The full volume is the tank's rated capacity — what fits when it is brimful. The filled volume is what is in there at a given depth. For an upright tank the two track in a straight line, but for a cylinder lying on its side they do not, and that gap is where most quick estimates go wrong.
What goes into a tank volume estimate
Three things set the answer: the shape, the dimensions, and the fill height. Get the shape wrong and even perfect measurements give the wrong volume, because each shape uses a different formula.
Shape — which formula applies
A tank standing upright is a vertical cylinder; one lying on its side is a horizontal cylinder. A box-shaped tank is rectangular. A long tank with rounded, dome-shaped ends is a capsule — a cylinder body capped by two hemispheres. The calculator picks the matching formula once you choose the shape.
Dimensions — diameter, length, height
Cylinders and capsules need a diameter and one length. A rectangular tank needs length, width and height. Measure the inside of the tank if you can; the wall thickness is small, but on a big tank it adds up. Every dimension here is in feet, with inches converted to feet by dividing by twelve.
Fill height — the liquid depth
The fill height is how deep the liquid sits, measured straight up from the lowest point of the tank. For an upright tank that depth maps directly to volume. For a tank on its side it does not, because the width of the liquid surface changes as the level rises.
A worked example using the tank volume calculator
Priya has a cylindrical water tank lying on its side: 8 ft across and 10 ft long. The dip stick reads 2 ft of water. She wants the full capacity and how many gallons are in it right now.
Step 1 — Find the full capacity
Radius is half the diameter: 8 ÷ 2 = 4 ft. Full volume = π × 4² × 10 = 502.65 cu ft. In gallons that is 502.65 × 7.48052 = 3,760.12 gal (14,233.58 L).
Step 2 — Find the filled cross-section
The liquid forms a circular segment. Its area = r²·acos((r − f) ÷ r) − (r − f)·√(2rf − f²), with r = 4 and fill f = 2: that is 9.83 sq ft of cross-section.
Step 3 — Multiply by the length
Filled volume = 9.83 × 10 = 98.27 cu ft, which is 98.27 × 7.48052 = 735.11 gal (2,782.68 L).
Step 4 — Read the fill fraction
That 2 ft of water is a quarter of the tank's 8 ft diameter, yet it is only 19.6% of the capacity — 735 of 3,760 gallons. The bottom of a round tank is narrow, so a low reading holds less than the depth suggests.
Tank volume by shape: full-capacity formulas
Every shape converts dimensions to cubic feet, then to gallons. This table is the full-capacity formula for each supported shape, with the gallons-per-foot conversion baked into the examples.
| Shape | Full-volume formula | Example → gallons |
|---|---|---|
| Vertical cylinder | π × r² × h | 6 ft dia × 10 ft tall → 2,115 gal |
| Horizontal cylinder | π × r² × L | 8 ft dia × 10 ft long → 3,760 gal |
| Rectangular | L × W × H | 6 × 4 × 5 ft → 898 gal |
| Capsule | π × r² × L + 4⁄3 × π × r³ | 4 ft dia × 12 ft long → 1,379 gal |
r = radius (half the diameter). Gallons = cubic feet × 7.48052. A capsule is a cylinder body plus two hemispherical end caps (one full sphere of the same diameter).
Volume-to-gallon and metric conversion factors follow the NIST customary-to-metric reference (US gallon = 231 in³ = 3.785 L). The capsule and cylinder formulas match the published CalculatorSoup and Omni tank-volume references.For partial fill, the upright cylinder and the box are linear — the liquid volume is just the full formula with the fill height in place of the full height. The horizontal cylinder is the exception, and it gets its own section below.
Why a half-full horizontal tank isn't half its gallons
Stand a cylinder upright and every inch of depth holds the same volume. Lay it on its side and that breaks. The liquid surface is narrow at the bottom of the circle, widest at the middle, and narrow again at the top, so each inch of depth holds a different amount.
The circular-segment formula
The cross-section of the liquid is a circular segment — the slice of a circle below a horizontal cut. The formula gives its area; multiply by the length and you have the volume. It is the part of tank math people get wrong, because it is tempting to assume depth and volume rise together. They do not.
What the curve looks like
Filling the first quarter of the diameter adds far fewer gallons than filling the middle quarter. In our 8 ft tank, 2 ft of water — a quarter of the height — is only 19.6% of capacity. Reach the halfway line at 4 ft and you are at exactly 50%, because the circle is symmetric about its centre. The shortfall at the bottom is mirrored by a surplus at the top.
How to read a tank level and convert it to gallons
A level reading is a height; capacity is a volume. To turn one into the other you need the tank's shape and dimensions, then the right formula.
- Measure the depth — drop a dip stick or read the sight gauge to get the liquid height from the bottom of the tank, in feet.
- Pick the shape — upright, on its side, box, or domed-end capsule. This sets which formula applies.
- Apply the fill formula — upright cylinder and box: full formula with fill height swapped in. Horizontal cylinder: the circular-segment formula above.
- Convert to gallons — multiply the filled cubic feet by 7.48052 for US gallons, or by 28.3168 for litres.
If you need the dry volume of bedding, backfill, or a base pad around the tank instead, the sand calculator and concrete calculator handle those.
Ullage and safe fill: why you don't fill to 100%
Rated capacity and usable capacity are not the same. Tanks are almost never filled to the brim. The empty space left on top is called ullage, or outage, and it exists for good reasons — so the rated gallons and the gallons you can safely store are two different numbers.
Why the headroom exists
Liquids expand as they warm, so a tank filled solid on a cold morning can overflow by afternoon. Vapour needs somewhere to go. Fittings and the filler neck sit above the true top. For these reasons a safe fill is often 80 to 95% of rated capacity, depending on the liquid and the code that governs it.
Working with usable volume
Take the full capacity from the calculator, then multiply by your safe-fill fraction to get usable gallons. An 80% safe fill on our 3,760-gallon tank leaves about 3,008 usable gallons. Always size to usable volume, not rated volume — the headroom is not yours to use.
Tank volume definitions
How accurate is this tank volume calculator?
The volume math is exact. Every shape formula is closed-form geometry, and the circular-segment formula for a horizontal cylinder is exact too — it is validated against the known cases of empty, half-full and full, where it returns zero, half the circle area and the full circle area to the decimal. If your dimensions are right, the volume is right.
The limits are in the measurements and the model. Real tanks have wall thickness, internal baffles, and sometimes dished or coned ends rather than true flat or hemispherical ones, and any of those shifts the true volume by a percent or two. Partial fill is computed for upright cylinders, boxes and horizontal cylinders; for capsules the calculator returns full capacity only, because the liquid level moving through the rounded end caps needs a separate formula. Measure the inside dimensions, confirm the end-cap shape, and treat the result as the precise volume of an idealised tank — which is within a hair of the real one for the vast majority of installations.
Frequently asked questions about the free Tank Volume and Fill calculator
About this tank volume and fill calculator
This calculator runs entirely in your browser — nothing you enter is sent anywhere. It computes full capacity for vertical and horizontal cylinders, rectangular, and capsule tanks, and the filled volume at a given depth, using exact geometry validated against known cases.
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