Aquarium Unit Converter
Convert the units that actually matter in fishkeeping: volume in US gallons, UK gallons, and liters, temperature in Fahrenheit and Celsius, water hardness in degrees and ppm, length in inches and centimeters, and reef salinity. Everything updates live as you type.
Volume
US gallons, UK (imperial) gallons, and liters. Type in any box.
Temperature
Most tropical tanks sit around 76 to 80 F (24 to 27 C).
Carbonate hardness (KH)
Your pH buffer. 1 dKH = 17.86 ppm calcium carbonate.
General hardness (GH)
Dissolved calcium and magnesium. 1 dGH = 17.86 ppm.
Length
Handy for tank dimensions and adult fish sizes. 1 in = 2.54 cm.
Salinity (saltwater)
Approximate, referenced near 77 F. Reef target is about SG 1.026, roughly 35 ppt. Confirm with a refractometer.
For accurate salinity, a hand refractometer beats a floating hydrometer and barely costs more.
Why Aquarium Unit Conversions Trip Everyone Up
Fishkeeping is a global hobby, and the gear, fish profiles, and care guides you rely on come from every corner of it. A heater is rated in US gallons, a care sheet lists a fish at 15 centimeters, a test kit reports hardness in ppm while your buffer product talks in degrees, and a European forum thread quotes liters. Getting these conversions wrong is not just annoying, it can cost fish. Dose to the wrong volume and you overdose. Buy a light for the wrong tank length and it does not fit. This page keeps the conversions that actually come up in day-to-day fishkeeping in one place, and every box updates live as you type so you never have to reach for a separate calculator.
US Gallons, UK Gallons, and Liters
The single most common mix-up is the gallon itself. A US gallon is 3.78541 liters, while a UK or imperial gallon is 4.54609 liters, which makes the UK gallon about 20 percent larger. If a care guide from across the Atlantic recommends a 20 gallon tank, that is roughly 24 US gallons, a meaningful difference when you are sizing a filter or planning stocking. In the United States almost all aquarium equipment is rated in US gallons, so unless a source clearly says otherwise, assume US gallons. When in doubt, convert everything to liters first, since liters are unambiguous, then work back to whichever gallon your gear uses.
Temperature: Small Numbers, Big Consequences
Most tropical community fish are comfortable around 76 to 80 F, which is about 24 to 27 C. The conversion is straightforward: Celsius equals Fahrenheit minus 32, times five, divided by nine. What matters more than the exact number is stability. Fish handle a steady temperature at the edge of their range far better than one that bounces around. Use the converter to translate a care sheet into your thermometer's units, set your heater, and then leave it alone. Sudden temperature changes, especially during water changes, are a leading trigger for stress and ich, so match new water to the tank before adding it.
Hardness: KH Versus GH, Degrees Versus ppm
Water hardness confuses people for two reasons. First, there are two kinds. General hardness, GH, measures dissolved calcium and magnesium, which fish and invertebrates need for healthy biology. Carbonate hardness, KH, measures the water's buffering capacity, which keeps pH from crashing. They are independent, so a tank can be soft in one and hard in the other. Second, they get reported in two unit systems. One German degree, written dGH or dKH, equals about 17.86 ppm of calcium carbonate. So 8 dKH is roughly 143 ppm. Rather than memorize that, drop your reading into the converter above and read the other unit instantly. Aim for a stable KH so your pH stays put, and match GH to the species you keep.
Length and Salinity
Length conversion is simple but constantly useful: one inch is 2.54 centimeters. It helps when a tank is advertised in centimeters, when a light is sized for a 90 cm aquarium, or when a fish profile lists an adult size that decides how big a tank it needs. Salinity is the trickiest of the group. The reef standard is a specific gravity around 1.026, which works out to roughly 35 ppt of salt. Our conversion uses ppt equal to specific gravity minus one, times 1281, referenced near 77 F, and it is deliberately labeled approximate because specific gravity shifts with temperature. It is perfect for planning a water change or a top-off, but for the precision a reef tank demands, read your salinity with a calibrated refractometer rather than a swing-arm hydrometer. Once you have your numbers, head back to our aquarium volume calculator or browse all of our aquarium calculators to size the rest of your setup.
Keep going: put these numbers to work.
Frequently Asked Questions
What is the difference between US gallons and UK gallons?
A US gallon is 3.78541 liters and a UK, or imperial, gallon is 4.54609 liters, so a UK gallon is about 20 percent larger. This trips up a lot of aquarists buying gear or reading care guides across regions. A 10 US gallon tank holds about 8.3 UK gallons of the same water. Most aquarium products in the United States are rated in US gallons, so always check which gallon a label means before you size a filter, heater, or dose.
How do I convert dKH to ppm?
One degree of carbonate hardness, written dKH, equals about 17.86 ppm of calcium carbonate. So a reading of 4 dKH is roughly 71 ppm and 8 dKH is about 143 ppm. Test kits and devices report KH in different units depending on the brand, which is why this converter is handy. Carbonate hardness is your buffer against pH swings, so keeping it in a stable range matters more than chasing a single perfect number.
Is dGH measured the same way as dKH?
They use the same conversion factor, about 17.86 ppm per degree, but they measure different things. General hardness, dGH, reflects dissolved calcium and magnesium, which most fish and invertebrates care about for their long-term health. Carbonate hardness, dKH, reflects buffering capacity that keeps pH steady. A tank can be high in one and low in the other, so test and adjust them separately rather than assuming one number tells you both.
Why does salinity matter so much for a reef tank?
Reef invertebrates and corals are adapted to a narrow salinity band, and drifting outside it stresses or kills them over time. The common reef target is a specific gravity around 1.026, which is roughly 35 ppt of salt. Freshwater evaporates and leaves the salt behind, so salinity slowly rises and needs topping off with fresh RODI water. Because the swings are small but important, serious reefkeepers measure with a refractometer rather than a floating hydrometer.
How accurate is the specific gravity to ppt conversion?
The conversion here is an approximation tied to a reference temperature of about 77 F, since specific gravity changes with temperature. The relationship between specific gravity and salinity is close to linear in the reef range, so the estimate is good for planning. For dosing and daily reef care, confirm with a calibrated refractometer, which reads salinity directly and is far less affected by temperature than a swing-arm hydrometer.
Should I dose to liters or gallons?
Dose to whatever unit the product instructions use, and convert your tank volume to match. Many aquarium medications and fertilizers list a dose per gallon or per liter, and mixing the two is a common way to overdose. Convert your real water volume, which is about 90 percent of the tank label after substrate and rock, into the product unit first, then measure. When in doubt, dosing slightly conservative is safer than overshooting.
Why convert inches to centimeters for an aquarium?
Equipment, fish profiles, and care guides come from all over the world, so you constantly bounce between inches and centimeters. A light rated for a 90 cm tank needs to match a roughly 36 inch tank, and an adult fish listed at 15 cm is about 6 inches, which changes the tank size it needs. Converting quickly helps you buy the right size gear and plan stocking around how big your fish will actually get.