Aquarium KH & GH Calculator
Work out how many grams of baking soda or another buffer will raise your KH, or how much calcium and magnesium salt will raise your GH, for your exact water volume.
How to use it
- Test first. Measure KH or GH with a drop test kit, and note the unit your kit uses.
- Enter the real water volume. For a whole tank, use the water left after substrate and decor (see the volume calculator). For remineralizing a bucket of RO water, enter the bucket volume.
- Weigh, don't guess. A cheap 0.01 g jewelry scale makes these doses accurate. Dissolve the powder in a cup of tank water before adding it.
- Go slowly in a stocked tank. Split large changes over several days and retest after each dose.
Formulas used
Both KH and GH are usually quoted in German degrees. One degree is defined as 10 mg/L of calcium oxide, which equals 17.848 mg/L as calcium carbonate.
1 dGH = 17.848 ppm CaCO3 = 0.1783 mmol/L calcium + magnesium
KH: each buffer supplies a fixed amount of alkalinity per gram. Bicarbonates give 1 equivalent per mole; carbonate gives 2.
| KH buffer | Molar mass | mg/L per 1 dKH |
|---|---|---|
| Sodium bicarbonate, NaHCO3 | 84.01 g/mol | 29.96 |
| Sodium carbonate, Na2CO3 | 105.99 g/mol | 18.90 |
| Potassium bicarbonate, KHCO3 | 100.11 g/mol | 35.71 |
GH: each degree needs 0.1783 mmol/L of calcium or magnesium ions. The calculator splits the rise between the two by your chosen calcium share.
| GH salt | Molar mass | mg/L per 1 dGH |
|---|---|---|
| Calcium chloride dihydrate, CaCl2·2H2O | 147.01 g/mol | 26.22 |
| Calcium chloride anhydrous, CaCl2 | 110.98 g/mol | 19.79 |
| Epsom salt, MgSO4·7H2O | 246.47 g/mol | 43.95 |
| Magnesium chloride hexahydrate, MgCl2·6H2O | 203.30 g/mol | 36.25 |
Teaspoon amounts are shown for baking soda only, using 4.6 g per level US teaspoon from USDA food data. Other powders vary too much in grain size to convert reliably.
Worked example: raising KH
A tank with 100 liters (26.4 US gallons) of real water has a KH of 2 and you want 4 dKH.
Rise = 2 dKH. Baking soda: 2 × 29.96 × 100 ÷ 1000 = 5.99 g, about 1.3 level teaspoons.
At a pace of 1 dKH per day, that is two doses of about 3 g on consecutive days.
As a check, a forum rule of thumb says one teaspoon in 50 L raises KH by "about 4 degrees". By the formula, 4.6 g in 50 L gives 3.07 dKH, so the rule of thumb runs a little high.
Worked example: raising GH
The same 100 L at 3 dGH, target 6 dGH, with 75% from calcium chloride dihydrate and 25% from Epsom salt:
Calcium: 3 × 0.75 × 26.22 × 100 ÷ 1000 = 5.90 g. Magnesium: 3 × 0.25 × 43.95 × 100 ÷ 1000 = 3.30 g.
Choosing a calcium to magnesium split
There is no single correct ratio. Natural fresh waters usually contain more calcium than magnesium, and many keepers who remineralize RO water aim for roughly 3 or 4 parts calcium to 1 part magnesium. TankMath's default of 75% calcium reflects that common practice. Shrimp and plant keepers often follow the ratio recommended by the remineralizer they use, so adjust the share to suit your livestock.
Frequently asked questions
How much baking soda raises KH by 1 degree?
About 30 mg per liter, or roughly 3 grams per 100 liters (about 26 US gallons). Baking soda is sodium bicarbonate, and each degree of KH needs 0.357 milliequivalents per liter, which works out to 29.96 mg/L of baking soda.
Does baking soda raise GH?
No. Baking soda adds bicarbonate and sodium, so it raises KH only. GH measures calcium and magnesium, so you need a calcium or magnesium salt, a GH booster product, or a calcium carbonate source such as crushed coral, which raises both slowly.
How fast can I raise KH?
A common hobby guideline is no more than about 1 dKH per day, and some keepers prefer 0.5 dKH per day. Raising KH also raises pH, so split a large correction over several days and test KH and pH after each dose.
What is the difference between dKH, ppm and meq/L?
They are three units for the same thing. One German degree (dKH or dGH) equals 17.848 ppm as calcium carbonate. For KH, one dKH also equals 0.357 meq/L of alkalinity. For GH, one dGH equals 0.178 mmol/L of calcium plus magnesium.
Which is better for KH: baking soda, soda ash or potassium bicarbonate?
Baking soda is cheap and gentle on pH. Soda ash (sodium carbonate) raises KH with less powder but pushes pH up harder. Potassium bicarbonate adds potassium instead of sodium, which suits planted tanks. All three give the same KH rise when dosed to the same meq/L.
Related calculators
- CO2 calculator: KH and pH together tell you your CO2 level
- Fertilizer calculator: dose potassium, magnesium and other nutrients
- Water change calculator: plan changes that keep hardness stable
Sources: German degree definition (10 mg/L CaO = 17.848 ppm CaCO3; 0.1783 mmol/L) from Hard water (Wikipedia). Molar masses from PubChem: sodium bicarbonate, sodium carbonate, potassium bicarbonate, calcium chloride dihydrate, magnesium sulfate heptahydrate. Baking soda weight per teaspoon (4.6 g) from USDA food data via ReciPal. Teaspoon rule of thumb from the FishProfiles forum. 0.5 dKH per day pacing from UltimateReef.
Last updated: October 2026