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Aquarium Gallons Calculator: Volume in US or Imperial

Card No. 06·65355 · Updated · Standard formula, shown below

The calculator on this card is switched off while it is checked. The formula is below, and the worked example shows how to do the sum by hand.

This aquarium gallons calculator works out the water volume of a rectangular, square or bow-front tank in US gallons, imperial gallons and litres. You enter the tank’s dimensions in inches or centimetres, choose the shape, and the calculator returns the total volume and the actual water capacity when you leave the usual 1–2 inches of air space at the top. The first result is the total volume in US gallons, followed by the water volume, imperial gallons and litres.

Formula

The calculator uses the standard geometric volume formula for a rectangular prism, then converts cubic inches to gallons. For a rectangular tank:

Volume (US gal) = (Length × Width × Height) / 231

For a bow-front tank, the volume is approximated as a rectangular tank with half the bow depth added to the width:

Volume (US gal) = Length × (Width + Bow Depth / 2) × Height / 231

Where:

  • Length – the longest horizontal dimension of the tank, in inches.
  • Width – the front-to-back dimension at the narrowest point (the flat back), in inches.
  • Height – the vertical dimension from bottom to top, in inches.
  • Bow Depth – the extra distance the curved front extends beyond the back, in inches.
  • Water Depth – the actual water height, usually 1–2 inches less than the tank height, in inches.
  • 231 – the number of cubic inches in one US gallon.
  • 277.42 – the number of cubic inches in one imperial gallon.
  • 0.0163871 – the number of litres in one cubic inch.

If you measure in centimetres, convert to inches first (1 cm = 0.3937 in) or use the conversion factor 3785.41 cm³ per US gallon.

Worked Example

Using the default values: a rectangular tank, 48 inches long, 13 inches wide, 20 inches tall, with water filled to 18 inches.

  1. Total volume in cubic inches: 48 × 13 × 20 = 12,480 in³.
  2. Convert to US gallons: 12,480 ÷ 231 = 54.0 US gal.
  3. Water volume: 48 × 13 × 18 = 11,232 in³; 11,232 ÷ 231 = 48.6 US gal.
  4. Imperial gallons: 12,480 ÷ 277.42 = 45.0 imp gal.
  5. Litres: 12,480 × 0.0163871 = 204.5 L.

The calculator rounds the first answer to one decimal place, so it shows 54.0 US gallons total and 48.6 US gallons of water.

When to Use This Calculator

Use this calculator when you need to know how much water a fish tank holds for stocking decisions, medication dosing, or choosing a heater and filter. It is also useful for estimating the weight of a filled tank (water weighs about 8.34 lb per US gallon) and for planning water changes.

For a standard rectangular aquarium, the calculation is straightforward. For bow-front or other curved tanks, the formula gives a good approximation but not an exact volume because the curve is not a perfect half-cylinder. If you need the exact volume of an irregularly shaped tank, measure the water you pour in with a bucket or use a flow meter.

How to Read the Results

The first number is the total volume of the tank when filled to the brim. The second number is the actual water volume when you leave the recommended air gap at the top. Most fish keepers use the water volume for stocking and maintenance because the tank is never filled completely.

The imperial gallon is the UK gallon, which is larger than the US gallon (1 imperial gallon = 1.20095 US gallons). If you are outside the US, check which gallon your equipment or literature uses.

Common Mistakes and How to Avoid Them

  • Mixing up US and imperial gallons – The difference is about 20%, which can lead to overdosing medication. Always confirm which gallon your product uses.
  • Using the tank height instead of water depth – A 20-inch tall tank with water at 18 inches holds 10% less water than the full volume. Use the actual water height.
  • Measuring outside dimensions – Glass thickness adds to the outside measurement. For accurate volume, measure the inside dimensions or subtract the glass thickness (usually 0.25–0.5 inches) from each side.
  • Forgetting the bow front – A bow-front tank is wider in the middle than at the back. The formula uses the average width, which is a good approximation.
  • Rounding too early – If you round each dimension to the nearest inch, a 48.5-inch tank becomes 48, losing about 1% of the volume. Keep the precise measurements until the final step.

Limitations of the Formula

This formula assumes the tank is a perfect rectangular prism or a rectangular prism with a half-cylinder bow. It does not account for:

  • Glass thickness – The volume is calculated from outside dimensions, so the actual water capacity is slightly less.
  • Displacement by decorations and substrate – Gravel, rocks, and ornaments reduce the water volume. A general rule is to subtract 10–15% for substrate and decor.
  • Water level changes – Evaporation and water changes alter the actual volume over time.
  • Non-standard shapes – Hexagonal, cylindrical, or custom tanks require different formulas.

For most practical purposes, the calculated volume is accurate within 5% of the true water capacity, which is acceptable for stocking and equipment sizing. For medication, always follow the product label and use the water volume, not the total volume.

Frequently Asked Questions

How many gallons is a 48x13x20 tank?
Using the default values, the total volume is 54.0 US gallons, and the water volume with 2 inches of air space is 48.6 US gallons.

How do I calculate gallons if my tank is in centimetres?
Convert each dimension to inches by multiplying by 0.3937, then use the same formula. Alternatively, use the conversion factor 1 US gallon = 3785.41 cubic centimetres.

What is the difference between US and imperial gallons?
A US gallon is 231 cubic inches (3.785 L), while an imperial gallon is 277.42 cubic inches (4.546 L). The imperial gallon is about 20% larger.

Does the calculator account for gravel and decorations?
No. The calculator gives the empty tank volume. Subtract roughly 10–15% for substrate and decor, or measure the actual water you add.

Why is my real tank volume less than the calculated volume?
Because the tank is not filled to the brim, glass takes up space, and decorations displace water. The calculator’s water volume field accounts for the air gap, but not for glass thickness or decor.

Can I use this for a cylindrical tank?
No. A cylindrical tank uses the formula π × radius² × height. This calculator is for rectangular and bow-front tanks only.

Related

Formula used
shape | Tank shape | Rectangular=1, Bow front=0.5 | 1
length | Length | in | 48 | 0
width | Width | in | 13 | 0
height | Height | in | 20 | 0
bow_in | Bow depth (front to back) | in | 4 | 0
water_h | Water depth (leave 1-2 in air) | in | 18 | 0
out total_us | Total volume | US gal | length * width * height * (shape + (1-shape)*0.5) / 231 | 1
out water_us | Water volume | US gal | length * width * water_h * (shape + (1-shape)*0.5) / 231 | 1
out total_imp | Total volume | imp gal | length * width * height * (shape + (1-shape)*0.5) / 277.42 | 1
out litres | Total volume | L | length * width * height * (shape + (1-shape)*0.5) * 0.0163871 | 1

Each line is one input (in, sel, list) or one result (out: label, unit, formula, decimals). How the calculators work

Method: the standard formula for this calculation, rounded as each result states. See how the calculators work or report an error.

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