Rectangular Tank Volume Calculator in Gallons

This calculator works out the volume of a rectangular tank (also called a rectangular prism or box) in US gallons. You enter the length, width, and height of the tank in inches, feet, centimeters, or meters, and it returns the volume in gallons, along with the volume in cubic feet and liters for reference. The calculation uses the standard formula for the volume of a rectangular prism, then converts the cubic result to gallons using the exact US gallon conversion factor.

Formula

The volume of a rectangular tank is found by multiplying its three internal dimensions:

V = L × W × H

Where:

• V = volume in cubic inches (in³)
• L = internal length of the tank (in inches)
• W = internal width of the tank (in inches)
• H = internal height of the tank (in inches)

To convert cubic inches to US gallons, divide by 231 (because 1 US gallon = 231 cubic inches exactly). To convert cubic inches to liters, multiply by 0.016387064 (the exact conversion factor). To convert cubic inches to cubic feet, divide by 1728 (12³).

Worked Example

Using the default values from the calculator above — length 48 inches, width 24 inches, height 18 inches:

1. Multiply the dimensions: 48 × 24 × 18 = 20,736 cubic inches
2. Convert to gallons: 20,736 ÷ 231 = 89.77 US gallons (rounded to two decimals)
3. Convert to cubic feet: 20,736 ÷ 1728 = 12.00 ft³
4. Convert to liters: 20,736 × 0.016387064 = 339.80 liters

So a tank that is 48 inches long, 24 inches wide, and 18 inches high holds about 89.77 US gallons when filled to the brim. The calculator returns 89.77 gallons for these inputs, matching the manual calculation.

Why Use Internal Dimensions?

The most common mistake when measuring a tank is using the outside dimensions. The walls of a tank take up space, so the actual water capacity is always less than what the outside measurements suggest. For example, a glass aquarium with 0.5-inch thick walls will lose about 1 inch in each dimension (0.5 inch on each side). For a 48×24×18 tank, that reduces the volume from 89.77 gallons to about 80.42 gallons — a difference of over 9 gallons. Always measure the inside from wall to wall, not the outside.

If you are measuring a tank that is already filled, you can also measure the water depth directly. The volume of water is simply the footprint area (length × width) times the water depth, converted to gallons. This is especially useful for irregular tanks where the bottom is not flat.

US Gallons vs. Imperial Gallons

There are two different “gallons” in common use, and confusing them can throw off your calculations by about 20%. The US gallon equals 231 cubic inches, while the imperial gallon (used in the UK and some Commonwealth countries) equals 277.42 cubic inches. This calculator returns US gallons. If you need imperial gallons, divide the cubic inches by 277.42 instead of 231. For the worked example above, the same tank would hold 74.75 imperial gallons — a significant difference when you are dosing chemicals or calculating weight.

Similarly, a US fluid ounce is about 29.57 mL, while an imperial fluid ounce is 28.41 mL. When a product label says “gallon,” check which one it means, especially for industrial chemicals or automotive fluids.

Common Mistakes and How to Avoid Them

• Mixing units: Always convert all dimensions to the same unit before multiplying. If you measure length in feet and width in inches, the result will be wrong. Use the calculator’s unit dropdowns to keep everything consistent.
• Using outside dimensions: As noted above, always measure the inside of the tank. If you only have outside dimensions, subtract twice the wall thickness from each dimension.
• Forgetting the tank isn’t filled to the top: Most tanks have a freeboard (empty space at the top) to prevent overflow. If you want the usable volume, measure the depth of water you actually plan to use, not the full tank height.
• Rounding too early: If you round the cubic inches to a whole number before dividing by 231, you can lose accuracy. Keep full precision until the final step.

What the Result Tells You

The volume in gallons is the maximum capacity of the tank when filled completely. For practical purposes, you will rarely fill it that high. Here are some common uses for the number:

• Aquarium stocking: Fish stocking guidelines are usually given in gallons per inch of fish. Knowing your tank’s volume helps you avoid overstocking.
• Water treatment dosing: Many water conditioners, medications, and fertilizers specify dosage per gallon. An accurate volume prevents under- or over-dosing.
• Weight calculations: Fresh water weighs about 8.34 pounds per US gallon. A 90-gallon tank weighs about 750 pounds when full — important for floor support.
• Heater sizing: A common rule is 3–5 watts per gallon for aquarium heaters. A 90-gallon tank would need a 270–450 watt heater.

Remember that the volume calculation assumes a perfectly rectangular shape. If your tank has rounded corners, a sloped bottom, or internal overflow boxes, the actual volume will be slightly less. For critical applications, measure the water you add rather than relying solely on the formula.

Alternatives to a Rectangular Tank

If your tank is not rectangular, the formula changes. A cylindrical tank uses π × r² × h. A hexagonal tank can be broken into triangles or rectangles. For oddly shaped tanks, the most reliable method is to fill it with a known volume of water (using a bucket or flow meter) and record the amount. This is especially common for custom acrylic tanks with curved fronts, where the rectangular formula overestimates volume by 10–20%.

For a rectangular tank with a sloped bottom (common in some industrial settling tanks), you can calculate the volume as a rectangular prism plus a triangular prism. Measure the depth at the shallow end and the deep end, then use the average depth in the formula. This gives a good approximation if the slope is uniform.

Frequently Asked Questions

How many gallons is a 48×24×18 tank?

Using the formula: 48 × 24 × 18 = 20,736 cubic inches. Divide by 231 to get 89.77 US gallons. This is a common “90-gallon” aquarium size, though actual capacity may be slightly less due to glass thickness and trim.

What is the difference between US and imperial gallons for tanks?

A US gallon is 231 cubic inches, while an imperial gallon is 277.42 cubic inches. Imperial gallons are about 20% larger. If you live in the UK or Canada, check which unit your tank’s specifications use. This calculator returns US gallons by default.

How do I convert cubic feet to gallons?

One cubic foot equals 7.48052 US gallons. To convert, multiply the cubic feet by 7.48052. For example, 12 cubic feet × 7.48052 = 89.77 gallons. This is useful when your tank dimensions are in feet and you want gallons directly.

Should I measure the inside or outside of the tank?

Always measure the inside for water capacity. Outside dimensions include the wall thickness, which reduces the actual volume. For a glass tank with 0.5-inch walls, subtract 1 inch from each dimension (0.5 inch per side) to get the internal measurements.

How much does a full rectangular tank weigh?

Fresh water weighs about 8.34 pounds per US gallon. Multiply the gallon volume by 8.34 to get the water weight, then add the empty tank weight. For a 90-gallon tank, the water alone weighs about 750 pounds. Saltwater is slightly heavier, about 8.56 pounds per gallon.

Can I use this formula for a tank with a rounded bottom?

No, the rectangular formula assumes flat, perpendicular sides. For a rounded or sloped bottom, you need to use the average depth or break the shape into simpler geometric forms. The most accurate method is to measure the actual water volume by filling the tank with a known quantity.

Related

• Carpet Area Calculator: Sq Ft Formula
• Roof Slope to Angle Calculator: Degrees & Ratio
• Volume Rectangular Prism Calculator

in l | Length | in | 48 | 0
in w | Width | in | 24 | 0
in h | Height | in | 18 | 0
out gal | Volume (US gallons) | gal | (l * w * h) / 231 | 2
out ft3 | Volume (cubic feet) | ft³ | (l * w * h) / 1728 | 2
out ltr | Volume (liters) | L | (l * w * h) * 0.016387064 | 2