Tank Volume Calculator

How to Use This Tank Volume Calculator

Pick the shape that matches your tank from the dropdown. A rectangular tank is a straight-sided box, common for indoor water storage, aquaculture and custom built tanks. A vertical cylinder stands on end, like a barrel, pressure tank or tall rain cistern. A horizontal cylinder lies on its side, the standard shape for heating oil, diesel, septic and many fuel tanks. A sphere covers round pressure and storage vessels.

Enter the dimensions and choose feet or inches. The calculator keeps the physical size correct when you switch units, so a 4-foot diameter stays 48 inches. The Total Capacity is the full geometric volume of the tank. To find what is actually inside, enter the liquid depth measured from the bottom, and the calculator returns current gallons, the fill percentage and the liquid weight.

The shape matters more than most people expect. In a rectangular tank or a vertical cylinder, half the liquid depth means half the volume, because the cross-section does not change with height. In a horizontal cylinder it does not, and the difference is large enough to misread a fuel gauge by hundreds of gallons. The section below explains why.

Tank Volume Formulas for Every Shape

All four formulas follow one idea: find the cross-sectional area in square feet, then multiply by the height or length to get cubic feet. Convert to gallons by multiplying by 7.48, because one cubic foot holds 7.48 US gallons.

The radius is always half the diameter, so a tank measured 4 feet across has a 2-foot radius. For a horizontal cylinder, note that the formula uses the flat end area of the circle times the shell length. A tank with domed or dished ends holds slightly more than this shell-only estimate, usually 5% to 10%, because the end caps add a small partial-sphere volume. For most residential measuring that margin is safety, not error.

Horizontal Cylinders: Why Half Full Is Not Half the Volume

Here is the counterintuitive fact that trips up anyone who measures a fuel or oil tank with a stick. In a horizontal cylinder, the cross-section of the liquid is a circular segment, a slice cut across the circle by the liquid surface. Near the bottom, that slice is narrow, so the first few inches of liquid add little volume. Near the middle, the slice is at its widest, so the same few inches add the most volume. The result is a fill curve that starts slow, speeds up through the middle, then slows again at the top.

Because of this curve, a stick reading at the halfway point of the diameter does not mean half the tank. In a cylinder lying on its side, the midpoint of the diameter sits below the midpoint of the volume. Reaching 50% of capacity actually requires the liquid level to sit at about 54% of the diameter, and the two only diverge more as you measure lower. A tank that reads 12 inches of liquid in a 24-inch diameter shell holds roughly 40% of its capacity, not 50%, a difference that can mean more than 100 gallons on a large tank.

The calculator handles this with the standard segment formula. For a cylinder of radius r and length L filled to depth d, the liquid volume is the segment area times the length. The segment area equals r² times the inverse cosine of (r minus d) divided by r, minus (r minus d) times the square root of (2rd minus d²). You never need to run that by hand, but it is worth knowing why your dipstick and your fuel gauge disagree, and why a stick chart printed on the tank wall is curved rather than evenly spaced.

Gallons, Liters and Cubic Feet: Conversions Explained

Every tank volume calculation eventually lands on a unit you can order fuel in or compare against a label. The three you will meet most often are cubic feet, US gallons and liters.

• One cubic foot holds 7.48052 US gallons, the conversion used on this page. A 100-cubic-foot tank holds about 748 gallons.
• One cubic foot holds 28.317 liters, so the same 100-cubic-foot tank holds about 2,832 liters.
• One US gallon holds 231 cubic inches, the conversion to use when you measure a tank in inches. A 275-gallon oil tank therefore holds about 63,525 cubic inches of space.
• One Imperial gallon holds 277.4 cubic inches and is about 1.2 US gallons. Canadian and UK tanks are often rated in Imperial gallons, so convert before comparing against a US-rated calculator.

A gallon of water weighs 8.34 pounds at room temperature. Diesel and heating oil weigh about 7.1 pounds per gallon, gasoline about 6.3, and milk is slightly denser than water at roughly 8.6 pounds per gallon. That is why the calculator asks which liquid you store: the volume is identical, but the load on the floor or trailer changes with the liquid type.

Common Residential Tank Sizes, Quick Reference

Dimensions are typical for commercially available tanks and vary by manufacturer. The "water weight when full" column is the reason these tanks sit on concrete pads, reinforced stands or dedicated trailers, never on an ordinary deck or floor without an engineer checking the load.

Rectangular vs Cylindrical Tanks: Which One Should You Buy?

A cylinder is the most material-efficient pressure vessel shape that exists. A round wall resists internal pressure in every direction at once, so a cylindrical tank can be thinner and lighter than a rectangular box holding the same volume. That is why pressure tanks, propane cylinders, fuel tanks and most large water tanks are round. The trade-off is space: a horizontal cylinder wastes the corners around it, and a vertical cylinder needs headroom.

A rectangular tank uses floor space far more efficiently and tucks flat against a wall, under a counter or into a cabinet. It is also easier to inspect, clean and repair, because the inside is a simple box. The downside is structural. Flat walls need bracing or thicker material to resist the outward push of the water, which is why rectangular tanks are usually rated for lower pressure and are less common above a few hundred gallons.

Match the shape to the site. Tight corner in a garage, choose a rectangular tank. Limited ceiling height but plenty of length along a wall, choose a horizontal cylinder. A small footprint with height to spare, choose a vertical cylinder. If the tank will be moved on a trailer or rolled into place, horizontal cylinders are the easiest to secure because they sit on a cradle and their center of gravity stays low.

Worked Example: Reading a 275-Gallon Heating Oil Tank

Take the classic 275-gallon oil tank, a horizontal cylinder roughly 44 inches wide and 60 inches long. The diameter is 44 inches, so the radius is 22 inches, about 1.83 feet. The shell length is 60 inches, or 5 feet. Full capacity is pi times the radius squared times the length, which works out to about 275 US gallons, which is where the tank's name comes from.

On a cold January morning the gauge stick shows 11 inches of oil in the bottom. Eleven inches is exactly one quarter of the 44-inch diameter, but the oil is not one quarter of the tank. Running the segment formula, 11 inches of depth in a 44-inch tank holds roughly 18% of capacity, about 50 gallons. The curved fill profile means the first quarter of the diameter stores less than a quarter of the oil, which is exactly when homeowners misjudge how much heating oil they have left before a refill.

Now imagine the same stick reads 33 inches, three quarters of the diameter. Because the tank is wide through its middle band, 33 inches holds roughly 88% of capacity, about 242 gallons. The same 22-inch swing between one quarter and three quarters of the diameter covers far more gallons at the top than at the bottom. Enter your own tank's dimensions and stick reading in the calculator, and you get the number without building a conversion chart.

5 Tank Sizing and Measuring Mistakes

1. Measuring the outside and ignoring wall thickness. A steel or plastic tank wall can be a quarter inch to half an inch thick, and on a large tank that removes several gallons of real capacity. Measure the inside dimensions whenever you can reach them, or subtract two wall thicknesses from each outside measurement.
2. Assuming a horizontal tank fills in a straight line. The liquid cross-section in a horizontal cylinder is a circular segment, not a rectangle, so depth and volume are not proportional. Half the diameter is only about 40% of the capacity. Use the segment formula or this calculator instead of a mental ratio.
3. Mixing units in one calculation. A tank that is 48 inches in diameter and 6 feet long needs both numbers in the same unit before the formula works. Mixing inches and feet silently changes the answer by a factor of 144. Pick feet or inches and convert every dimension to it first.
4. Forgetting the end caps. Many horizontal and vertical tanks have domed or dished ends that add 5% to 10% more volume than the shell-only cylinder formula. If you are ordering fuel or filling a tank to the brim, add that margin instead of ordering short.
5. Underestimating the weight of a full tank. A 1,000-gallon water tank weighs more than 8,000 pounds when full, before the tank itself. Setting that on an ordinary deck, a wooden stand or a pickup bed without checking the load rating can end with a collapsed structure. Size the base for the full weight, not the empty weight.