Hot Tub Calcium Hardness Calculator: Grams to Add

Calcium hardness is the hot tub reading most people ignore until something goes wrong: too low and the water can start to eat at heater parts and seals, too high and you get cloudy water and a crust of scale on the shell and jets. The fix depends on which way you are out. Raising it takes a measured dose of calcium chloride; lowering it takes fresh water, because no chemical takes calcium back out. Enter your litres and your test reading below and the calculator gives the grams of calcium increaser to add, or the litres to drain and refill, worked out from the chemistry rather than the scoop on the tub.

How the dose is worked out

Test strips report calcium hardness "as calcium carbonate", so a reading of 1 ppm means the calcium you would get from 1 milligram of calcium carbonate in every litre. Calcium chloride carries the same single calcium atom in a heavier molecule (about 110.98 against 100.09 for calcium carbonate), so it takes about 1.109 milligrams of pure calcium chloride per litre to lift the reading by 1 ppm. Products are not pure: flakes are usually the dihydrate form, labelled around 77%, and anhydrous granules are stronger, often around 94%. The calculator divides by the percentage you choose.

Worked example: an 800 litre tub reading 80 ppm with a target of 150 ppm needs to rise by 70 ppm. That is 70 × 800 × 1.109 = 62,100 milligrams, or 62.1 grams, of pure calcium chloride. Divided by 0.77 for flakes, it comes to about 81 grams. The same sum gives the familiar pool rule of roughly 1.2 lb of 77% flakes per 10,000 US gallons for a 10 ppm rise, which is a useful cross-check.

Why you cannot dose it down

Calcium only leaves the water when water leaves the tub. If the reading is too high, the fix is to drain part of it and top up with water that is softer than your target. The calculator uses a simple mixing sum: if you replace a fraction of the water, the new reading is the old reading for the part you keep plus the tap water reading for the part you add. Solve that for the target and you get the fraction to drain: (current minus target) divided by (current minus tap water).

If your tap water is already harder than your target, no amount of draining gets you there, and the calculator will say so. That is common in hard water areas. Options are to accept a higher reading and keep the pH and alkalinity in check so scale is less likely, fill through a hose-end softening or pre-filter cartridge, or ask your hot tub dealer what your model tolerates. A full drain and refill is due anyway at intervals, and the hot tub water change calculator works out when.

Tap water hardness units

UK water suppliers publish hardness in several units. The calculator converts them all to ppm as calcium carbonate: 1 degree Clark is 14.25 ppm, 1 German degree is 17.85 ppm, 1 French degree is 10 ppm, and 1 mg/l of calcium is 2.497 ppm as calcium carbonate. A supplier's "total hardness" figure includes magnesium as well as calcium, so it slightly overstates the calcium your tap water adds. Using it means the calculator errs towards draining a little more than strictly needed, which is the safe side.

Adding calcium chloride safely

• Dissolve it first. Calcium chloride gets hot as it dissolves, the anhydrous kind especially. Add the product to a bucket of tub water, never water onto the product, then pour it in slowly with the jets running.
• Go in stages. Add about half, let it circulate, retest, then add the rest. Strips are only accurate to a band, so a second reading is worth more than a precise first dose.
• Get the order right. Many makers advise adjusting total alkalinity first, then pH, then calcium. The hot tub alkalinity calculator covers that step, and the chlorine calculator comes after.
• Follow the label. Branded increasers may contain more than calcium chloride or give their own dose. If the label and this calculator disagree, use the label and check your figures.