The Tank Volume Calculator returns total capacity, filled volume and remaining capacity for horizontal and vertical cylinders, rectangles, ovals, capsules and dished-end tanks. Enter the tank's shape and dimensions, then a fill depth, and the calculator returns gallons, litres and cubic feet at that fill level alongside the tank's full capacity. A fill chart shows how volume relates to depth in ten percent increments, which is not a straight line for most tank shapes lying on their side.
Calculate total and filled tank volume
Total volume depends only on the tank's shape and outer dimensions; filled volume depends on those same dimensions plus how deep the liquid inside currently sits. The two are related but not proportional for every shape, which is the single most useful thing this calculator demonstrates beyond a simple capacity figure.
Total volume = shape-specific formula (below)
Fill volume = portion of that shape occupied up to the fill depth
Ullage = Total volume − Fill volume
Fill percent = Fill volume ÷ Total volume × 100A vertical tank's fill volume scales directly with depth, since every horizontal slice through a vertical cylinder or a rectangle is identical. A horizontal tank does not share this property, because the cross-section at the bottom of a lying-down cylinder is a thin sliver while the cross-section through the middle is the full diameter.
Select a tank shape
Tank shape determines which formula applies, and the calculator supports nine common shapes covering nearly every above-ground and underground storage tank in residential and light commercial use. Selecting a shape switches the input fields to match its specific dimensions.
- Horizontal cylinder: a cylinder lying on its side, the most common shape for fuel and water tanks. Diameter and length.
- Vertical cylinder: standing upright. Diameter and height.
- Rectangle: a rectangular tank or cistern. Length, width and height.
- Horizontal oval: a flattened cylinder lying on its side. Width, height and length.
- Vertical oval: the same cross-section standing upright.
- Horizontal capsule: a cylinder with two rounded, hemispherical ends, lying on its side.
- Vertical capsule: the same shape standing upright.
- Horizontal 2:1 elliptical: a cylinder with shallow elliptical end caps rather than full hemispheres.
- Horizontal dish ends: a cylinder with shallow dished, rather than flat, end caps.
Fuel storage and septic tanks are most often the horizontal cylinder or horizontal capsule shapes; cisterns and mixing tanks are more often vertical cylinders or rectangles.
Apply the tank volume formulas
Every shape's total volume follows directly from its cross-sectional area multiplied by its length or height, with oval and capsule shapes combining a rectangle or cylinder with curved end sections. Filled volume for a shape lying on its side also needs the circular segment formula, covered in the next section.
Horizontal / vertical cylinder = π × (Diameter ÷ 2)² × Length or Height
Rectangle = Length × Width × Height
Horizontal oval = (π × r² + 2 × r × a) × Length, r = Height ÷ 2, a = Width − Height
Horizontal capsule = π × r² × [(4 ÷ 3) × r + a], a = straight cylindrical section lengthA rectangular cistern's volume needs no curved-shape adjustment at all; it is length times width times height, identical to any rectangular prism. A capsule or oval tank's extra terms account for the rounded end caps, which hold proportionally more volume relative to their length than the straight cylindrical middle section does.
Measure fill depth in a horizontal cylinder
Fill depth in a horizontal tank is normally read with a dip stick lowered through a fill port, and converting that single depth reading into a volume needs the circular segment formula, since the liquid's cross-section is a curved sliver rather than a full circle or a simple rectangle. This is the calculation every horizontal-tank fill chart is built from.
m = Radius − Fill depth
θ = 2 × arccos(m ÷ Radius)
Segment area = ½ × Radius² × (θ − sin θ)
Fill volume = Segment area × LengthThe angle θ, measured in radians, defines how much of the circle's circumference the liquid surface spans. A fill depth exactly equal to the radius, half the tank's diameter, gives θ of exactly π radians (180°) and a segment area of exactly half the circle, which is the only fill depth at which a horizontal cylinder's percent-full figure matches its percent-depth figure.
Read the fill chart
Horizontal cylindrical tanks fill non-linearly: the bottom and top of the tank hold far less volume per inch of depth than the middle does, because the circular cross-section is narrowest at both ends and widest across the centre.
The chart below reports volume as a percentage of total capacity at each ten percent increment of depth, independent of the tank's actual size.
| Depth (% of diameter) | Volume (% of total) |
|---|---|
| 10% | 5.2% |
| 20% | 14.2% |
| 30% | 25.2% |
| 40% | 37.3% |
| 50% | 50.0% |
| 60% | 62.7% |
| 70% | 74.8% |
| 80% | 85.8% |
| 90% | 94.8% |
| 100% | 100.0% |
A tank at 20 percent of its full depth holds only 14.2 percent of its total volume, not 20 percent, and a tank at 80 percent depth already holds 85.8 percent of its volume. The curve is symmetric around the halfway point, where depth percentage and volume percentage are the only pair on the table that match exactly.
Estimate a 1,000 gallon horizontal tank
A horizontal cylindrical tank 64 inches in diameter and 72 inches long has a total capacity of about 1,003 gallons, and at a fill depth of 40 inches holds roughly 659 gallons, about 65.8 percent full. Every step below carries the units through in feet.
1. Convert dimensions to feet. Diameter 64 in = 5.333 ft (radius 2.667 ft); Length 72 in = 6 ft; Fill depth 40 in = 3.333 ft
2. Find the total volume. π × 2.667² × 6 = π × 7.111 × 6 ≈ 134.04 ft³ ≈ 1,003 gallons
3. Since fill depth exceeds the radius, find the empty segment above the liquid. f′ = 2 × 2.667 − 3.333 = 2.0 ft; m = 2.667 − 2.0 = 0.667 ft
4. Find the empty segment's angle and area. θ = 2 × arccos(0.667 ÷ 2.667) = 2 × arccos(0.25) ≈ 2.636 rad Segment area = 0.5 × 2.667² × (2.636 − sin 2.636) ≈ 7.65 ft² Empty segment volume = 7.65 × 6 ≈ 45.91 ft³
5. Subtract the empty segment from the total. 134.04 − 45.91 ≈ 88.13 ft³ ≈ 659 gallons filled
That leaves an ullage, the remaining empty capacity, of about 45.91 ft³, or roughly 343 gallons. A tank this size and shape corresponds to a common nominal 1,000 gallon residential fuel or water tank; the calculated 1,003 gallon total is the geometric capacity before any manufacturer's fill-limit derating, which most suppliers apply for safety margin.
Convert tank volume between units
Tank volume converts to whichever unit a fill gauge, a delivery invoice or a spec sheet uses, and the calculator reports several at once so a single result serves a US delivery ticket and a metric equipment spec without a second lookup.
| Convert from ft³ | Factor | Example (134.04 ft³) |
|---|---|---|
| To US gallons | × 7.4805 | 1,002.8 gal |
| To imperial gallons | × 6.2288 | 834.9 imp gal |
| To litres | × 28.317 | 3,796 L |
| To cubic metres | × 0.02832 | 3.80 m³ |
| To barrels | × 0.1781 | 23.87 bbl |
A US gallon and an imperial gallon are not the same volume; the imperial gallon is about 20 percent larger, which matters when a tank spec sheet from outside the United States lists a capacity in gallons without specifying which one.
Frequently asked questions
How do I calculate the volume of a horizontal cylindrical tank?
Multiply pi by the square of the radius, then by the length: π × r² × length. A tank 64 inches in diameter (a radius of 32 in, or 2.667 ft) and 72 inches (6 ft) long holds π × 2.667² × 6, about 134 cubic feet, or roughly 1,003 gallons.
Why does a horizontal tank fill non-linearly?
Because the cross-section of a cylinder lying on its side is narrowest near the bottom and top and widest across the middle. A given inch of depth near the bottom adds far less volume than the same inch of depth near the middle, which is why 20 percent depth holds only about 14.2 percent of the total volume rather than 20 percent.
How much does a 1,000 gallon tank hold at 40 inches of fill?
For a tank 64 inches in diameter and 72 inches long, a 40 inch fill depth holds about 659 gallons, roughly 65.8 percent of the tank's total capacity of about 1,003 gallons.
What is ullage?
Ullage is the remaining empty capacity in a tank, found by subtracting the current fill volume from the total volume. A 1,003 gallon tank holding 659 gallons has an ullage of about 344 gallons, the volume still available before the tank reaches full.
How do I measure fill depth with a dip stick?
Lower a stick or gauge through a vertical fill port until it touches the bottom of the tank, then read the wetted length once withdrawn. That reading is the fill depth, which converts to a volume using the circular segment formula for a horizontal tank or a direct depth-times-area calculation for a vertical or rectangular one.
How many gallons are in a cubic foot?
About 7.48 US gallons, or about 6.23 imperial gallons. A 100 cubic foot tank holds roughly 748 US gallons or 623 imperial gallons.
What is the difference between a US gallon and an imperial gallon?
An imperial gallon is larger, at about 4.546 litres compared with 3.785 litres for a US gallon, a difference of roughly 20 percent. A tank rated at 1,000 imperial gallons holds more liquid than one rated at 1,000 US gallons.
How do I calculate volume for a tank with rounded dish ends?
Add the volume of the straight cylindrical body to the volume of the two shallow dished end caps, which hold less volume than a full hemisphere would. The calculator's horizontal dish-ends mode performs this combination directly once the tank's diameter, straight length and dish depth are entered.
How do I find fill percentage from a dip stick reading?
Divide the fill volume calculated from the dip stick reading by the tank's total volume, then multiply by 100. A horizontal tank's fill percentage does not track its depth percentage directly, which is why the fill chart above is the more reliable reference than assuming depth and volume percentages match.
Can this calculator handle a vertical oval tank?
Yes. A vertical oval tank uses the same oval cross-section as a horizontal one, standing on end, and its fill volume is calculated in three zones, the bottom curved section, the straight middle section, and the top curved section, rather than the single circular-segment formula used for a horizontal cylinder.
Summary
The Tank Volume Calculator returns total and filled volume for nine tank shapes, converting a fill depth into a volume through the circular segment formula for horizontal shapes and direct depth-times-area for vertical and rectangular ones.
A 64 by 72 inch horizontal cylinder holds about 1,003 gallons total and about 659 gallons at a 40 inch fill depth, close to 66 percent full despite the depth reading alone suggesting otherwise.
Horizontal tanks fill non-linearly, most steeply through the middle of the tank and most slowly near the bottom and top, which is why the fill chart, not the depth reading alone, is the reliable way to estimate how much liquid remains.