Home › Home & Construction › Tank Volume Calculator

Tank Volume Calculator

Use this free tank volume calculator to find the capacity of any tank in gallons, liters, and cubic feet. Pick a vertical cylinder, a horizontal cylinder, or a rectangular tank, enter the dimensions, and set the fill level to see both the total capacity and the volume actually in the tank. It is handy for water storage tanks, fuel tanks, rain barrels, aquariums, and chemical dosing, where the difference between full capacity and current contents matters.

How it works

Tank volume starts with the same idea as any container: cross-sectional area times the dimension the liquid extends along. For a vertical cylinder, the cross-section is a circle and the liquid extends up the height, so the capacity is pi times the radius squared times the height. For a rectangular tank, the cross-section is a rectangle and the capacity is length times width times height. For a horizontal cylinder, the cross-section is still a circle, but the tank extends along its length, so the capacity is pi times the radius squared times the length of the tank.

Once the capacity is known in cubic feet, the conversions are simple: multiply by 7.48 to get gallons, and by 28.32 to get liters. The calculator asks for the diameter rather than the radius because the diameter is what a tape measure actually gives you — the straight distance across the tank through the center. It halves the diameter to get the radius before doing the math, so there is no need to divide anything yourself.

The fill level is where the three tank types start to behave differently. For a vertical cylinder and a rectangular tank, the filled volume is a straight fraction of the capacity: a tank that is 50 percent full by depth holds exactly 50 percent of its volume, because the cross-section is the same at every height. Enter 60 percent and the filled volume is 60 percent of capacity. Simple, and true at every level.

A horizontal cylinder is different, because its cross-section is a circle lying on its side. Near the bottom the tank is narrow, so the first few inches of depth add very little volume; near the middle the tank is at its widest, so each inch of depth adds the most volume; near the top it narrows again. A horizontal tank that is 25 percent full by depth holds only about 20 percent of its total volume, while one that is 75 percent full by depth holds about 80 percent. Only at exactly 50 percent do depth and volume agree. The calculator handles this with the circular segment formula, which works out the exact area of the filled part of the circle from the depth and multiplies it by the tank length.

Measuring a tank well is worth a minute of care. Measure the outside diameter and subtract twice the wall thickness if you need the inside volume — for thin steel or plastic tanks the difference is small, but for thick concrete or insulated tanks it matters. Measure the height or length along the straight section and do not include domed ends, which add a little extra volume that this calculator does not count. For a horizontal tank, measure the fill depth straight down from the inside top to the liquid surface, or work out the fill percentage from a sight gauge or a dipstick reading.

The two results answer two different questions. Total capacity tells you what the tank can hold when full, which is what you need when buying a tank, sizing a replacement, or checking a rated capacity against a spec sheet. Filled volume tells you what is actually in the tank right now, which is what you need for ordering a fuel delivery, dosing a chemical, or knowing how many days of water you have left. Many tanks are never filled to the brim — fuel tanks leave room for expansion and water tanks have an overflow below the top — so the filled volume is usually the more useful number day to day.

Keep in mind that the result is the volume of liquid, and it assumes the tank walls are straight and the ends are flat. Domed or dished ends, internal baffles, pipes, and fittings all displace a little liquid, and a tank that is not level will read differently at each end. For dosing and ordering, the calculator is accurate enough for planning; for billing or regulatory reporting, use the tank's certified calibration chart.

Worked example

A vertical cylindrical water tank has a diameter of 4 feet and a height of 4 feet, and it is completely full.

Step 1: Find the radius — 4 ÷ 2 = 2 feet.
Step 2: Find the cross-sectional area — π × 2² = 3.1416 × 4 = 12.57 square feet.
Step 3: Find the capacity in cubic feet — 12.57 × 4 = 50.27 cubic feet.
Step 4: Convert to gallons — 50.27 × 7.48 = 376 gallons.
Step 5: Convert to liters — 50.27 × 28.3168 = 1,423 liters.

The tank holds 50.27 cubic feet, which is 376 gallons or about 1,423 liters. If the same tank were only 50 percent full, the filled volume would be half of that — 188 gallons — because a vertical cylinder fills in a straight line with depth.

FAQ