Aquarium math, done right

Free calculators for planted and reef tanks. Every formula is written out, sourced and tested, and every tool works in metric or US units. Switch units at the top of any page.

• Tank volume & water weight

  Rectangular, cylinder and bow-front tanks, in litres and gallons, plus how much the water weighs.

• Water change & dilution

  What a water change does to nitrate, salinity or hardness, how much to change to hit a target, and RO/tap blends.

• Fertilizer dosing (dry salts)

  Grams to ppm and back for KNO₃, KH₂PO₄, K₂SO₄ and Epsom salt, plus stock solutions and an EI-style planner.

• CO₂ from pH & KH

  Dissolved CO₂ from your pH and KH readings, with temperature correction, and the pH that matches your target.

• Salinity & specific gravity

  Convert hydrometer SG to ppt with temperature correction (EOS-80), and see what your instrument should read.

• Heater wattage

  How many watts your tank needs, from volume, room and target temperature, and optional evaporation.

Why ScapeMath

No sign-up, no clutter. The tools run in your browser and work offline once loaded. Nothing you type is sent anywhere.

Shown working. Each page explains the method, gives worked examples you can check by hand, and links the primary sources: the UNESCO seawater equation, published carbonate chemistry constants, and IUPAC atomic weights.

Honest about limits. Where a number is an estimate, like heater wattage or CO₂ from pH, we say so and tell you what can throw it off.