Tank Volume Calculator

A tank volume calculator answers one question precisely: how many gallons does this tank actually hold? Enter the tank's shape and dimensions, and the tool computes full capacity in gallons, liters, cubic feet, and cubic meters. Add a dipstick reading and it also tells you how much fuel is sitting in the tank right now, both as gallons and as a percent of capacity.

Property owners can use it to identify a mystery tank before a sale. Fuel oil customers can use it to decide whether there is room for a full delivery. Facility managers can use it to check that the capacity on file matches the steel in the ground. And contractors can use it during bids, because a 1,000 gallon tank is a very different removal job than a 275.

The calculator supports five shapes: horizontal cylinders, vertical cylinders, rectangular tanks, true elliptical tanks, and the flat-sided oval used on 275 and 330 gallon heating oil tanks. It accepts dimensions in inches, feet, centimeters, or meters, and it includes presets for the most common residential and commercial tanks so you can skip measuring entirely when your tank matches a standard size.

How the math works

Every result starts from the same conversion: one US gallon equals 231 cubic inches. The calculator converts your dimensions to inches, computes the tank's volume in cubic inches, then divides by 231.

For a full horizontal or vertical cylinder, the volume is the circular cross section times the length: pi times the radius squared times the tank's length (or height, for a vertical tank). A rectangular tank is simply length times width times height. A flat-sided oval, the shape of a 275 or 330 gallon heating oil tank, is a rectangle with a half circle on each end: for a 27 by 44 inch end, pi times 13.5 squared plus 27 times 17, about 1,032 square inches, times the length. A true elliptical tank is pi times half the width times half the height, times the length.

Partial fills are where the real math lives. When you enter a dipstick reading for a horizontal cylinder, the calculator uses the exact circular segment formula rather than an approximation. The liquid's cross section equals the radius squared times the inverse cosine of (radius minus liquid height) over radius, minus (radius minus liquid height) times the square root of (2 times radius times height minus height squared). Multiply that cross section by the tank length and you have the exact partial volume. This matters because fill height and volume are not proportional in a horizontal cylinder. The first few inches at the bottom hold very little, the middle inches hold the most, and the curve flattens again near the top.

Vertical cylinders and rectangular tanks are easier: volume scales linearly with liquid height, so half the height really is half the volume. For true elliptical tanks, the calculator stretches the circular segment math to fit the ellipse, which is exact. For flat-sided ovals, it adds the circle segments at the rounded ends to the rectangle in the straight middle. The tool also offers a printable strapping chart generator.

The weight figure shown with each result multiplies gallons by the density of the fuel you select, using values stored in the tool's data file. Diesel #2 and heating oil #2 weigh about 7.1 pounds per gallon, for example. For weight math across every fuel and any volume, the fuel weight calculator goes deeper.

When to use this

Before a fuel delivery. You have a 275 gallon heating oil tank reading 12 inches on the stick. The calculator tells you what those 12 inches mean in gallons, so you can order the right amount instead of guessing and having the driver stop short.

Selling or buying a house with a tank. A buyer's inspector reports an oil tank of unknown size. Measuring the tank's length and diameter takes two minutes, and the calculator turns those measurements into a capacity figure. Capacity also drives removal pricing, so once you know the gallons you can estimate removal costs by state before negotiating.

Facility compliance records. Federal UST rules require owners to notify the implementing agency of each regulated tank (40 CFR 280.22). If a tank's registered capacity looks wrong, computing volume from measured dimensions is the fastest sanity check before you involve anyone else.

Contractor bidding. A removal or installation bid depends on tank size, and the customer often does not know it. A contractor on site with a tape measure can confirm capacity in the driveway. If the project includes a new tank, the directory lists tank installation contractors in Texas and every other state.

Gas station underground tank sizes

Gas station tanks are far larger than most people expect. Retail underground storage tanks are built in standard sizes from about 6,000 to 20,000 gallons, and larger, buried under or beside the forecourt.

The common sizes follow standard dimensions. A steel 6,000 gallon UST is about 8 feet in diameter and 16 feet long. An 8,000 gallon tank keeps the 8 foot diameter and stretches to about 21 feet. The 10,000 gallon size runs about 8 feet by 27 feet. A 12,000 gallon tank is roughly 8 feet by 32 feet. Above that, diameters step up to 10 feet: a 15,000 gallon tank is about 25 and a half feet long, and a 20,000 gallon truck stop tank is about 34 feet long. Fiberglass tanks of the same capacity run about 4 to 5 feet longer overall because of their domed ends: a fiberglass 10,000 gallon tank is about 8 by 31 and a half feet. All of these steel sizes are available as one-click presets in the calculator.

How many tanks a station has depends on volume and product mix. The sizing tool in this calculator uses illustrative tiers rather than survey data: two 6,000 gallon tanks for a small rural station selling around 1,500 gallons a day, three 10,000 gallon tanks at 4,000 gallons a day, four 12,000 gallon tanks at 8,000 gallons a day, and four 20,000 gallon tanks for a truck stop selling 15,000 gallons a day. Real tank choices depend on sales by grade, delivery schedule, site space, and local code.

The throughput sizing tool is built on those same tiers: enter daily gallons sold and it shows the nearest example tank size and count. A companion days-of-supply tool divides tank size by daily throughput.

What this estimate doesn't account for

The calculator computes ideal geometric volume. Real tanks deviate from the ideal in a few specific ways.

Wall thickness is ignored. Dimensions are treated as internal, so if you measured the outside of the tank, actual capacity is slightly less than the result. On a small residential tank the difference is a few gallons. On a large UST it is about 1 percent, roughly 100 gallons on a 10,000 gallon tank with a quarter inch wall.

End caps are assumed flat. Many commercial cylindrical tanks have dished or rounded heads that add volume beyond the straight shell. The preset dimensions are calibrated to nominal capacities, but custom entries can be off in either direction. A steel tank measured along its straight shell reads a little low. A fiberglass tank measured by its outside diameter and overall length can read 10 to 27 percent high because of its domed ends and ribs, so use the chart from the manufacturer for fiberglass tanks.

Internal hardware is invisible to the math. Fill tubes, suction lines, gauge probes, and baffles inside the tank all take up a little space.

Temperature is not corrected. Petroleum products expand and contract with temperature, so warm fuel weighs less per gallon than cold fuel: gasoline grows about 0.07 percent and diesel about 0.05 percent per degree Fahrenheit. What counts is the temperature of the fuel, not of the air, and fuel in a buried tank stays close to ground temperature. The math here is volume only.

Finally, a tilted tank throws off dipstick math. The partial fill formula assumes the tank is level; a tank pitched toward one end reads differently depending on where you drop the stick. For anything feeding a compliance record, federal inventory control rules call for measuring equipment readable to the nearest one-eighth inch and a monthly water check (40 CFR 280.43(a)), and EPA says a tank that is not level needs a corrected tank chart.