How much salt do I need per gallon for 35 ppt?
From physics alone, 137 grams per US gallon (36.2 g per liter): 35 ppt means 35 grams of dissolved salt in every kilogram of finished seawater, and a gallon of fresh water is 3.77 kg. Every real bag needs a little more than that, because a "salt" mix isn't dry sodium chloride — it's a blend of hydrated salts that carries water of crystallization and picks up moisture from the air. So the makers who publish a yield land at 38.2–40.8 g/L, 6–13% above the pure-salt figure — and a 7% spread from one brand to the next, which is why "half a cup per gallon" is advice, not a recipe.
| Mix | Yield at 35 ppt | Per US gallon | 5 gal batch | vs pure salt |
|---|---|---|---|---|
| Pure salt (physics) | 36.2 g/L | 137 g | 684 g | — |
| Red Sea Coral Pro | 38.2 g/L | 145 g | 723 g · 1.59 lb | +6% |
| Tropic Marin Pro-Reef | 40 g/L | 151 g | 757 g · 1.67 lb | +11% |
| Instant Ocean Reef Crystals | 40.8 g/L | 154 g | 772 g · 1.7 lb | +13% |
Brand yields from the makers' labels and spec sheets (Tropic Marin quotes 37–40 g/L for 32–35 ‰; Reef Crystals' label is 155 g per gallon for SG 1.025). Moisture varies bag to bag — verify each new bucket with a refractometer.
Salt mix chart: grams per batch at 35 ppt
| Fresh water | Generic | Red Sea Coral Pro | Tropic Marin Pro-Reef | Reef Crystals |
|---|---|---|---|---|
| 1 gal · 4 L | 144 g | 145 g | 151 g | 154 g |
| 2 gal · 8 L | 287 g | 289 g | 303 g | 309 g |
| 5 gal · 19 L | 719 g | 723 g | 757 g | 772 g |
| 10 gal · 38 L | 1,437 g | 1,446 g | 1,514 g | 1,544 g |
| 15 gal · 57 L | 2,156 g | 2,169 g | 2,271 g | 2,317 g |
| 20 gal · 76 L | 2,875 g | 2,892 g | 3,028 g | 3,089 g |
| 25 gal · 95 L | 3,593 g | 3,615 g | 3,785 g | 3,861 g |
| 30 gal · 114 L | 4,312 g | 4,338 g | 4,542 g | 4,633 g |
| 40 gal · 151 L | 5,749 g | 5,784 g | 6,057 g | 6,178 g |
| 50 gal · 189 L | 7,187 g | 7,230 g | 7,571 g | 7,722 g |
Grams of mix into that much RO/DI water for 35 ppt (SG 1.0264 on a refractometer). For other salinities scale in proportion — 33 ppt is 33/35 of the number. Generic = pure-salt physics + 5 % moisture allowance. Weigh; a cup of hygroscopic salt can vary 10 %.
Dissolved salt takes up room. Mix 684 g into exactly 5 gallons of RO and you get about 5.05 gallons of seawater — 1% more. Small, but it means the honest way to hit a salinity is to measure the water, weigh the salt, and let the volume be whatever it is, not to fill a bucket to the 5-gallon mark and add "5 gallons' worth" of salt (that's a 1% overshoot before moisture even enters). It also means a batch mixed to the brim will be over the brim.
How to mix saltwater so it tests right the first time
Fill the vessel with RO/DI and note the volume — a marked container or a flow meter, not a guess. Get it near tank temperature with a heater and a powerhead running. Weigh the salt from the calculator on a kitchen scale and add the salt to the water, slowly, into the flow — never water onto a pile of salt, which sets like plaster and precipitates calcium carbonate. Let it circulate until it is completely clear, usually 1–2 hours for a good mix; some makers say to use it within a day rather than leaving it churning for a week, because prolonged aeration can drive off CO₂ and raise the pH enough to precipitate. Then measure at temperature: a refractometer calibrated with a 35 ppt standard should read 1.0264 for 35 ppt; a 60/60 glass hydrometer floating in 78 °F water should read about 1.0242 for the same batch (see the salinity converter for why those differ). Missed? The next section.
My batch came out too salty (or too fresh) — how do I fix it?
Salt doesn't leave the bucket, so this is arithmetic on a fixed amount of salt. Too fresh: add salt — a 20 gallon batch that tests 33 ppt needs about 162 g more of pure salt to reach 35 (the calculator scales that up for your brand's moisture). Too salty: add fresh water — the same batch at 37 ppt needs 4.5 L (1.19 gal) of RO to come down to 35. Both are the "fix a batch" mode above. Note the asymmetry: going up costs a few grams, going down costs volume you may not have room for, so if you're unsure, mix a little fresh and add salt. And for a running tank that's off, the same math gives the "remove X gallons and replace with RO" number — the salinity adjustment tool is built for that case, sump and all.
If you've saved a tank, the batch size is prefilled at 10% of its true system volume — the water you'd actually change, not the label on the display. Change it to what your mixing container holds. Salt for a water change should be mixed to the tank's salinity, not to 35 by reflex: putting 35 ppt water into a 33 ppt tank nudges it up every week. Match first; correct on purpose, separately, with the adjustment tool.