Aquarium unit converter
Gallons ↔ litres, inches ↔ cm, °F ↔ °C, dGH ↔ ppm — the four conversions every fishkeeper ends up needing.
26.4 US gal
23.6 in
77 °F
142.8 ppm
Most aquarium advice on the internet was written by someone using different units from you, and two of the conversions have traps in them. The gallon is the worst: a US gallon is 3.785 litres and an imperial gallon is 4.546, a 20% gap, so a British article recommending "one gallon per inch of fish" and an American one saying the same sentence do not mean the same thing at all. Hardness is the other. Degrees of general hardness (°dGH) and parts per million both describe dissolved calcium and magnesium, and 1 °dGH is 17.848 ppm as CaCO₃ — so a test kit reading 8 °dGH and a water report quoting 143 ppm are the same water, and a keeper comparing them without converting concludes their supply changed overnight. Temperature is the familiar one and still catches people out at the low end, where the two scales run in opposite directions from anything intuitive: 25 °C is 77 °F, but a 2-degree drop in Celsius is a 3.6-degree drop in Fahrenheit, which is why "keep it within a couple of degrees" means something stricter in one language than the other. This converter does the four that matter for a tank, and nothing else.
Common questions
- How many ppm is one degree of hardness (dGH)?
- 1 °dGH is about 17.85 ppm (mg/L as calcium carbonate equivalent). A liquid test reading of 8 °dGH therefore matches roughly 143 ppm on a strip that reports ppm.
- Are US and UK gallons the same?
- No — a US gallon is 3.785 litres, a UK gallon 4.546 litres. Aquarium sizes quoted online are almost always US gallons.
- Is dGH the same as total hardness on my water report?
- Usually yes, but check the unit. Water companies mostly publish total hardness as mg/l or ppm of CaCO₃, which converts at 17.848 ppm per °dGH. Some publish French degrees (°fH, 10 ppm each) or English degrees (°e, 14.25 ppm each), and a few report calcium alone rather than calcium plus magnesium — that last one reads low and will make soft-water species look safer than they are.