Reef salt mix calculator
How much salt to weigh for a batch of new saltwater, and what your refractometer or hydrometer should read when it is right.
Formulas and sources last checked . How we check
Calculator
Result
Salt to weigh996 grams
- Add first (90 per cent)
- 897 g
- Grams per litre
- 39.8
- Target salinity
- 35.0 ppt (SG 1.0264 at 25 C)
- Refractometer should read
- SG 1.0264
Mix the first 90 per cent, wait until it has dissolved and warmed, measure, then add the rest a spoonful at a time.
How does the salt calculator work?
Salt needed is the water volume times the grams per litre for your target. Brand charts give grams per litre at a few salinities; we draw a straight line between them. The most accurate figure comes from your own last batch: if a known weight gave a measured salinity, the same salt needs proportionally more or less for the target. The reading your instrument should show comes from the standard UNESCO seawater equation (EOS-80), which gives 35 ppt as SG 1.0264 at 25 C.
grams = litres x grams per litrefrom your batch: grams per litre = (last grams / last litres) x (target ppt / measured ppt)hydrometer reading = seawater density at water temperature / pure water density at the calibration temperature
Worked example: a 25 litre batch
- Last time, 950 g of salt in 25 litres measured 34 ppt. That is 38 g per litre.
- For 35 ppt you need 38 x 35 / 34 = 39.1 g per litre, so 25 x 39.1 = 978 g.
- Add 880 g first, mix until clear and at temperature, measure, then add the last 98 g a little at a time.
- A refractometer should read SG 1.0264. A hydrometer calibrated at 25 C, in water at 22 C, should read about 1.0272, because cooler water is denser.
Which salinity should I aim for?
Most sources centre on 35 ppt, which is natural seawater. Red Sea’s 2021 Foundation manual sets 33 to 35 ppt depending on the type of tank (with 30 for a fish-only tank). Fauna Marin’s Balling Light method gives 33 to 35. Charterhouse Aquatics, a UK shop, gives SG 1.025 to 1.026 in its 2024 testing guide. Pick one figure and hold it: a steady 34 is better than a number that wanders.
| Topic | What sources say | Our reading |
|---|---|---|
| Grams per litre for reef salinity |
| Brands differ and labels assume a measuring convention they do not always state. Use the chart to start, then switch to your own last batch, and always finish with a calibrated refractometer. |
What are the limits?
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