# Aquarium salinity and salt mix calculator

> Enter your target specific gravity and water volume to get the Instant Ocean salt mix needed and the matching ppt reading. Defaults sit at the reef standard SG 1.025 (≈32.5 ppt). The conversion is the standard (SG − 1) × 1300 = ppt, valid across the reef range.

## Who stands behind this calculator

Written by [Patrick Li](/authors/patrick-li/). Editorially reviewed 2026-09-01 · Evidence checked 2026-09-01.

Evidence: Multiple sources agree · Evidence from this species.

The calculator performs salinity arithmetic — SG↔ppt conversion and salt-mass math — on the numbers you enter. It does not measure your aquarium and it does not acclimate livestock; verify the mixed water with a calibrated refractometer or hydrometer before it touches your animals.

Use the interactive calculator on the page: https://petguidecalc.com/aquarium/salinity-calculator/. Enter your target SG and water volume to get the Instant Ocean pounds or grams, and ppt.

## What salinity should a reef tank be?

A reef tank typically targets SG 1.025–1.026 (about 32–34 ppt). For a 100 US gal tank at SG 1.025 you need about 34 lb (≈ 15.5 kg) of Instant Ocean mix — the calculator gives you the pounds, grams, and ppt for any tank size in the reef range, and reminds you to verify the final reading with a refractometer.

## Reference points at a glance

| Target SG | ≈ ppt | Use case | Mix per 5 US gal |
|---|---|---|---|
| 1.022 | 28.6 | FOWLR — Instant Ocean label anchor (1.5 lb / 5 gal) | 1.50 lb (36.0 g/L) |
| 1.025 | 32.5 | Reef standard | 1.70 lb (40.9 g/L) |
| 1.026 | 33.8 | Reef upper bound | 1.77 lb (42.5 g/L) |
| 1.030 | 39.0 | Highest reef-safe target | 2.05 lb (49.0 g/L) |

Confirm the final reading with a temperature-compensated refractometer calibrated to RO water at 77 °F (25 °C).

## How the grams are derived

The SG↔ppt conversion is the well-known aquarium approximation (SG − 1) × 1300 = ppt. SG 1.025 maps to about 32.5 ppt; SG 1.026 maps to about 33.8 ppt.

Salt mix amounts anchor to the Instant Ocean label: 1.5 lb per 5 US gallons reaches SG ≈ 1.022. That works out to 1.50 lb per 5 US gal (35.95 g/L) at SG 1.022, and the calculator scales linearly by (SG − 1) / (1.022 − 1) for any target in the reef range.

Worked examples: a 100 US gal tank at SG 1.025 needs 34.1 lb (≈ 15.5 kg); a 55 US gal tank at SG 1.026 needs 19.5 lb (≈ 8.8 kg); a 20 US gal FOWLR tank at SG 1.022 needs 6.0 lb (≈ 2.7 kg).

## Salinity & salt mix questions

**Q: What SG should a reef tank be?**

SG 1.025 (≈32.5 ppt) is the standard reef target; 1.026 (≈33.8 ppt) is the upper end used by some reef keepers. SG 1.022 (≈28.6 ppt) suits fish-only-with-live-rock tanks.

**Q: How much Instant Ocean for a 100 gallon reef tank?**

About 34 lb (≈ 15464 g) of Instant Ocean mix to reach SG 1.025 in 100 US gal of fresh water. Mix into a separate container, verify with a refractometer, then top up to the tank.

**Q: How do you convert SG to ppt?**

ppt = (SG − 1) × 1300, the standard reef-range approximation. For SG 1.025 that is 32.5 ppt; for SG 1.026 it is 33.8 ppt. Conversely SG = 1 + ppt / 1300, so 35 ppt maps to SG 1.027.

**Q: How accurate is the Instant Ocean label anchor?**

The label ratio (1.5 lb per 5 US gal ≈ SG 1.022) is the manufacturer’s published mix instruction. Real-world readings vary with temperature, salt brand, and impurities; always verify with a refractometer and adjust in small steps rather than trusting the calculated grams alone.

**Q: Can I use a hydrometer instead of a refractometer?**

Yes for a quick check, but a refractometer is more accurate. Swing-arm hydrometers drift with age and read lower at higher temperatures — calibrate or replace them. The calculator gives the starting mix; the refractometer reading tells you whether to add salt or fresh water to hit the target.

## Tools that pair well

- [Water Change Scheduler](/aquarium/water-change-scheduler/)
- [Aquarium fish compatibility checker](/aquarium/mix-calculator/)
- [Reptile care tools](/reptile/)

## Sources

The mix ratio and SG↔ppt conversion come from the references below.

- [Study on survival, growth, haematology and body composition of Cyprinus carpio under different acute and chronic salinity regimes](https://doi.org/10.1016/j.sjbs.2018.12.013) — Mubarik MS, Asad F, Zahoor MK, Abid A, Ali T, Yaqub S, et al., Saudi J Biol Sci, 2019.
- [How Often Do You Have to Change the Water in a Fish Tank?](https://www.aquariumcoop.com/blogs/aquarium/water-changes) — Aquarium Co-Op, aquariumcoop.com. Same load-level cadence used for water-change planning and salinity top-ups after evaporation.
- [Instant Ocean — Salt mix instructions](https://www.instantocean.com/) — Instant Ocean, instantocean.com. Source for the 1.5 lb per 5 US gal ≈ SG 1.022 anchor that calibrates the grams-per-liter constant.

_Educational resource, not veterinary advice. PetGuideCalc — https://petguidecalc.com_