Aquarium unit converter

How the conversions work

Every conversion except temperature is a single multiplication: multiply one way, divide the other. Temperature also needs an offset of 32, because the Fahrenheit and Celsius scales start at different points.

• Volume: litres = US gallons × 3.785411784 (a US gallon is defined as 231 cubic inches). Litres = UK gallons × 4.54609. One UK gallon is 1.20095 US gallons.
• Temperature: °C = (°F − 32) ÷ 1.8, and °F = °C × 1.8 + 32.
• Length: centimetres = inches × 2.54.
• Hardness: ppm = dGH (or dKH) × 17.848. Grains per gallon = ppm ÷ 17.118.
• Flow: L/h = GPH × 3.785411784.
• Weight: kilograms = pounds × 0.45359237.

The volume, temperature, length and weight factors are exact by definition. The converter rounds results to one or two decimal places so they are easy to read.

Worked example: a 29 gallon tropical tank

Say you have a 29 gallon tank, the heater is set to 78°F, and your GH test reads 8 dGH.

• Volume: 29 × 3.785411784 = 109.78 litres. Divide by 4.54609 and you get 24.15 UK gallons.
• Temperature: (78 − 32) ÷ 1.8 = 46 ÷ 1.8 = 25.6°C.
• Hardness: 8 × 17.848 = 142.8 ppm, which the USGS scale calls hard water (121–180 ppm).

Why 1 dGH equals 17.848 ppm

One German degree of hardness (°dH) is defined as 10 milligrams of calcium oxide (CaO) per litre. Hardness in ppm is usually reported as if it were all calcium carbonate (CaCO₃). A mole of CaCO₃ weighs 100.09 g and a mole of CaO weighs 56.08 g, so 10 mg/L of CaO matches 10 × 100.09 ÷ 56.08 = 17.848 mg/L of CaCO₃. In fresh water, 1 mg/L is the same as 1 ppm.

The same factor works for GH (general hardness, mostly calcium and magnesium) and KH (carbonate hardness), because both are expressed as calcium carbonate. Some test kits round the factor to 17.8 or 17.9. That changes the answer by less than 0.3%, far less than the one-drop step of a drop test.

US water reports sometimes give hardness in grains per gallon (gpg). One grain per US gallon is 17.118 ppm, so it is close to one German degree but not the same: 1 dGH is 1.04 gpg.

Tips for getting the units right

• Check which gallon. Tanks sold in the US are measured in US gallons. UK and European tanks are usually labelled in litres, but older UK books and forum posts use UK gallons, which are about 20% bigger. A 50 UK gallon tank holds 227 litres, or 60 US gallons.
• Dose by the unit on the label. Water conditioner, fertiliser and medicine doses are given per US gallon, per litre or sometimes per UK gallon. Mixing up US and UK gallons puts the dose off by about 20%.
• Your tank holds less than its label. A standard 55 gallon tank measures 48 × 13 × 21 inches on the outside: 13,104 cubic inches, or 56.7 US gallons. Glass, substrate, rock and the gap below the rim mean the real water volume is lower. Measure the inside and use the aquarium volume calculator for a closer figure.
• Convert temperature changes differently. A change of 1°C is a change of 1.8°F, so raising the heater by 2°C is a 3.6°F rise. Don't add 32 when you convert a difference.
• Match hardness units before comparing. Our fish profiles list GH in dGH. If your test strip reads in ppm, divide by 17.848 first.
• Treat filter ratings as a maximum. Flow ratings are usually measured with an empty filter and no height to pump against, so real flow is lower. The filter flow calculator shows what your tank needs.
• Water is heavy. A US gallon of fresh water weighs about 8.34 lb, and a litre weighs about 1 kg. A 20 gallon tank holds about 167 lb (76 kg) of water before glass, gravel and rock. Check your stand with the tank weight calculator.

Related calculators

• Aquarium volume calculator: Work out how many gallons or litres your tank holds, for rectangular, cylinder, bow front, corner and hexagon tanks.
• Aquarium weight calculator: Estimate the full weight of your aquarium with water, glass, substrate and rock, so you can check your stand and floor.
• Aquarium gravel and sand calculator: Find how many pounds or kilograms of gravel, sand or aquasoil you need for the depth you want.
• Aquarium heater size calculator: Find the right heater wattage for your tank size, room temperature and target water temperature.
• Aquarium filter flow rate calculator: See what filter flow rate (GPH or L/h) your tank needs, and check whether your current filter is enough.
• Water change and nitrate calculator: See how much a water change lowers nitrate, and how many changes it takes to reach a target.
• Aquarium CO2 calculator (pH and KH): Estimate dissolved CO2 in a planted tank from your pH and KH readings.
• Fish tank stocking calculator: check how many fish your tank can hold, and whether they get along.

See every tool on the calculators page, compare common tank sizes in fish by tank size, or read our fishkeeping guides.

Sources

1. NIST Handbook 44 (2026), Appendix C: General tables of units of measurement. US gallon = 231 cubic inches; inch = 2.54 cm; pound = 0.45359237 kg; grain = 64.79891 mg.
2. NIST Special Publication 811, Appendix B.8: Factors for units listed alphabetically. UK (imperial) gallon = 4.54609 L; °C = (°F − 32) ÷ 1.8.
3. U.S. Geological Survey, Hardness of water. Soft 0–60, moderately hard 61–120, hard 121–180 and very hard over 180 mg/L as calcium carbonate.
4. Wikipedia, Hardness of water. German degree = 10 mg/L CaO = 17.848 ppm; 1 grain per US gallon = 17.118 ppm.