Conversion factors used
These are exact definitions or standard values.
| Convert | Multiply by |
|---|---|
| US gallons → litres | 3.785 |
| UK (imperial) gallons → litres | 4.546 |
| US gallons → UK gallons | 0.833 |
| Cubic inches → US gallons | divide by 231 |
| Cubic feet → US gallons | 7.48 |
| Inches → centimetres | 2.54 |
| Pounds → kilograms | 0.4536 |
| US GPH → litres per hour | 3.785 |
| °dH (dGH or dKH) → ppm as CaCO₃ | 17.85 |
| °dKH → meq/L alkalinity | divide by 2.8 |
Temperature is the one conversion that is not a simple multiplication:
US gallons and UK gallons are different
A UK (imperial) gallon is 4.546 litres; a US gallon is 3.785 litres. The UK gallon is 20% larger. American tank sizes ("55 gallon") are US gallons. If you follow a dose written "per gallon", check which country the product comes from. When in doubt, convert to litres, which mean the same everywhere.
Water hardness units
Test kits report hardness in two ways: German degrees (°dH, written dGH for general hardness and dKH for carbonate hardness) or parts per million of calcium carbonate (ppm CaCO₃, the same as mg/L). One degree equals 17.85 ppm.
| Description | dGH | ppm |
|---|---|---|
| Very soft | 0–4 | 0–70 |
| Soft | 4–8 | 70–140 |
| Medium | 8–12 | 140–210 |
| Hard | 12–18 | 210–320 |
| Very hard | over 18 | over 320 |
Handy temperatures
| °F | 68 | 72 | 75 | 78 | 80 | 82 | 86 |
|---|---|---|---|---|---|---|---|
| °C | 20 | 22.2 | 23.9 | 25.6 | 26.7 | 27.8 | 30 |
Common mistakes
- Treating US and UK gallons as the same. The difference is 20%.
- Converting a temperature difference like a temperature. A rise of 10 °F is a rise of 5.6 °C, not −12 °C: for differences, just multiply by 5 ÷ 9.
- Confusing GH and KH. They use the same units but measure different things: GH is calcium and magnesium, KH is carbonate buffering.
Questions and answers
How many litres is a 55 gallon fish tank?
55 US gallons is 208 litres. In UK gallons, 208 litres is 45.8.
How many gallons is 100 litres?
100 litres is 26.4 US gallons, or 22 UK gallons.
How do I convert dGH to ppm?
Multiply degrees of hardness by 17.85. For example 8 dGH is about 143 ppm. The same factor converts dKH to ppm.
What is 78 °F in Celsius?
78 °F is 25.6 °C, a typical temperature for tropical community fish.
How do I convert GPH to LPH?
Multiply US gallons per hour by 3.785 to get litres per hour. 300 GPH is about 1,136 L/h.
Sources
- NIST, unit conversion factors (1 US gallon = 231 cubic inches = 3.785 L)
- 1 °dH = 17.848 mg/L as CaCO₃ (10 mg/L CaO)