Guide

How much salt per litre for a saltwater aquarium?

By the Aquarium Calculator team Updated 9 October 2026

You need about 35 g of reef salt mix per litre of water for a salinity of 1.025 SG (about 33 ppt), and about 36–37 g per litre for 1.026 SG, close to natural seawater. That's roughly 133–138 g per US gallon. Brands vary by a few percent, so always confirm with a refractometer.

Salt amounts for common volumes

At 1.025 SG you need about 0.70 kg of salt for 20 litres, 1.75 kg for 50 litres, 3.5 kg for 100 litres, 7.0 kg for 200 litres and 10.5 kg for 300 litres. At 1.026 SG, add about 4%: 0.73 kg for 20 litres, 1.82 kg for 50 litres, 3.64 kg for 100 litres, 7.3 kg for 200 litres and 10.9 kg for 300 litres. These come straight from the salt calculator.

In US measures, that's about 133 g (just under half a cup) per gallon at 1.025, and 138 g at 1.026. A 55-gallon tank needs roughly 7.3–7.6 kg of salt to fill from scratch. For water changes, you only need salt for the new water you mix, not the whole tank.

Why the amount varies, and SG vs ppt

Salt mixes differ slightly in moisture and composition, so one brand may need a little more or less than another for the same salinity. Temperature also affects readings: specific gravity is lower in warm water, which is why salinity is normally measured at about 25 °C. Treat any per-litre figure as a starting point, then measure and fine-tune.

Hydrometers measure specific gravity (SG), while many refractometers also show parts per thousand (ppt). Near aquarium temperature, 1.020 SG is about 26.5 ppt, 1.025 about 33 ppt and 1.026 about 34.5 ppt. Natural seawater is about 35 ppt. Reef tanks are best kept at 1.025–1.026, fish-only tanks at 1.020–1.025.

How to mix saltwater properly

Start with RO/DI water rather than tap water, which can contain nitrate, phosphate, copper and silicate that feed algae and harm corals. Heat it to tank temperature in a clean food-safe container, then add the salt gradually with a powerhead or pump running to keep it moving.

Let it mix for at least a few hours, ideally overnight, until it is completely clear. Then measure the salinity with a calibrated refractometer, add a little salt or fresh water to reach your target, and check that the temperature matches the tank before using it.

Topping up and adjusting salinity

Only water evaporates, never salt, so always top up evaporation with fresh RO/DI water, not saltwater. Topping up with saltwater slowly raises the salinity. An auto top-off unit keeps salinity rock steady, which matters for corals and invertebrates.

To correct salinity, change it slowly, by no more than about 0.001 SG per day. To lower a 200-litre tank from 1.028 to 1.025, replace about 21 litres of tank water with fresh water; to raise it from 1.022 to 1.025, dissolve about 0.84 kg of salt in some tank water and add it gradually over several hours.

Work it out for your tank

Enter your volume and target salinity in the free salt calculator to get the exact amount in kg and lb. Add your current salinity and it also tells you how much salt to add or how much water to replace to reach the target safely.

Check your own tank

Enter your real dimensions to get exact figures for your setup in a few seconds.

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