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Calcium Hardness Calculator
Low calcium eats plaster; high calcium leaves scale. Two answers give the fix.
1 of 2: your gallons
Your dose appears here after the last question.
- 200 to 400 ppm
- calcium hardness range
- 77%
- calcium chloride flakes
- Water swap
- the only way to lower it
This calcium hardness calculator figures out the amount of calcium chloride your pool needs so the calcium level lands inside the safe range. When calcium runs low, it can wear at plaster. When it climbs too high, you get scale, that crusty white film that forms on pool surfaces.
- Aim for: 200 to 400 ppm
- To raise it: use calcium chloride flakes at 77% strength, about 1 lb 3 oz per 10 ppm in a 10,000-gallon pool
- To lower it: trade out part of your water for softer water
What does calcium hardness mean?
Calcium hardness is the amount of calcium held in the water. It gets expressed as ppm, short for parts per million. That is the figure your test kit or test strip gives you.
The site's target is 200 to 400 ppm. If you're under 200, calcium needs to go in. If you're over 400, the only fix is draining off some water.
Pool gallons for the dose
All doses on this site are based on US gallons, so your pool's size is the first thing to pin down. Once you enter your gallons, the site stores them in your browser and plugs them into each dose calculator for you.
These common pool sizes assume water about 4.5 feet deep on average:
- A 12 x 24 ft pool: roughly 9,690 gallons
- A 16 x 32 ft pool: roughly 17,200 gallons
- A 20 x 40 ft pool: roughly 26,900 gallons
Got a sloped floor? Average depth sits halfway between your shallow end and your deep end.
How this is worked out
Take the surface area in square feet, multiply by average depth in feet, then times 7.48. Round or oval? Use 5.9 in that spot instead. To get average depth, add the shallow depth to the deep depth and divide by 2.
Raising the calcium in your pool
The answer is calcium chloride, a salt that comes as flakes. Every dose on this site assumes flakes at 77% strength. It's sold by the bag, and you only weigh out what a dose calls for, so one bag usually covers you more than once.
- 1. Test the water first so you know where you're starting.
- 2. Weigh the dose that matches your pool size and the jump you're after.
- 3. Mix the flakes into a bucket of water before anything else. The mix gets hot while it dissolves, so handle the bucket gently.
- 4. Run the pump, since moving water carries the calcium around evenly.
- 5. Walk the edge of the pool and pour the mix in slowly, beginning near a return jet.
Only ever add one chemical at a time. Otherwise two products can react with each other before they've had a chance to spread out.
Calcium chloride doses by pool size
The base dose is 545 grams (that's 1 lb 3 oz) of 77% calcium chloride, and it lifts 10,000 gallons by 10 ppm. More water or a bigger lift means more product, in even steps. Double the water, double the dose. A 5,000-gallon pool takes half.
- 5,000 gal: 10 oz gets you 10 ppm, 3 lb gets 50 ppm, 6 lb gets 100 ppm
- 10,000 gal: 1 lb 3 oz gets you 10 ppm, 6 lb gets 50 ppm, 12 lb gets 100 ppm
- 15,000 gal: 1 lb 13 oz gets you 10 ppm, 9 lb gets 50 ppm, 18 lb gets 100 ppm
- 20,000 gal: 2 lb 6 oz gets you 10 ppm, 12 lb gets 50 ppm, 24 lb gets 100 ppm
- 25,000 gal: 3 lb gets you 10 ppm, 15 lb gets 50 ppm, 30 lb gets 100 ppm
- 30,000 gal: 3 lb 9 oz gets you 10 ppm, 18 lb gets 50 ppm, 36 lb gets 100 ppm
Say your pool holds 17,200 gallons and needs a 50 ppm lift to reach 200 ppm. Getting to 200 ppm takes roughly 10 lb.
Bringing the calcium back down
There's no chemical that pulls calcium back out. Lowering it means draining a portion of the pool and topping it back up with softer water.
Check your tap water before you begin. If the fill water is also hard, a refill won't move the number much. When that happens, ask a pool store about getting softer water delivered by truck.