How do I calculate aquarium volume accurately?

Multiply the internal length by width by height and divide by 1000 for liters (cm) or by 231 for gallons (inches). Usable water volume is typically 85-90% of gross, accounting for substrate depth and the air gap at the surface.

Aquarium Volume Calculator — Liters & Gallons

Q: Should I subtract substrate depth from the height measurement?

For gross volume used in dosing calculations, use the full internal height. For actual water volume in a running setup, subtract substrate depth from the height — a 45 cm tank with 5 cm substrate uses 40 cm in the calculation.

Q: How many liters is a standard 55-gallon aquarium?

A standard 55-gallon aquarium (122x33x51 cm) holds approximately 205 liters gross. With 5-7 cm of substrate, usable water volume is closer to 175-185 liters.

Q: What is the difference between gross and usable aquarium volume?

Gross volume is the total geometric capacity of the tank filled to the rim. Usable volume is the actual water volume in a running setup — typically 85% of gross, accounting for substrate depth, hardscape, equipment displacement, and the air gap kept at the surface for gas exchange.

Quick Answer

For a rectangular tank in centimetres: multiply Length × Width × Height and divide by 1,000 to get liters. Divide liters by 3.785 for gallons. Usable water volume is typically 85% of gross — accounting for substrate depth and the air gap at the surface.

Calculate Tank Volume

Length (cm)

Internal dimension

Width (cm)

Front to back

Height (cm)

Bottom to rim

Please enter valid dimensions greater than 0 in all three fields.

Gross Volume— LTotal capacity

Gross Volume— galTotal capacity

Usable (~85%)— LAfter substrate & gap

Usable (~85%)— galAfter substrate & gap

§ 01 · Reference

Standard Tank Sizes at a Glance

Common Name	Dimensions (L×W×H)	Gross Volume	Usable (~85%)
Loading reference data…

§ 02 · Method

How the Calculation Works

Volume (L) = Length × Width × Height (cm) ÷ 1,000 Volume (gal) = Length × Width × Height (in) ÷ 231

Both formulas return gross volume — the total capacity of the tank filled to the rim. Usable water volume is typically 85% of gross, accounting for substrate (5–8 cm in most planted setups), hardscape, equipment displacement, and the air gap at the surface. Use gross volume for CO2, fertilizer and medication dosing. Use usable volume for stocking calculations and water change percentages.

PlantedGuide · Recommended Accessories Essential gear for your tank

🧪 API Master Test Kit Test ammonia, nitrite, nitrate and pH — parameters you dose by tank volume. See on Amazon → 🚿 Python No Spill Gravel Vacuum Plan water change volume from your gross figure, then remove it cleanly — no buckets. See on Amazon → 💧 Seachem Prime Dosed by volume — Prime treats 100L per 5ml. Your gross tank figure sets the dose exactly. See on Amazon → 🌡️ Digital Aquarium Thermometer Monitor temperature across the full water column — increasingly relevant as volume grows. See on Amazon →

§ 03 · FAQ

Common Questions

Multiply the internal length by width by height and divide by 1000 for liters (cm), or by 231 for gallons (inches). Usable volume is typically 85–90% of gross, accounting for substrate, hardscape, and the air gap. For planted tanks, measuring internal dimensions gives a more accurate figure than converting from the label size.

For gross volume used in dosing calculations, use full internal height. For actual water volume in a running setup, subtract substrate depth. A 45 cm tank with 5 cm of substrate uses 40 cm in the calculation. This calculator shows both gross and estimated usable volume at 85%.

Volume is the basis for CO2 injection rate, EI fertilizer doses (grams per 100L per week), heater wattage (1–3W per liter), water conditioner dosing, and medication volumes. Knowing your exact volume prevents under- and over-dosing — both cause problems in planted setups.

A standard 55-gallon aquarium (122 × 33 × 51 cm) holds approximately 205 liters gross. With 5–7 cm of substrate, usable water volume is closer to 175–185 liters. Tank labels use nominal sizes that round down, so measuring and calculating directly gives a more precise figure.

Gross volume is the total geometric capacity filled to the rim. Usable volume is the actual water volume in a running setup — typically 85% of gross, accounting for substrate depth, hardscape, equipment displacement, and the air gap at the surface kept for gas exchange and overflow prevention.

🌿

The Planted Tank Blueprint Free 11-page guide — substrate, lighting, CO₂ and first plants