Fish Tank Gallon Calculator: Volume, Stocking and Substrate Math

Getting the maths right before filling a tank saves water, money and frustration. Whether you are calculating how many litres your aquarium actually holds, how much substrate to buy or how many fish it can support, accurate volume figures are the starting point. This fish tank gallon calculator guide from Gensou Aquascaping Singapore — with over 20 years of hands-on experience at 5 Everton Park — gives you the formulas and practical shortcuts you need.

Rectangular Tank Volume

Multiply length x width x height in centimetres, then divide by 1,000 to get litres. A tank measuring 60 x 30 x 36 cm holds 64.8 litres gross. To convert to US gallons, divide litres by 3.785 — that same tank is about 17.1 gallons. Simple, but many hobbyists forget to account for displacement.

Substrate, rocks and driftwood typically reduce actual water volume by 8–15 %. A 4 cm substrate bed across a 60 x 30 cm footprint displaces roughly 7.2 litres alone. Always use the net water volume — not the gross figure — when calculating stocking density and medication doses.

Cylindrical and Bowfront Tanks

For a cylinder, use the formula: pi x radius squared x height, all in centimetres, divided by 1,000. A cylindrical tank 30 cm in diameter and 40 cm tall holds about 28.3 litres. Bowfront tanks are trickier — measure the deepest point of the bow as the width and multiply by 0.8 to approximate the average width, then calculate as a rectangle.

Cube Tanks

Cubes are simply length cubed divided by 1,000. A 30 cm cube holds 27 litres; a 45 cm cube holds 91.1 litres. Cube tanks are popular in Singapore for desktop aquascapes and shrimp colonies — their equal proportions create a pleasing visual balance.

Stocking Calculations

The old “one inch of fish per gallon” rule is a rough starting point but ignores fish body mass, waste output and activity level. A better approach: aim for 1 cm of slim-bodied adult fish per 2 litres of net water volume. Adjust downward for heavy-bodied species like goldfish or plecos and upward for minimal-waste species like small rasboras.

A 60-litre net-volume tank comfortably supports around 30 cm of total adult fish length — for example, ten neon tetras at 3 cm each. Always research individual species bioload rather than relying on a single formula. Our community tank compatibility guide goes deeper on pairing species by temperament and waste output.

Substrate Volume and Weight

Multiply tank length x width x desired depth in centimetres, then divide by 1,000 for litres. For weight, most aquarium soils weigh roughly 1–1.2 kg per litre dry. Gravel and sand run heavier at 1.5–1.8 kg per litre. A 60 x 30 cm tank with a 4 cm bed needs about 7.2 litres of substrate — approximately 7–9 kg of soil or 11–13 kg of sand.

Buy an extra 10–15 % to account for sloping the substrate toward the back, which adds depth and perspective to your aquascape.

Water Change Volume

Knowing your net water volume makes water changes precise. A 25 % change on 55 litres of actual water means removing and replacing about 14 litres — easily managed with two standard buckets. In Singapore, dechloraminate with a product that handles chloramine and match the replacement water to tank temperature before pouring.

Medication Dosing

Most aquarium medications dose per volume of water. Using gross tank volume instead of net leads to overdosing, which can stress or kill sensitive species like shrimp and scaleless fish. Subtract substrate and hardscape displacement for an accurate figure. When in doubt, dose slightly under rather than over — you can always add more, but you cannot easily remove excess medication. This fish tank gallon calculator guide ensures every number you plug in is reliable.

Related Reading

• Fish Tank Dimensions and Weight Chart: Every Standard Size
• Best Substrate Calculator for Aquariums: Depth, Volume and Weight
• Best 10-Gallon Fish Tank Setup Guide: Stocking, Filter and Plants
• Best 20-Gallon Fish Tank Setup Guide: Community Tank Blueprint
• Freshwater Aquarium Setup for Beginners: Step-by-Step First Tank