Marine tanks run on salt water mixed to a steady strength, and the strength is measured in parts per thousand, or ppt, which is simply grams of marine salt per litre of water. Mixing by guesswork leads to swings that stress corals and fish, while weighing the salt takes the guesswork out. This free calculator tells you how many grams to weigh for a full tank and for a water change, plus the rough specific gravity to expect.
How to use this calculator
Enter your tank size in litres, your target salinity in ppt, and the size of your planned water change in litres. Press calculate and the tool shows total grams for the tank and grams for the change, plus an approximate specific gravity. Weigh the salt on kitchen scales, dissolve it fully in a separate container of fresh water, and always check the finished mix with a refractometer before it goes near livestock.
How the calculation works
The maths is simple: grams equal litres times ppt, since one ppt means one gram per litre. A 100 litre tank at 35 ppt needs 3,500 grams of salt, and a 20 litre change at the same strength needs 700 grams. Specific gravity is estimated as 1 plus ppt times 0.00075, which gives about 1.026 at 35 ppt. Real mixes vary a little by brand and temperature, which is why the refractometer check still matters.
A worked example
For a 100 litre reef tank at 35 ppt, you weigh 3,500 grams for the full fill and 700 grams for each 20 litre weekly change. For a brackish tank at 10 ppt, the same 100 litres needs only 1,000 grams, and a 20 litre change needs 200 grams. If you top up evaporation, use fresh water with no salt, since evaporation leaves the salt behind and topping up with salt water would push salinity higher every day.
Reading your result
Use the gram figures as your weighing guide and the specific gravity as a sanity check, not as a replacement for testing. Most reef tanks aim near 35 ppt, while fish-only marine tanks often run a little lower, and brackish tanks vary widely by species, so check your livestock needs first. If your refractometer disagrees with the maths by a wide margin, trust the refractometer, check your scales, and re-mix before adding the water.
Tips for stable salinity
Mix salt water at least a few hours before use, with a small pump or air stone to dissolve it fully and bring it to tank temperature. Mark a fill line on your tank or sump so top-ups stay consistent. Keep a refractometer calibrated with calibration fluid, not just tap water, since a wrong reading is worse than no reading. Store mixed salt water with a lid and a pump if you keep it for days, and re-check it before use.
Common mistakes
The classic mistake is topping up evaporation with salt water, which creeps salinity up week after week. The second is pouring undissolved salt straight into a stocked tank, where it can burn corals and irritate fish gills. The third is trusting a cheap swing-arm hydrometer without checking it against a refractometer. Weigh, dissolve, test, and only then add, and your livestock will thank you with steady behaviour.
Frequently asked questions
What salinity do reef tanks need? Most run near 35 ppt, which is about 1.026 specific gravity, but check the needs of your corals and fish. Can I use table salt? No, use proper marine salt mix, since table salt lacks the minerals marine life needs and may hold additives. How often should I test salinity? Test every new mix before use and test the tank weekly, plus after any big top-up or water change.
A note on safety
Keep salt mix dry and out of reach of children and pets, and wipe up spills, since salt water corrodes metal stands and equipment over time. Rinse pumps and heaters after contact with strong salt mixes if the maker advises it. This calculator is a mixing aid only: species needs vary, so confirm the target salinity for your fish, corals, and invertebrates before you mix a batch.
Versión en español: Calculadora de salinidad para acuarios – Gramos de sal marina.