About this tool
The Marine Salt Mix Calculator is a free, in-browser tool that tells you how much synthetic reef or marine salt to dissolve in RO/DI water to hit a target salinity for a saltwater or reef aquarium. Enter the water volume and your target as either salinity in ppt or specific gravity, and it returns grams, kilograms, and approximate US cups of salt. All calculations run locally in your browser, so nothing you enter is uploaded.
Specific gravity is converted to salinity with the 25 °C/77 °F reference relation salinity (ppt) = (SG − 1) ÷ 0.000754, so the reef target of SG 1.0264 maps to about 35 ppt. Salt required is grams = volume_L × salinity_ppt × mix_rate, where the mix rate defaults to 1.03 g per (L·ppt) — roughly 36 g/L for a 35 ppt batch, a typical synthetic-salt figure that you can edit between about 0.97 and 1.09 to match your brand. In adjustment mode, to raise an existing volume the tool adds volume_L × (target_ppt − current_ppt) × mix_rate; to lower it, it computes the RO/DI top-off volume = volume_L × (current ÷ target − 1) needed to dilute down.
As a worked example, mixing 100 L to 35 ppt at the default rate: grams = 100 × 35 × 1.03 = 3,605 g, about 3.6 kg or roughly 16 cups. Always mix and aerate salt in a separate, livestock-free container until it is clear and fully dissolved, let it reach tank temperature, and confirm salinity with a calibrated refractometer before use — never pour dry salt into a tank with animals.