Saltwater Aquarium Salinity Calculator

Setup	Specific Gravity	Salinity (ppt)
Fish-only (FOWLR) tank	1.020–1.023	~27–31 ppt
Mixed reef (fish + soft/LPS corals)	1.024–1.026	~32–35 ppt
SPS-dominant display reef	1.025–1.026	~34–35 ppt
Natural seawater reference	~1.0264	35 ppt

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Walk-through

How to Use This Calculator

3 steps

1

Enter your tank size and current SG

Enter your tank's volume in gallons and its current specific gravity, measured with a hydrometer or refractometer. If you only know salinity in ppt, switch to the SG ↔ ppt Converter tab first to translate it.

2

Pick a mode and target

Choose whether you're adding salt mix to raise salinity or adding freshwater to dilute it, then enter your target specific gravity. Reef tanks commonly target 1.025–1.026 SG; fish-only tanks often run a bit lower.

3

Read the amount to add

The result shows how much marine salt mix (in pounds) or freshwater (in gallons) to add, along with the resulting salinity in both specific gravity and ppt. Add gradually and re-test before adding more.

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Reference

Formula & Methodology

3 formulas

Salt mix amount (raising salinity)

salt (lb) = gallons × 0.5 lb/gal × (targetSG − currentSG) ÷ (1.025 − 1.000)

Scales linearly off a standard marine-salt-mix manufacturer reference: roughly 0.5 lb of dry salt mix per gallon of RODI freshwater brings a batch from 1.000 SG (plain freshwater) up to full reef-standard strength (about 1.025 SG / 35 ppt). To top up an already-wet tank from its current SG rather than mixing from scratch, the amount scales with how far you need to move (targetSG − currentSG) relative to that full 0.025 SG reference span.

Freshwater amount (diluting salinity)

freshwater (gal) = gallons × (currentSG − targetSG) ÷ (targetSG − 1.000)

Derived from conservation of dissolved salt: treating (SG − 1) as proportional to salt concentration, the total dissolved salt stays constant as you add plain freshwater — only the volume changes. Solving V₁ × (currentSG − 1) = V₂ × (targetSG − 1) for the freshwater added (V₂ − V₁) gives this formula. This mass-balance approach is more accurate than approximating the dilution fraction off the current SG alone, especially since reef salinity ranges are narrow (1.020–1.026) where that shortcut can be off by 15–20%.

Specific gravity ↔ salinity (ppt)

ppt ≈ (SG − 1) × 1344 (equivalently, ÷ 0.744)

A linear approximation calibrated near 75–77°F (24–25°C), the standard temperature swing-arm hydrometers are calibrated at. It reproduces commonly cited reef-hobby conversion values closely (1.020 SG ≈ 27 ppt, 1.025 SG ≈ 33.6 ppt, 1.026 SG ≈ 35 ppt / natural seawater) to within about ±1 ppt. Refractometer readings, which are temperature-compensated, may differ slightly.

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Glossary

Key Terms Explained

7 terms

Specific gravity (SG) The density of water relative to pure freshwater at a reference temperature, used as the standard way aquarists measure salinity with a hydrometer or refractometer. Natural seawater is about 1.0264 SG; reef tanks commonly target 1.025–1.026.

Salinity (ppt) Salt concentration expressed in parts per thousand by weight — the other common way to express saltwater strength, especially on refractometers. Natural seawater is about 35 ppt, which corresponds to roughly 1.025–1.026 SG.

Reef tank standard The salinity range most commonly recommended for mixed and SPS-dominant reef tanks: about 1.025–1.026 SG (34–35 ppt), close to natural seawater, for coral and invertebrate health.

Marine salt mix A commercial blend of salts and trace elements (e.g. Instant Ocean, Red Sea) dissolved in RO/DI freshwater to create synthetic saltwater. Manufacturers publish a rough dosing rate — typically around 0.5 lb per gallon for full reef-standard strength — that this calculator uses as its scaling reference.

Hydrometer A tool that measures specific gravity via buoyancy, either as a floating glass bulb or a swing-arm plastic device. Swing-arm hydrometers are affordable but less precise than a refractometer and can drift with age or calibration.

Refractometer An optical instrument that measures salinity by how light bends through a water sample. Generally more accurate and temperature-stable than a hydrometer, and the preferred tool for reef keepers who need precise, repeatable readings.

Dilution Lowering a tank's salinity by adding plain freshwater (RO/DI, not tap water) without adding more salt, which increases total volume while keeping the dissolved salt amount constant.

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Scenarios

Real-World Examples

3 worked examples

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40-gallon reef tank, slightly under target

Raising salinity from 1.020 to 1.025 SG

Tank size 40 gal Current → target SG 1.020 → 1.025

Using the salt-mix formula: 40 × 0.5 × (1.025 − 1.020) ÷ 0.025 = 40 lb-equivalent scaling × 0.2 = 4.0 lb of salt mix. Dissolve it in some tank water first, add gradually over a few hours, and re-test with a hydrometer before adding more — the resulting salinity is about 33.6 ppt.

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55-gallon tank, accidentally over-salted

Diluting from 1.030 down to 1.025 SG

Tank size 55 gal Current → target SG 1.030 → 1.025

Using the dilution (mass-balance) formula: 55 × (1.030 − 1.025) ÷ (1.025 − 1.000) = 55 × 0.005 ÷ 0.025 = 11 gallons of RO/DI freshwater to add. Add it gradually over several hours rather than all at once — a sudden 20% volume change can stress fish and invertebrates even though the salinity itself is moving in a safe direction.

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Reading a refractometer in ppt

Converting 35 ppt to specific gravity

Salinity (ppt) 35 Formula SG = 1 + ppt ÷ 1344

35 ÷ 1344 ≈ 0.0260, so SG ≈ 1.026 — matching the commonly cited natural-seawater reference point. Use the SG ↔ ppt Converter tab to flip between the two units instantly instead of doing this by hand.

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Deep Dive

Understanding the Saltwater Aquarium Salinity Calculator

3 chapters · 3 min read

Keeping saltwater and reef tanks at a stable, correct salinity is one of the most important routine maintenance tasks in marine aquarium keeping. This calculator translates a current-to-target salinity change into a concrete amount of marine salt mix or freshwater to add, so you're not guessing at dosing.

How the Saltwater Aquarium Salinity Calculator works

When raising salinity, the calculator scales a standard marine-salt-mix manufacturer reference (about 0.5 lb of dry salt per gallon of RODI water to reach full reef-standard strength, ~1.025 SG) proportionally to how far you need to move your tank's specific gravity. When diluting, it uses conservation of dissolved salt mass: adding freshwater increases volume while the total salt stays the same, so the calculator solves for how much freshwater brings your concentration down to the target. Both formulas are documented in the Formula section above with the exact equations and assumptions.

Inputs and what they mean

Tank size in gallons should reflect your actual system water volume, including sump and refugium if you have one — not just the display tank. Current specific gravity comes from your hydrometer or refractometer reading; target specific gravity is where you want to end up, commonly 1.025–1.026 for reef tanks or a bit lower (1.020–1.023) for fish-only setups. The mode toggle switches between the two formulas above depending on whether you're raising or lowering salinity.

Limits and edge cases

This calculator gives a starting estimate, not an exact dose — actual marine salt mixes vary somewhat by brand and batch, and hydrometers can drift out of calibration over time, so always re-test with your own instrument before and after adjusting. Change salinity gradually (over hours, not all at once) in an established, stocked tank; sudden swings in either direction stress fish and invertebrates far more than the same total change made slowly. This tool is informational only and is not a substitute for species-specific husbandry research or professional aquaculture guidance.

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Questions

Frequently Asked Questions

6 questions

What's the reef-standard specific gravity?+

Most reef keepers target about 1.025–1.026 SG (roughly 34–35 ppt), close to natural seawater's ~1.0264 SG / 35 ppt. Fish-only tanks are sometimes run a bit lower, around 1.020–1.023 SG, for cost and simplicity.

What's the difference between SG and ppt?+

Specific gravity (SG) is a density ratio measured with a hydrometer, while salinity in parts-per-thousand (ppt) is a direct concentration measurement often shown on refractometers. They describe the same underlying salt concentration — use the SG ↔ ppt Converter tab to switch between them.

Should I change salinity all at once or gradually?+

Gradually. Add salt mix or freshwater over several hours rather than dumping the full amount in at once, especially in a stocked tank — sudden osmotic swings stress fish and invertebrates even when the salinity is moving toward a safer target.

Hydrometer or refractometer — which is more accurate?+

A refractometer is generally more accurate and temperature-stable than a swing-arm or floating hydrometer, which can drift with age or calibration. Many experienced reef keepers use a refractometer for routine testing and a hydrometer only as a rough backup.

What water should I use to top off or dilute?+

Use RO/DI (reverse osmosis / deionized) freshwater, not tap water. Tap water often contains chlorine, chloramine, phosphate, and metals that can harm fish, corals, and beneficial bacteria.

Is this calculator aquarium-expert or veterinary advice?+

No — this calculator provides general, informational rule-of-thumb estimates only. It is not a substitute for species-specific husbandry research, professional aquaculture guidance, or veterinary advice for sick livestock.

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Common Salinity Targets

How to Use This Calculator

Enter your tank size and current SG

Pick a mode and target

Read the amount to add

Formula & Methodology

Key Terms Explained

Real-World Examples

40-gallon reef tank, slightly under target

55-gallon tank, accidentally over-salted

Reading a refractometer in ppt

Understanding the Saltwater Aquarium Salinity Calculator

How the Saltwater Aquarium Salinity Calculator works

Inputs and what they mean

Limits and edge cases

Frequently Asked Questions

Get a branded PDF of your results

Saltwater Aquarium Salinity Calculator

Common Salinity Targets

How to Use This Calculator

Enter your tank size and current SG

Pick a mode and target

Read the amount to add

Formula & Methodology

Key Terms Explained

Real-World Examples

Understanding the Saltwater Aquarium Salinity Calculator

How the Saltwater Aquarium Salinity Calculator works

Inputs and what they mean

Limits and edge cases

Frequently Asked Questions

Get a branded PDF of your results

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