Saltwater Mix Calculator by Brand
Enter the amount of RODI water, by weight or volume, choose your salt and the salinity you want, and get the grams of salt mix to add. Doses are set per brand, since salts differ.
Salt mix to add
700 g
24.7 oz · for 5 US gal of water at SG 1.025
Add the salt slowly with a powerhead running, give it a few hours to dissolve, then check with a refractometer. Batches and temperature move the result a little, so treat this as a starting dose and trim it if it reads off.
Salt per gallon of RODI water, by brand
| Salt mix | 1 gal at 1.025 | 5 gal at 1.025 | 20 gal at 1.025 | 1 gal at 1.026 |
|---|---|---|---|---|
| Red Sea Coral Pro | 140 g | 700 g | 2,798 g | 146 g |
| Instant Ocean Sea Salt | 158 g | 789 g | 3,157 g | 164 g |
| Instant Ocean Reef Crystals | 152 g | 762 g | 3,047 g | 159 g |
| Tropic Marin Pro-Reef | 146 g | 732 g | 2,930 g | 153 g |
| Aquaforest (Reef Salt, Reef Salt+, Hybrid Pro Salt, Sea Salt) | 151 g | 753 g | 3,013 g | 157 g |
The amounts are for gallons of RODI water, before the salt goes in. For other salinities, other amounts or litres, use the calculator above.
Why every salt mix needs a different amount
A bag of salt mix weighs more than the ions that end up dissolved. Mixes carry water of hydration and other weight, and each brand blends them differently. The same weight of two brands therefore doesn’t give the same salinity.
The calculator starts from the pure-salt maths for the salinity you pick, then scales it by a factor for the brand. The factor comes from the brand’s own dose for 35 ppt.
- Red Sea Coral Pro: Red Sea's published 35 ppt dose: 38.2 g/L, or 1.6 lb per 5 gal.
- Instant Ocean Sea Salt: From the bag label: 18 lb makes 50 gal at 35 ppt (1.026). A 1.5 lb to 5 gal at 1.022 label agrees.
- Instant Ocean Reef Crystals: From the label: 1.4 lb makes 5 gal at 1.021, scaled up to 35 ppt.
- Tropic Marin Pro-Reef: Tropic Marin's 37–40 g/L for 32–35 ppt, taken at the 35 ppt end.
- Aquaforest (Reef Salt, Reef Salt+, Hybrid Pro Salt, Sea Salt): Aquaforest's dose for all its salts: about 390 g in 10 L of RODI water for 33 ppt (1.025), as printed on the label and quoted by retailers.
If your brand isn’t listed, choose “Another brand”. Enter the dose on the bag in grams per litre or per gallon, together with the specific gravity it makes, and the calculator scales it to your target. Or mix a small batch, weigh the salt you used and enter that.
How to mix saltwater
- Fill a clean, food-safe container with RODI water and put a heater and a pump or powerhead in it.
- Weigh the salt. A scale is more reliable than a cup, because a cup of salt varies with the grain and how it packs.
- Add the salt to the water, never the water to the salt, slowly, with the pump running, so the salt dissolves instead of clumping.
- Keep it circulating until the water is clear and the salinity stops changing. Some brands take a couple of hours, so follow the bag.
- Check salinity with a calibrated refractometer, then add a little salt or water to correct it.
- Bring it to the tank’s temperature before the water change.
How the salt amount is calculated
Salinity in ppt is grams of salt per kilogram of finished water. So for a salinity of s ppt, pure salt for w grams of water is w × s ÷ (1000 − s), and the brand factor multiplies that. Weighing the RODI water makes it exact, because the volume the salt takes up doesn’t matter. Volumes are turned into weight at about 998 g per litre.
Specific gravity is converted to ppt on the reef-chart scale, where 35 ppt is 1.026. The Salinity converter uses the physical density, which puts 35 ppt at 1.0264, so the two give slightly different ppt for the same SG.
Frequently asked questions
How much salt do I add per gallon for 1.025?
To reach 1.025 in one gallon of RODI water you need about: Red Sea Coral Pro 140 g, Instant Ocean Sea Salt 158 g, Instant Ocean Reef Crystals 152 g, Tropic Marin Pro-Reef 146 g, Aquaforest (Reef Salt, Reef Salt+, Hybrid Pro Salt, Sea Salt) 151 g. Use the calculator for other amounts.
How much salt do I need for 5 gallons of saltwater?
At 1.025 it is 700 g of Red Sea Coral Pro, 789 g of Instant Ocean Sea Salt, 762 g of Instant Ocean Reef Crystals, 732 g of Tropic Marin Pro-Reef, 753 g of Aquaforest (Reef Salt, Reef Salt+, Hybrid Pro Salt, Sea Salt).
How many cups of salt per gallon?
Instant Ocean and Reef Crystals give half a cup per gallon for their 1.020 to 1.023 range, so a reef at 1.025 needs more. A cup of salt varies with the brand and how it packs, so a scale is more reliable than a cup.
Should I measure the salt by weight or by volume?
By weight. Weigh the RODI water too, if you can, and the amount of salt for any salinity comes straight from the calculator.
Why does my salinity come out different from the calculator?
Batches differ a little, the salt may not be fully dissolved, the water volume may be off, and your refractometer may need calibrating. Treat the result as a starting dose, check it, and trim with a little salt or RODI.
What salinity should a reef tank be?
Most reef keepers run 1.025 to 1.026, about 33.5 to 35 ppt. Natural seawater is about 35 ppt. A steady value matters more than the exact number.
More calculators
These calculators give estimates for general guidance. Confirm what matters, like salinity and alkalinity, with a test and your own equipment before you adjust a tank.