Quick answer: a cylindrical tank's capacity is V = π (d/2)² × length; multiply cubic feet by 7.4805 for US gallons, or divide cubic inches by 231. Partial fills in a horizontal tank are the tricky part — liquid at a quarter of the diameter fills only about 19.6% of the tank, not 25%. The calculator above handles that properly.
Which tank shape do you have?
Getting the geometry right matters more than any other single decision here. These seven cover essentially every tank in domestic and light-industrial use:
| Shape | Measure | Typically used for |
|---|---|---|
| Vertical cylinder | Diameter, height | Water storage, brewing, chemical dosing, rain barrels |
| Horizontal cylinder | Diameter, length | Fuel and diesel tanks, underground storage, air receivers |
| Rectangular | Length, width, height | Aquariums, sumps, IBC totes, cisterns, troughs |
| Horizontal oval | Width, height, length | Truck-mounted fuel tanks, low-profile tanks wider than they are tall |
| Vertical oval | Width, height, length | Home heating oil tanks, slimline garden water tanks |
| Horizontal capsule | Diameter, total length | Propane and LPG tanks, pressure vessels, tankers |
| Vertical capsule | Diameter, total height | Upright pressure vessels, some process tanks |
If your tank is round-ended, look carefully before choosing: a cylinder has flat ends, while a capsule has domes. On a 500-gallon propane tank those domes account for roughly a fifth of the capacity, so the distinction is worth the extra glance. Real pressure vessels often use shallower dished or 2:1 elliptical heads rather than true hemispheres, which makes them hold slightly less than the capsule figure — see the accuracy note below.
The chart every tank owner needs: depth versus volume
This is where intuition fails. In a vertical tank, half the depth is half the volume — the cross-section never changes. In a horizontal cylinder, the cross-section is a circle: narrow at the bottom, widest in the middle, narrow again at the top. A dipstick reading of 25% therefore does not mean a quarter of a tank.
| Liquid depth (% of diameter) | Horizontal cylinder — % full | Vertical tank — % full | Difference |
|---|---|---|---|
| 5% | 1.9% | 5% | −3.1 |
| 10% | 5.2% | 10% | −4.8 |
| 15% | 9.4% | 15% | −5.6 |
| 20% | 14.2% | 20% | −5.8 |
| 25% | 19.6% | 25% | −5.4 |
| 30% | 25.2% | 30% | −4.8 |
| 40% | 37.4% | 40% | −2.6 |
| 50% | 50.0% | 50% | 0 |
| 60% | 62.6% | 60% | +2.6 |
| 70% | 74.8% | 70% | +4.8 |
| 75% | 80.4% | 75% | +5.4 |
| 80% | 85.8% | 80% | +5.8 |
| 90% | 94.8% | 90% | +4.8 |
| 95% | 98.1% | 95% | +3.1 |
Read the practical consequence carefully: a horizontal fuel tank showing a quarter of its depth has already burned through 80% of its contents. The last quarter of the depth is only about a fifth of the fuel. Anyone running a generator or a heating system off a horizontal tank should be ordering long before the stick says 25%.
The chart is symmetrical around the halfway mark, and the maximum error from assuming depth equals volume is 5.8 percentage points, at 20% and 80%.
The formulas behind the calculator
Every result here is reproducible with a pencil. Total capacities first:
Rectangular: V = l × w × h
Horizontal oval: V = [π (h/2)² + (w − h) × h] × L
Vertical oval: V = [π (w/2)² + w × (h − w)] × L
Capsule: V = π (d/2)² × (L − d) + (4/3) π (d/2)³
The oval tanks are stadium-shaped in cross-section: a rectangle with a semicircle stuck on each end. Adding the two semicircles gives one full circle, which is why the formulas above pair a single π term with a rectangular term. The capsule works the same way in three dimensions — two hemispherical heads make exactly one sphere.
Partial fill: the circular segment
For a horizontal cylinder filled to depth d, the wetted cross-section is a circular segment:
Vfilled = A × L
Worked example — a 4 ft diameter, 10 ft long horizontal tank with 1 ft of diesel in it. Here r = 2 and d = 1:
- arccos((2 − 1)/2) = arccos(0.5) = 1.0472 radians
- r² × 1.0472 = 4 × 1.0472 = 4.1888
- (r − d) × √(2rd − d²) = 1 × √(4 − 1) = 1.7321
- Segment area = 4.1888 − 1.7321 = 2.4567 ft²
- Volume = 2.4567 × 10 = 24.57 ft³ ≈ 184 US gallons
Full capacity is π × 4 × 10 = 125.66 ft³ ≈ 940 gallons, so that foot of diesel is 19.6% of the tank — exactly the figure in the chart above. For capsule tanks the calculator adds a spherical cap term, π d² (3r − d) / 3, to account for the liquid sitting in the two domed ends.
Common tank sizes and their real capacities
| Tank | Shape | Typical dimensions | Capacity |
|---|---|---|---|
| Home heating oil tank | Vertical oval | 27 wide × 44 tall, 60 in long | 275 gal |
| Residential propane tank | Horizontal capsule | 37 in ⌀, 119 in long | 500 gal (400 usable) |
| Small propane tank | Horizontal capsule | 30 in ⌀, 94 in long | 250 gal (200 usable) |
| IBC tote | Rectangular | 48 × 40 × 46 in | 275 gal / 1,040 L |
| 55-gallon drum | Vertical cylinder | 22.5 in ⌀, 33.5 in tall | 57.7 gal brim-full |
| Rain barrel | Vertical cylinder | 24 in ⌀, 34 in tall | ≈ 66 gal |
| Poly water tank (slimline) | Vertical oval | 30 × 72 in, 72 in long | ≈ 600 gal |
| Septic tank (residential) | Rectangular | 90 × 51 × 51 in | ≈ 1,000 gal |
| Air compressor receiver | Vertical cylinder | 24 in ⌀, 60 in tall | ≈ 117 gal |
Nameplate capacities are nominal. Manufacturers round, and nearly every tank includes headspace it is never meant to use — which is why your own measurements and the calculator above will usually land a few percent away from the label.
Three jobs this calculator actually does
Reading a heating oil tank
A standard 275-gallon tank has an upright oval cross-section 27 in wide and 44 in tall (choose vertical oval above). The stick reads 12 in. Because the tank narrows toward the bottom, 12 in is 27% of the height but only about 24% of the volume — roughly 64 gallons. At a January burn rate of 5 gallons a day, that is under thirteen days, not the sixteen the naive reading suggests. Getting this wrong is how people run dry in a cold snap.
Sizing a rainwater system
Roof catchment is easy to underestimate. A 1,500 ft² roof sheds about 935 gallons per inch of rain (roof area × 0.623 gal per ft² per inch). One 2-inch storm therefore overwhelms a 1,000-gallon tank — which is the calculation that decides whether you buy one tank or two, and how big an overflow you need. Full sizing guidance is on the water tank volume calculator.
Dosing a treatment chemical
Chemical dose rates are per unit volume, so an error in tank capacity becomes an error in concentration. If you need 50 ppm in a tank you believed held 1,000 gallons but which actually holds 940, you have overdosed by 6%. Measure inside dimensions, and if the tank is partially full, use the wetted volume rather than the nameplate.
Common mistakes & pro tips
- Assuming depth is proportional to volume. Only true for vertical tanks with straight sides. For anything horizontal or round-bottomed, use the fill-depth field above.
- Measuring outside dimensions. Capacity comes from the inside cavity. Wall thickness comes off a diameter twice — once at each side.
- Confusing a cylinder with a capsule. Flat ends versus domed ends. On a propane tank the domes are roughly 20% of the capacity.
- Ignoring the unusable bottom. Most tanks draw from an outlet sitting above the floor, and sediment collects below it. Subtract that dead volume from anything you plan to actually use.
- Forgetting expansion space. Propane fills to 80%; heating oil deliveries stop around 90%; water tanks need overflow headroom.
- Pro tip — calibrate your dipstick once. Run the calculator at 2-inch increments, write the gallon figures on the stick, and you never have to do the maths again in the dark with cold hands.
- Pro tip — verify by filling. Add a known quantity — say ten 5-gallon buckets — and check the depth change against the calculator. It catches wrong assumptions about shape immediately.
A note on accuracy
The geometry here is exact; the assumptions are where error enters. Real tanks depart from ideal shapes in ways worth knowing about: pressure vessels often use 2:1 elliptical or torispherical dished heads that are shallower than hemispheres, so a capsule calculation runs a little high on those. Poly tanks bulge when full. Buried tanks may not sit level, which tilts the liquid surface and skews any dipstick reading. Internal baffles, heaters and sumps all displace liquid.
For custody transfer, regulated fuel inventory or anything where the number has legal weight, use the manufacturer's calibration chart or a professionally strapped tank table. For ordering fuel, sizing a pump, dosing a chemical or planning a water system, the figures here are more than accurate enough — and considerably better than the guess they replace.
How to use this calculator
- Pick the tank shape. Choose the geometry that matches your tank — vertical or horizontal cylinder, rectangular, horizontal or vertical oval, or a capsule with rounded ends.
- Enter the inside dimensions. Measure inside the tank where possible. Wall thickness reduces capacity, and on small tanks the difference is significant.
- Add the liquid depth (optional). Leave the depth blank for total capacity, or enter a dipstick reading to get the volume actually in the tank right now.
- Read capacity and fill level. The calculator shows the volume, percent full and remaining space, and converts between gallons, liters and cubic units instantly.
Frequently asked questions
How do I calculate the volume of a tank in gallons?
Compute the volume in cubic feet, then multiply by 7.4805 to get US gallons. For a vertical cylinder, capacity = π × (diameter/2)² × height. A tank 4 ft across and 5 ft tall holds π × 4 × 5 = 62.83 ft³ ≈ 470 US gallons. If you measured in inches, divide cubic inches by 231 instead.
Why is a half-full horizontal tank not half its depth?
It actually is — a horizontal cylinder filled to exactly half its diameter holds exactly half its capacity, because the circle is symmetrical about its center line. The surprise comes at other depths: at one quarter of the diameter the tank holds only about 19.6% of capacity, not 25%, because the bottom of a circle is much narrower than its middle.
How do I calculate how much is left in a partially filled tank?
Measure the liquid depth with a dipstick, enter it in the fill-depth field, and the calculator returns the wetted volume using the circular segment formula: A = r²·arccos((r−d)/r) − (r−d)·√(2rd−d²), multiplied by the tank length. It also reports percent full and remaining space.
Should I measure the inside or outside of the tank?
Inside, whenever you can reach it. Capacity is set by the internal cavity, and wall thickness is subtracted twice from any diameter. On a poly water tank with a 12 mm wall, using the outside diameter overstates a 1,000-litre tank by roughly 25 litres.
How many gallons is a 275 gallon oil tank really?
A standard 275-gallon home heating oil tank measures about 27 in wide, 44 in tall and 60 in long with an upright obround (oval) cross-section — use the vertical oval setting, which gives roughly 268 gallons brim-full against the 275 nameplate. Usable oil is less — most tanks draw from a few inches above the bottom and are only filled to about 90%, so plan on about 240 usable gallons.
Why do propane tanks only fill to 80 percent?
Liquid propane expands substantially as it warms — roughly 1.5% for every 10 °F. The 20% vapour space is a safety margin so that a tank filled on a cold morning does not become liquid-full and over-pressurised in the afternoon sun. So a 500-gallon tank is charged with about 400 gallons of propane.
What is wetted volume?
Wetted volume is the volume of liquid actually in a partially filled tank — the portion touching, or wetting, the tank walls. It is what a dipstick reading converts to, and for horizontal tanks it requires the circular segment formula rather than simple proportional scaling.