Tank Volume Calculator
Assumes flat tank ends. Propane and gas tanks with curved or dished ends will hold slightly more, depending on the manufacturer's end shape.
Working out how much liquid is left in a tank gets tricky with horizontal cylinders, where the volume does not rise evenly with the fill level. Pick your tank shape below and enter its dimensions and current fill depth to get the volume in U.S. gallons or liters.
Standard Tank Fill Capacities
| Tank Shape | Dimensions | Total Cubic Inches | Max Capacity (Gallons) | Max Capacity (Liters) |
|---|---|---|---|---|
| Horizontal Cylinder | 48" Diameter x 120" Length | 217,146 cu in | 940.0 gallons | 3,558.4 liters |
| Vertical Cylinder | 36" Diameter x 72" Height | 73,287 cu in | 317.3 gallons | 1,201.0 liters |
| Horizontal Cylinder | 24" Diameter x 48" Length | 21,714 cu in | 94.0 gallons | 355.8 liters |
| Rectangular Cube | 24" W x 24" L x 24" H | 13,824 cu in | 59.8 gallons | 226.5 liters |
The math behind horizontal cylinder tanks
Finding the volume in a partially filled horizontal tank, like a buried propane cylinder or an outdoor oil tank, is harder than it looks. Because the tank curves out to the sides, the depth near the top and bottom changes a lot for only a small change in volume, while the middle is the opposite.
Why the inverse cosine comes in
A vertical drum or a rectangular box is simple: the volume tracks the fill depth directly, so 50 percent depth means 50 percent full. A horizontal drum doesn't behave that way. To find its fill volume you slice the circular cross-section into a segment, which the calculator does like this:
- Step 1: Take the tank radius from the diameter.
- Step 2: Find the circular segment where the liquid line cuts across the circle.
- Step 3: Solve that segment's area with an
arccosfunction based on the fill depth, then multiply by the tank length.
This is also why depth sensors read unevenly on horizontal tanks compared to vertical ones. Getting the shape right is the key to an accurate number.
How the volume formula changes by tank shape
Every tank calculation is the same idea — cross-sectional area times length — but the cross-section is what differs, so picking the right shape above is what makes the number accurate. Here's how each one is figured.
Rectangular and square tanks
The simplest case: volume is length × width × height, and a square tank is just the version where all sides match. Fill tracks depth directly, so a rectangular tank at half depth holds half its capacity. This is the shape to use for aquariums, box-style water and stock tanks, and most fabricated steel tanks.
Vertical cylinders (round tanks)
A round tank standing on end — a water storage tank, a poly stock tank, a water heater — uses π × radius² × height. Like a rectangle, it fills evenly with depth because every horizontal slice is the same circle. Measure the internal diameter, not the outside, since wall thickness eats into capacity.
Horizontal cylinders
A cylinder lying on its side, the shape of most propane, fuel oil, and buried tanks, holds the same total as the vertical version (π × radius² × length) but fills unevenly, which is why the partial-fill math above uses a circular segment rather than a straight percentage. This is the one case where reading a dipstick and estimating "about half" goes badly wrong.
Cone-bottom and oval tanks
These fall outside the three shapes here and are worth flagging honestly. A cone-bottom (conical) tank is a vertical cylinder with a cone at the base — estimate the cylinder portion above and add the cone (⅓ × π × radius² × cone height) separately. An oval or "capsule" tank has flattened sides, so the horizontal-cylinder figure runs slightly high; use it as a ceiling, not an exact number, and confirm against the maker's rating.
Common tank types and how to measure them
- Fish tanks and aquariums. Rectangular shape, measured inside the glass. Stop the height at the intended waterline, not the rim — a tank is never filled to the top edge. Interior measurement matters more here than anywhere, since glass thickness and the gravel bed both cut into the real water volume.
- Water tanks. Usually vertical cylinders (poly storage, stock tanks) or rectangular. Straightforward, even-fill shapes, so depth and percentage line up.
- Fuel, oil, gas, and propane tanks. Most are horizontal cylinders, so use that shape and the partial-fill math to read a dipstick correctly. Propane and heating-oil tanks often have curved (dished or hemispherical) ends that add a little capacity beyond the flat-end formula — treat the result as a close working figure and defer to the tank's rated capacity for billing.
- Septic tanks. Typically rectangular or horizontal-cylinder concrete. To size a new one, most codes start at a 1,000-gallon minimum for three bedrooms or fewer and add roughly 250 gallons per extra bedroom, but the requirement is set locally — confirm with your health department. To check an existing tank, measure the interior and run the matching shape.
Gallons, liters, and reading the result
The calculator reports both U.S. gallons and liters. To convert by hand, multiply gallons by 3.78541 for liters. The one trap is the U.K. imperial gallon, which is about 4.546 liters — larger than the U.S. gallon — so a tank spec'd in imperial gallons holds roughly 20 percent more liquid than the same number reads in U.S. gallons. Match the unit on your spec sheet before you order or bill against a figure.
About these figures: the horizontal-cylinder partial-fill method and the exact 231-cubic-inch-per-gallon conversion follow the Engineering ToolBox and NIST references cited below. Septic minimums are common U.S. code figures (state health codes vary — for example North Carolina and several states set a 1,000-gallon minimum with about 250 gallons added per bedroom over three); always confirm sizing with your local authority before buying or installing a tank.
Frequently asked questions
How do you calculate the volume of a partially filled horizontal tank?
Find the area of the liquid's cross-section, then multiply by the tank's length. Because a horizontal cylinder bulges in the middle, the fill volume doesn't rise evenly with depth, so the cross-section is a circular segment found with an arccosine (inverse cosine) formula. The calculator above runs that math for you once you enter the diameter, length, and fill depth.
What if my tank has curved or rounded ends?
This calculator models cylinders with flat ends. If your tank has hemispherical, elliptical, or dished heads, like a propane tank, the flat-end formula will slightly underestimate the true capacity, and the gap grows with how curved the ends are. For a rough working figure it's still close, but check the manufacturer's rated capacity for exact numbers.
Why does this calculator use inches?
Tank volume changes fast with small measurement errors, so inches keep the result precise. Fill depth is usually read off a dipstick graded in inches anyway, and using fractional inches avoids rounding that would throw off fuel logs or chemical monitoring. You can enter other units in each field and the calculator converts to inches internally.
How many cubic inches are in a U.S. gallon?
There are exactly 231 cubic inches in one U.S. gallon. That clean conversion is why measuring a rectangular tank in cubic inches and dividing by 231 gives an accurate gallon figure with no messy rounding.
What is the difference between a vertical and a horizontal cylinder?
A vertical cylinder fills evenly, so a tank at 50 percent depth holds 50 percent of its capacity. A horizontal cylinder does not, because it's widest through the middle. Going from 40 to 60 percent depth adds far more liquid than going from 0 to 20 percent, since the middle slices are larger than the ones near the top and bottom.
Can I use this for my water heater?
Yes. A residential water heater is a vertical cylinder, so it works fine here. Measure the external circumference to work out the internal diameter, enter the tank height, and you'll get an accurate capacity in gallons.
How do I convert tank volume from gallons to liters?
Multiply U.S. gallons by 3.78541 to get liters, or divide liters by 3.78541 to go back. The calculator shows both at once, so a 100-gallon tank reads as about 379 liters. Watch the unit if you're working from a spec sheet: a U.K. imperial gallon is larger, about 4.546 liters, so a tank rated in imperial gallons holds roughly 20 percent more than the same number of U.S. gallons.
What size septic tank do I need?
Most codes set a 1,000-gallon minimum for a home of three bedrooms or fewer, then add about 250 gallons for each additional bedroom, so a four-bedroom house typically needs 1,250 gallons and a five-bedroom 1,500. These are minimums and the exact figure is set by your local health department, so confirm before you buy or dig. To check the capacity of an existing tank, measure its interior and run the rectangular or horizontal-cylinder shape above.
How do I find the volume of a fish tank or aquarium?
A standard rectangular aquarium is length times width times height; enter those interior dimensions under the rectangular shape above for the capacity in gallons and liters. Measure inside the glass, not the outside, and stop the height at the waterline rather than the rim, since the tank is never filled to the very top. A 48-by-13-by-21-inch tank works out to roughly 55 gallons, the common aquarium size.
Does this calculator handle cone-bottom or oval tanks?
Not directly. It covers horizontal cylinders, vertical cylinders, and rectangular tanks, which fit most fuel, water, and storage tanks. A cone-bottom tank is a vertical cylinder plus a cone at the base, so estimate the cylinder here and add the cone separately; an oval or capsule tank isn't a true cylinder, so the horizontal-cylinder figure will be close but not exact. For those shapes, check the manufacturer's rated capacity.
Related calculators
Sources
- Volume in Horizontal Pipes or Cylindrical TanksThe Engineering ToolBoxCircular-segment method for the volume of a horizontal cylinder filled to a given depth, used for partial-fill tank calculations.
- NIST Handbook 44 — Appendix C, General Tables of Units of MeasurementNational Institute of Standards and Technology (NIST)Basis for the exact volume conversions: 231 cubic inches per U.S. gallon and 1,728 cubic inches per cubic foot.