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Reef Salt Mix Calculator

Salt-mix math is straightforward in principle — grams of salt per litre of RO — but every brand publishes its own dose because grain size, density, and supplemental package vary. A scale beats a measuring cup every time; cup measures drift ±10% with brand and how compactly the salt was packed at the factory.

Salt mix calculator

Mixed reef: 1.025–1.026 SG (33–35 ppt). FOWLR: 1.022–1.024.

Dry salt — grams
6812g
Dry salt — kg
6.81kg
Dry salt — lb
15.02lb
Approx. cups
23.7US cups

Volumetric estimate — weigh whenever possible.

Target salinity
33.1ppt
Brand
Red Sea Salt

Red Sea Salt — Red Sea Aquatics datasheet

Resulting batch parameters (Red Sea Salt)

Alkalinity

8.2 dKH

Calcium

430 ppm

Magnesium

1340 ppm

Published manufacturer values at this brand's reference mix (~35 ppt). Test the finished batch — real values drift ±5%.

How this is calculated

grams_per_L      = brand_g_per_L_at_35ppt × (target_ppt / 35)
total_grams_salt = grams_per_L × water_litres
cups_salt        = brand_cups_per_gal_at_1.025 × gallons × (target_ppt / 35)
target_ppt       = (target_SG - 1) × 1326    // hobbyist approximation

Per-brand dose constants

Each salt has a published "reference dose" on the bucket — usually grams per litre or cups per gallon — to reach roughly 35 ppt / 1.025 SG. The calculator scales linearly from that reference to your target salinity. Real chemistry is slightly non-linear at the extremes (below 1.020 or above 1.030), but ±2% accuracy is plenty for a reef batch you're going to verify with a refractometer anyway.

  • Instant Ocean / Reef Crystals — Spectrum Brands. IO targets 33–35 ppt; Reef Crystals is the same recipe with elevated calcium and trace elements for reef tanks. Instant Ocean product page.
  • Red Sea Salt and Red Sea Coral Pro — Red Sea Aquatics. Coral Pro is the "elevated" version: 12.2 dKH, 465 ppm Ca, 1390 ppm Mg at the reference mix. Red Sea Salt range.
  • Tropic Marin Classic / Pro-Reef — German lab salt, lower published alk (7.5–8 dKH) than Red Sea but with a very clean trace-metal profile. Tropic Marin sea salts.
  • Fritz RPM Reef Pro Mix — US-made, mixed-reef parameters (8.5 dKH, 450 ppm Ca, 1350 ppm Mg). Fritz RPM product page.

Why weigh instead of measure cups

Cup measures assume a fixed bulk density. Real salt buckets settle in shipping: the top of a fresh bucket is about 10–15% lighter per cup than the bottom of an older bucket. The same "½ cup" can deliver 1.024 or 1.027 depending on how the bag was packed, shaken, or stored. A scale removes that error. For batches under 10 gallons it's the difference between coral pop and a slow-burn drift in salinity over a year of top-offs.

FAQ

How much salt do I need for a 50-gallon water change?
About 7.2 kg / 16 lb of Red Sea Salt or 7.0 kg of Reef Crystals to mix 50 US gallons at 1.025 SG. Roughly 25 US cups by volume — but weigh it for accuracy.
Should I use 1.025 or 1.026 SG for a reef tank?
1.025–1.026 SG (34–35 ppt) matches the open Pacific reefs most aquacultured corals come from. Below 1.023 stresses LPS/SPS; above 1.027 is fine for fish-only but slows coral growth slightly. Pick a number and hold it within ±0.001.
Why does Red Sea Coral Pro need more salt per gallon?
Coral Pro is formulated with elevated alkalinity (12.2 dKH), calcium (465 ppm), and magnesium (1390 ppm). More dissolved solids means more grams of dry salt per litre — roughly 41 g/L vs. 38 g/L for standard reef salts.
Can I use kitchen tap water?
No. Tap water carries chlorine, chloramine, phosphate, silicate, copper, and dissolved organics. Even one mix with municipal tap can seed a months-long algae bloom. RO/DI only — every reefer learns this twice.
How long should I let the salt mix before using it?
Minimum 24 hours with a powerhead running, heated to display temperature. Some salts (Red Sea Coral Pro, Tropic Marin syn-biotic) precipitate calcium carbonate if you skip the mix-time — you'll see a snow effect that costs you 1 dKH per batch.
Refractometer or hydrometer?
Refractometer, calibrated monthly with 35-ppt calibration fluid. Swing-arm hydrometers are accurate to ±0.002 SG out of the box and drift fast with salt creep. The math here assumes refractometer-level accuracy.
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Sources & references

  1. 01
    Instant Ocean / Reef Crystals — mixing instructions
    https://www.instantocean.com/products/instant-ocean.aspx
  2. 02
    Red Sea Salt — product page and parameters
    https://www.redseafish.com/salt/red-sea-salt/
  3. 03
    Red Sea Coral Pro — elevated reef-salt datasheet
    https://www.redseafish.com/salt/coral-pro-salt/
  4. 04
    Tropic Marin sea salts — Classic and Pro-Reef
    https://www.tropic-marin.com/en/products/sea-salts/
  5. 05
    Fritz RPM Reef Pro Mix — manufacturer page
    https://fritzaquatics.com/product/rpm-reef-pro-mix-fritz/
  6. 06
    Randy Holmes-Farley — Specific Gravity in reef aquaria
    https://reefs.com/magazine/specific-gravity-the-most-important-water-chemistry-parameter/

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