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 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.
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.
Quick answers
How much salt mix do I need per gallon?
For natural-seawater reef salinity (about 35 ppt, SG ≈1.0264 at 25 °C), the widely-used figure is roughly 36 g of dry marine salt per litre of finished saltwater — that is about 136 g per US gallon (brands span ~35–38 g/L). Instant Ocean's practical instruction is about ½ US cup per gallon, but that aims a bit lower (~1.022), so reef tanks usually add a little more. Grams is the accurate measure; cup scoops vary a lot by brand and packing.
How much salt for a 25 gallon batch of saltwater?
Twenty-five US gallons is about 94.6 litres, so at ~36 g/L for 35 ppt that's roughly 3.4 kg (about 7.5 lb) of dry salt. This calculator works out the exact grams for any volume and target salinity, because the amount scales with your target — but always mix slightly under, then confirm with a refractometer and top up.
How much salt for a water change?
Only enough for the volume of new water you're replacing. Enter your real system volume (display plus sump, minus rock and sand — not the rated size) and your change percentage, and the tool mixes salt for just that fraction. For a water change, match the new water's temperature AND salinity to the tank before it touches livestock.
How do I mix reef salt correctly?
Mix into RO/DI water only, never tap water. Add salt to the water (not water onto salt), gradually, with a powerhead running, until fully dissolved and clear — about 0.5 to 2 hours — then bring it to tank temperature (~77 °F / 25 °C). Measure with a refractometer and adjust (more salt if low, RO/DI if high) before use. A salt-mix number is a starting point, never the last word.