Chlorine Dose Calculator for Pools and Hot Tubs

How the calculator works

"Parts per million" in pool water means milligrams per litre. To raise free chlorine by 1 ppm, you need 1 mg of available chlorine for every litre of water. One US gallon is 3.785 litres, so 10,000 gallons is 37,854 litres, and a 1 ppm rise needs 37,854 mg, or about 37.9 grams, of available chlorine.

Products differ only in how much available chlorine each gram or millilitre contains:

• Liquid chlorine and bleach are labeled by "trade percent", the grams of available chlorine in 100 mL. A 10% product has 10 g per 100 mL, so 37.9 g needs 379 mL, or 12.8 fl oz.
• Dry products are labeled by weight percent. 65% cal-hypo has 0.65 g of available chlorine per gram, so 37.9 g needs 58 g of product, about 2.1 oz.

The calculator multiplies the ppm rise by your volume and divides by the product strength. The table shows the result for the most common strengths.

The side effects follow from the chemistry. Dichlor and trichlor are cyanuric acid molecules with chlorine attached, so every dose leaves cyanuric acid behind once the chlorine is used: about 0.9 ppm CYA per 1 ppm FC from dichlor and 0.6 ppm from trichlor, based on their molecular weights. Cal-hypo carries calcium with it, about 0.7 ppm of calcium hardness per 1 ppm FC.

Worked example

Choosing a target

The right free chlorine level depends on your stabilizer (cyanuric acid, CYA) level, because CYA holds most of the chlorine in reserve.

• Public-health minimums (CDC): at least 1 ppm in pools, at least 2 ppm in pools that use stabilizer or stabilized chlorine, at least 3 ppm in hot tubs. CDC also notes that chlorine above 10 ppm can bleach DPD test reagents and give a false low reading, and recommends not using cyanuric acid in hot tubs.
• Pool-owner rule of thumb: many residential pool owners, following the Trouble Free Pool method, keep FC at no less than about 7.5% of the CYA level. With CYA at 40 ppm that is 3 ppm, and they aim a few ppm higher so the level doesn't fall below that minimum between doses.

Both point the same way: the more stabilizer in the water, the higher the free chlorine you need. Our free chlorine and CYA guide explains why. If you enter a CYA value, the calculator shows the 7.5% figure for context.

Adding chlorine safely

• Pour liquid chlorine slowly in front of a return jet, close to the water surface.
• Follow the label for dry products. Some say to broadcast granules over the deep end; others require a feeder.
• Trichlor tablets belong in a floater or a dedicated feeder, never loose in the skimmer with other products.
• Wait at least 30 minutes with the pump running, then retest before adding more.

Why your result might differ

• Liquid chlorine weakens over time, faster when it is warm. A jug that has sat in a hot garage for months can be well below its labeled strength. If doses seem to do less than expected, enter a lower custom strength.
• Chlorine demand: algae, debris and sunlight consume chlorine while it mixes, so the measured rise can be smaller than the calculated one.
• Volume errors: if the measured rise is consistently bigger or smaller, check your pool volume.

Common questions

How much bleach do I add to raise chlorine 1 ppm?

In 10,000 US gallons, about 15.5 fl oz of 8.25% plain unscented bleach or about 21.3 fl oz of 6% bleach raises free chlorine by 1 ppm. Scale up or down in proportion to your volume.

Can I use scented or splash-less bleach?

No. Use plain, unscented bleach only. Scented, splash-less and gel products contain additives that do not belong in pool water.