Free tool
Reef salt-mix calculator
How much salt for a fresh batch of saltwater — or your next water change? Enter the volume and your target salinity and we'll give you the amount to add. Then always confirm with a refractometer and adjust — a salt number is a starting point, never the last word.
⚠️ Mix it right — and never trust the number blind.
- Mix into RO/DI water only — never tap water.
- Add salt to the water (not water onto salt), gradually, with a powerhead running.
- Mix until fully dissolved and clear (about 0.5–2 hours), then bring it to tank temperature (~77 °F/25 °C).
- Measure with a refractometer and adjust — add salt if low, RO/DI if high — before it touches livestock.
- For a water change, match the new water's temperature AND salinity to the tank.
An honest tool, not a promise. It uses ~36 g of salt per litre for 35 ppt — the widely-used figure, though brands run about 35–38 g/L and enriched salts (e.g. Red Sea Coral Pro) can need a touch more. Instant Ocean's own ½-cup/gallon instruction aims at a lower ~1.022, so reef tanks usually add a little more for 1.026. The cups figure is a rough guide (cup measures vary — grams is the accurate one). Your refractometer has the final say: mix slightly under, test, top up.
Sources & method
- ~35–38 g salt per litre → ~35 ppt / SG ≈1.0264 (25 °C); consistent with ppt = g salt per kg of solution.
- Instant Ocean — Sea Salt mixing instructions (½ US cup per US gallon; add salt to water, stir, aerate to equilibrium).
- Red Sea — Coral Pro Salt mixing (mix in RO/DI, dissolve fully, bring to temperature, measure and adjust).
A free Coralkin tool — no sign-up, nothing saved. It does the arithmetic; your refractometer and your salt's own label have the final say. We never guess a care number.