Doses from your real volume and your product’s real strength
Pool dosing is mass balance with consequences: the amount of chlorine, alkalinity, calcium, stabiliser or salt a pool needs is its volume times the change you want, divided by your product’s strength, and every error in those three numbers lands in the water. This calculator takes the volume (use the pool volume calculator linked below if yours is a guess), your tested current level and your target, and the product you actually own, and returns the dose with its assumptions printed. Two safety rules stand above all the arithmetic and appear before any result does: never mix different pool chemicals together or add water to concentrated chemicals, and the product label’s instructions govern over any calculator, this one included.
Disclaimer: This calculator gives estimates for general information only. It is not financial, medical or other professional advice. Pool chemicals are hazardous: read and follow the product label, wear the stated protective equipment, add chemicals one at a time with circulation running, never mix products, and store them away from children. If in doubt, a licensed pool professional or your product’s manufacturer is the authority.
What this calculator does
Most pool trouble is a dose that was right for a different pool, a different product strength or a different starting point: the same jug that maintains one pool shocks another and barely stirs a third. The mass-balance adjustments here (chlorine, alkalinity, calcium, stabiliser, salt) are exact arithmetic once volume, change and product strength are honest, and the results show the pure-chemical figure as well as the product figure so substitutions can be checked rather than guessed. pH is handled differently and says so: it is logarithmic and buffered, so the acid mode runs on the published planning approximation scaled by your alkalinity, labels itself an estimate, and prescribes the method that makes estimates safe (dose partially, circulate, retest).
How to use it
- Volume first, and measured. A guessed volume poisons every dose downstream by the same percentage. The pool volume calculator on this site works it out from shape and depth; unusually, an honest 15,000 beats a hopeful 20,000.
- Test, then dose, then retest. Current levels come from a fresh test (liquid kit or strips read promptly), targets from your sanitiser system’s range, and the confirmation test after circulation is part of the dose, not an optional review of it.
- Additions only. These tools raise levels. Lowering chlorine, alkalinity, calcium, CYA or salt is done by partial draining and refilling (or time, for chlorine), and the calculator refuses those directions rather than inventing a negative chemical.
- One chemical at a time. Add each product separately, brush and circulate, and keep products apart in storage and in the bucket: the never-mix rule is chemistry, not tidiness, since several pool products react violently together.
The method
- Free chlorine: pure chlorine needed (oz by weight) = gallons × ppm change × 0.00013344. Product amount = pure figure / the product’s available-chlorine fraction. Liquids are converted to fluid ounces at the product’s specific gravity; dry products stay in weight ounces and pounds.
- Alkalinity: 1.4 lb of sodium bicarbonate per 10,000 gallons raises alkalinity about 10 ppm. Calcium: 1.2 lb of calcium chloride per 10,000 gallons raises calcium hardness about 10 ppm. CYA: 1 lb per 10,000 gallons raises stabiliser about 10 ppm. Salt: pounds = gallons × ppm change × 8.34 / 1,000,000.
- pH (acid mode): planning approximation only: about 6.3 fl oz of 31.45% muriatic acid per 10,000 gallons lowers pH about 0.1 at alkalinity 100, scaled by your alkalinity relative to 100. Real demand varies with buffering and is established by an acid-demand test; dose partially and retest.
- Stabilised chlorines carry a passenger: dichlor and trichlor add CYA with every dose (roughly 0.9 and 0.6 ppm of CYA per 1 ppm of chlorine respectively, at typical strengths), which the results print when those products are chosen.
Worked examples
The evening top-up. A 20,000 gallon pool tests at 1 ppm free chlorine with a target of 3: the change is 2 ppm, needing 5.34 oz of pure chlorine, which from 12.5% liquid (specific gravity 1.17) is 34.99 fl oz of product. The same pool on 65% cal hypo needs 8.21 oz by weight instead, which is why the product selector matters more than the jug’s size: strength, not volume, is what a pool drinks.
The alkalinity correction. The same pool raising alkalinity from 80 to 100 ppm needs 20,000 / 10,000 × 20 / 10 × 1.4 = 5.60 lb of sodium bicarbonate, added in partial doses with circulation, because alkalinity is pH’s anchor and overshooting it starts a second correction in the opposite direction.
The salt cell’s appetite. A thousand ppm of salt in that pool is 166.80 lb, four-plus 40 lb bags: salt adjustments are freight, not jugs, and the figure explains itself the first time it is computed rather than discovered at the shop.
Reading your results
Read the dose in the unit your product is measured by (liquids in fluid ounces, dry goods in weight), and read the pure-chemical line as the invariant: change products and that figure stays, which makes it the check against any second opinion. Read stabilised-chlorine results together with their CYA passenger line, because a pool can reach its chlorine target while quietly raising the stabiliser that will blunt next month’s chlorine. Read any result that looks surprisingly large as a prompt to re-check the volume before doubting the arithmetic.
Limitations
- pH movement is not linear, and this page says so in its own mode. Buffering, temperature, aeration and total dissolved solids all move acid demand; the acid mode is a planning approximation with a partial-dose method attached, and a demand test on your own water outranks it.
- Targets are system-specific. Salt cells, biguanide systems, spas and commercial pools run different ranges and sometimes different chemistry entirely; this calculator doses amounts for conventional chlorine pools and sets no targets.
- Shock and algae treatment are protocols, not single doses. Breakpoint chlorination and algae remediation follow staged procedures with their own testing rhythm; the calculator supplies the dose arithmetic for a stated ppm change and does not prescribe treatment.
- Product strengths and gravities vary by brand and age. Liquid chlorine in particular weakens in storage and heat; the label’s percentage is the input, and an old jug’s real strength is lower than its print.
Frequently asked questions
How much chlorine does my pool need?
As many ounces of available chlorine as the mass balance says: volume times the ppm change you intend, at your product’s strength, which the worked example computes for a typical evening top-up. The reason no flat answer exists is that pools differ in volume by factors of five and products in strength by factors of fifteen, so any jug-based rule of thumb is a dose for somebody else’s pool and product. The inputs are the answer: measure the volume, test the water, read the label.
Why does my chlorine keep disappearing?
Sunlight and load consume it: ultraviolet breaks down unstabilised chlorine within hours, and swimmers, heat and organic debris spend it continuously. The chemistry checks are stabiliser (too little and the sun takes the dose; too much and the chlorine that remains works slowly), and a combined-chlorine reading that suggests the chlorine present is already spent on contaminants. The calculator doses the replacement; the diagnosis uses the pattern of losses, which is why testing before and after belongs to the method.
Can I just use household bleach?
Plain unscented bleach at a stated sodium hypochlorite percentage is chemically the same active ingredient as liquid pool chlorine, and it appears in the product list for that reason, with two conditions: no scents, thickeners or splash-less additives, and a fresh, dated bottle, since bleach weakens noticeably in storage. Pool-labelled product remains the safer default because its strength is printed for exactly this purpose and its label carries the pool directions that govern.
What order do I add chemicals in?
One at a time, with circulation running, allowing each to disperse before the next, and never combining products in the bucket or the skimmer. Balance order in practice runs alkalinity first (it anchors pH), then pH, then calcium and stabiliser, with chlorine adjustments made on balanced water because its test and its effectiveness both depend on pH. The label of each product states its own addition method (broadcast, pre-dissolved, skimmer-fed), and those instructions outrank any general order, including this paragraph.
Related calculators
Method: mass-balance dosing from measured volume, tested current level and the product’s stated strength, trade planning figures for alkalinity, calcium and stabiliser, a labelled approximation for acid only, and the product label governing throughout. Last reviewed: 15 October 2026.