Tank Volume Calculator

Free Online Tank Volume Calculator

This tank volume calculator works out exactly how much liquid a storage tank holds, whether it's completely full or only partly filled. It covers the five tank shapes that show up most often in real life — vertical cylindrical tanks, horizontal cylindrical tanks, rectangular tanks, and the rounded-end capsule tanks used for propane and other pressurized liquids. Enter your tank's dimensions once, and you'll get the total capacity plus the current filled volume in US gallons, imperial gallons, liters, cubic meters, cubic feet, and oil barrels.

Most tank capacity calculators online only handle a tank that's completely full, which isn't very useful day to day. This one goes a step further and lets you set a fill level — either by percent or by an actual measured height — so you can figure out how much is really in a tank that's half empty, three-quarters full, or sitting at any level in between. That's the number people actually need when they're checking a fuel tank, planning a delivery, or sizing a new water tank for a property.

Tank Shapes This Calculator Supports

Storage tanks aren't all built the same way, and the shape changes the math quite a bit. This calculator covers:

• Vertical Cylinder — an upright cylindrical tank standing on its flat end, like a typical water tower base or a plastic vertical storage tank.
• Horizontal Cylinder — a cylindrical tank lying on its side, like most septic tanks, underground fuel tanks, and many farm water tanks.
• Rectangular — a box-shaped tank, like an IBC tote, a fish tank, a concrete cistern, or a rectangular water storage tank.
• Vertical Capsule — an upright cylinder with a rounded, dome-shaped cap on the top and bottom.
• Horizontal Capsule — a cylinder lying on its side with rounded ends on both sides, the classic shape used for propane and butane gas tanks.

The Tank Volume Formulas

Each shape uses its own formula for total volume, based on standard solid geometry:

• Vertical Cylinder: Volume = π × Radius² × Height
• Horizontal Cylinder: Volume = π × Radius² × Length
• Rectangular: Volume = Length × Width × Height
• Vertical or Horizontal Capsule: Volume = (π × Radius² × Straight Length) + (4⁄3 × π × Radius³)

Why the Capsule Formula Has Two Parts

A capsule tank is really a straight cylinder with a hemisphere stuck on each end. The straight cylindrical middle section is calculated the normal way — circular area times length. The two rounded end caps are each exactly half a sphere, and since two halves make a whole, they're simply added together as one full sphere using the standard sphere volume formula. That's why the capsule formula looks like a cylinder formula with an extra sphere term tacked on at the end.

Vertical vs Horizontal Tanks — Why It Matters for Partial Volume

If a tank is completely full or completely empty, the shape barely matters — you just need the total capacity. But the moment a tank is partly filled, orientation changes everything. In a vertical cylindrical or rectangular tank, the cross-section is the same no matter how high the liquid sits, so volume rises in a straight line with height — 25% of the height means 25% of the volume.

A horizontal tank doesn't work that way. Picture a cylinder lying on its side: near the bottom and top, the circular cross-section is narrow, and it's widest in the middle. That means the first few inches of liquid going into an empty horizontal tank fill it up more slowly (in percentage terms) than the middle few inches do. This calculator uses the correct circular segment formula for horizontal cylinders, and the matching spherical cap formula for the rounded ends of capsule tanks, so the partial-fill numbers come out accurate instead of just being a rough straight-line guess.

How to Measure Your Tank

A few tips for getting accurate dimensions before you calculate:

• Measure diameter across the widest inside point of the tank, not the outer shell or any insulation wrapped around it.
• For a horizontal tank, diameter and length are swapped from what you might expect — length runs along the axis the tank is lying on, and diameter is the measurement straight across it.
• For a vertical or horizontal capsule tank, measure the straight cylindrical section only, not including the rounded end caps — this calculator adds the caps in automatically using the same diameter.
• Fill height is always measured straight up from the lowest point of the tank, using a dipstick or measuring tape lowered through an access port, not along the curved wall.
• If your tank has legs, a sump, or a sloped bottom, measure from the true lowest interior point for the most accurate fill height reading.

Worked Example: A Horizontal Propane Tank

Take a common 500-gallon horizontal capsule propane tank with a 30-inch diameter and a 96-inch straight cylindral section. The radius is 15 inches, or 1.25 feet. The straight section volume is π × 1.25² × 8 = 39.27 cubic feet, and the two rounded end caps together add a full sphere: 4⁄3 × π × 1.25³ = 8.18 cubic feet. That's a total of 47.45 cubic feet, which converts to roughly 355 US gallons — this calculator would show that as the tank's total capacity. If the tank gauge reads 60% full, the calculator works out the correct curved fill shape rather than simply taking 60% of 355, giving a more realistic number for how much propane is actually left.

Common Tank Sizes and Their Uses

A few tank sizes and shapes that come up often, all of which you can plug straight into this calculator:

• 55-gallon drum — vertical cylinder, common for chemicals, oil, and water storage.
• 275 to 330-gallon IBC tote — rectangular, widely used for bulk liquid transport and rainwater harvesting.
• 275 to 1,000-gallon heating oil tank — usually a horizontal cylinder.
• 500 to 1,000-gallon propane tank — horizontal capsule shape with rounded ends.
• 1,000 to 1,500-gallon septic tank — typically a horizontal cylinder or a rectangular concrete tank buried underground.
• Vertical water storage tanks (500 to 10,000+ gallons) — tall vertical cylinders, common on farms and rural properties.

Converting Between Gallons, Liters, Cubic Meters, and Barrels

Different industries and countries prefer different volume units, so this calculator shows results in several at once. One US gallon equals about 3.785 liters, one imperial (UK) gallon equals about 4.546 liters, one cubic meter equals about 264.17 US gallons, and one oil barrel — the unit used in the petroleum industry — equals exactly 42 US gallons. Having all of these on one screen saves the trouble of running separate unit conversions after you already have a volume figure.

Estimating the Weight of the Liquid Inside

Volume alone doesn't tell you how much a tank weighs, since different liquids have different densities. This calculator includes a liquid weight estimate using typical densities for water, diesel, gasoline, crude oil, milk, and seawater, or you can enter a custom density if you're working with something else, like a chemical solution. This is useful for checking whether a foundation, trailer, or lifting equipment can safely handle a full tank, since a liquid noticeably lighter or heavier than water can change the total weight by a meaningful margin.

Common Uses for a Tank Volume Calculator

People reach for a tank volume calculator for all kinds of practical, everyday reasons — sizing a new water storage tank for a home or farm, checking how much fuel or heating oil is left before ordering a refill, working out the capacity of a septic tank during a home inspection, planning a rainwater harvesting system, calculating dosing volumes for a chemical or agricultural tank, or figuring out how much water a fish tank or aquarium actually holds before adding livestock.

Common Mistakes When Calculating Tank Volume

A few errors show up again and again when people work out tank volume by hand:

• Using a flat percentage of total volume for a horizontal tank's fill level, which overestimates or underestimates the true volume because the cross-section isn't uniform.
• Mixing up radius and diameter, which doubles the error once it gets squared in the formula.
• Measuring the outside of the tank shell or insulation instead of the true inside dimension.
• Forgetting to include the rounded end caps on a capsule-shaped tank, which noticeably understates the total capacity.
• Measuring fill height along a sloped or curved wall instead of straight up from the lowest point.

Limitations of This Calculator

This calculator gives an accurate estimate for standard, regularly shaped tanks — it assumes a true cylindrical, rectangular, or capsule shape with a level installation. Tanks with internal baffles, sloped or coned bottoms, irregular dents, or unusual custom shapes will hold a slightly different volume than the formula predicts. The liquid density figures used for the weight estimate are typical averages and can vary by temperature, concentration, and grade, so treat that number as a planning estimate rather than an exact reading. For safety-critical uses, such as checking a tank's structural load rating, always confirm figures against the manufacturer's specification sheet or a professional measurement.