Free aquarium calculator
Aquarium Salt Calculator
Find how much marine salt to mix for any volume and target salinity, convert specific gravity to ppt, and work out how to raise or lower the salinity of your tank.
For water changes, enter the volume of new water you are mixing. To adjust your tank, enter its net water volume.
Reef tanks: 1.025–1.026 SG (33–35 ppt). Fish-only: 1.020–1.025. Brackish: 1.005–1.015.
Salt needed
Salt for this volume of new water
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Adjusting your tank
Enter your current salinity to see how much salt to add or how much water to replace.
Enter a volume and a target salinity.
Based on about 37 g of reef salt per litre for natural seawater (35 ppt). Brands vary by a few per cent, so mix, let it dissolve fully, then check with a calibrated refractometer before use.
How the salt calculator works
Enter the volume of water you want to mix and your target salinity, either as specific gravity (the number on a hydrometer, such as 1.025) or in parts per thousand (ppt, as shown on many refractometers). The calculator shows how much salt mix you need, converts between SG and ppt, and tells you which range your target falls in. Add your tank's current salinity and it also tells you how much salt to add or how much water to replace to reach the target.
The formula
Salt needed = water volume × target salinity × about 1.06 g per litre per ppt, which is about 37 g of reef salt per litre for natural seawater at 35 ppt. Specific gravity and ppt are converted with ppt ≈ (SG − 1) × 1325 at around 25 °C, so 1.025 is about 33 ppt and 1.026 about 34.5 ppt. To lower salinity, replace a share of tank water equal to 1 − target ÷ current with fresh water; to raise it, dissolve salt for the difference.
Worked example
To mix 200 litres of new saltwater at 1.025 SG (about 33 ppt) you need about 7.0 kg (15.4 lb) of salt. If a 200-litre tank has crept up to 1.028 through evaporation, replacing about 21 litres with fresh RO/DI water brings it back to 1.025. If it has dropped to 1.022, dissolving about 0.84 kg of salt in tank water and adding it slowly raises it to 1.025.
What salinity should my tank be?
Natural seawater is about 35 ppt, or 1.026 SG at 25 °C. Reef tanks with corals and invertebrates are best kept at 1.025–1.026 and held steady. Fish-only tanks can run slightly lower, at 1.020–1.025. Brackish species such as mollies kept in brackish water, figure-eight puffers or archerfish need 1.005–1.015 depending on the species. Stability matters more than the exact number: change salinity by no more than about 0.001 SG per day.
How to mix saltwater properly
Use RO/DI water, which removes the nitrates, phosphates and metals in tap water that feed algae and harm corals. Heat the water to tank temperature, add the salt with a pump or powerhead running, and let it mix for at least a few hours, ideally overnight, before measuring and using it. Always top up evaporation with fresh water, not saltwater: only the water evaporates, so adding saltwater would slowly raise the salinity. Calibrate your refractometer with a 35 ppt calibration fluid.
How much salt for common tank sizes
Salt mix needed to make new saltwater at the two most common reef targets, worked out with the calculator above. Use these as a starting point and fine-tune with a refractometer.
| Volume | At 1.025 SG | At 1.026 SG |
|---|---|---|
| 20 L | 0.70 kg (1.5 lb) | 0.73 kg (1.6 lb) |
| 40 L | 1.40 kg (3.1 lb) | 1.46 kg (3.2 lb) |
| 60 L | 2.10 kg (4.6 lb) | 2.19 kg (4.8 lb) |
| 100 L | 3.50 kg (7.7 lb) | 3.64 kg (8.0 lb) |
| 150 L | 5.25 kg (11.6 lb) | 5.46 kg (12.0 lb) |
| 200 L | 7.00 kg (15.4 lb) | 7.28 kg (16.1 lb) |
| 300 L | 10.5 kg (23.2 lb) | 10.9 kg (24.1 lb) |
| 500 L | 17.5 kg (38.6 lb) | 18.2 kg (40.1 lb) |
| 10 US gal | 1.33 kg (2.9 lb) | 1.38 kg (3.0 lb) |
| 20 US gal | 2.65 kg (5.8 lb) | 2.76 kg (6.1 lb) |
| 40 US gal | 5.30 kg (11.7 lb) | 5.51 kg (12.2 lb) |
| 55 US gal | 7.29 kg (16.1 lb) | 7.58 kg (16.7 lb) |
| 75 US gal | 9.94 kg (21.9 lb) | 10.3 kg (22.8 lb) |
| 125 US gal | 16.6 kg (36.5 lb) | 17.2 kg (38.0 lb) |
Specific gravity to ppt conversion
Specific gravity and salinity in ppt at about 25 °C, the usual aquarium temperature.
| Specific gravity (SG) | Salinity (ppt) |
|---|---|
| 1.018 | 23.9 ppt |
| 1.020 | 26.5 ppt |
| 1.021 | 27.8 ppt |
| 1.022 | 29.2 ppt |
| 1.023 | 30.5 ppt |
| 1.024 | 31.8 ppt |
| 1.025 | 33.1 ppt |
| 1.026 | 34.5 ppt |
| 1.027 | 35.8 ppt |
Frequently asked questions
- How much salt do I need per litre of saltwater?
- About 35–37 g of reef salt per litre for natural seawater salinity (35 ppt, about 1.026 SG), or about 3.5 kg per 100 litres at 1.025. Brands vary slightly, so always confirm with a refractometer.
- How much salt per gallon for a saltwater aquarium?
- About 0.13–0.14 kg (roughly half a cup) of salt mix per US gallon for 1.025–1.026 SG. A 55-gallon tank needs about 7.3–7.6 kg of salt to fill from scratch.
- What is the ideal salinity for a reef tank?
- 1.025–1.026 specific gravity, or about 33–35 ppt. Keep it as stable as possible and change it slowly, no more than about 0.001 SG per day.
- How do I lower the salinity in my aquarium?
- Remove some tank water and replace it with fresh RO/DI water at the same temperature. To go from 1.028 to 1.025 in a 200-litre tank, replace about 21 litres, spread over a day or two.
- Can I use this for freshwater aquarium salt?
- No. This calculator is for marine salt mix used to make saltwater. Freshwater aquarium salt is a treatment used at much lower doses; follow the product label and never add it routinely to tanks with scaleless fish, plants or shrimp.