How much boric acid per 10 ppm
lb of boric acid = (target − current) ÷ 10 × 4.77 × gallons ÷ 10,000
4.77 lb of boric acid raises borates by 10 ppm in 10,000 gallons, because boric acid is 17.5 % boron and 10 ppm in 10,000 gallons is 378.5 g of boron. A 10,000-gallon pool needs 23.9 lb to go from 0 to 50 ppm, a 20,000-gallon pool 28.6 lb from 0 to 30 ppm, and a 15,000-gallon pool 21.5 lb from 20 to 50 ppm. Pour it slowly over the deep end with the pump running, in two or three portions, and test the next day.
| Pool | 30 ppm | 40 ppm | 50 ppm |
|---|---|---|---|
| 5,000 gal | 7.2 lb | 9.5 lb | 11.9 lb |
| 10,000 gal | 14.3 lb | 19.1 lb | 23.9 lb |
| 15,000 gal | 21.5 lb | 28.6 lb | 35.8 lb |
| 20,000 gal | 28.6 lb | 38.2 lb | 47.7 lb |
| 25,000 gal | 35.8 lb | 47.7 lb | 59.7 lb |
| 30,000 gal | 43 lb | 57.3 lb | 71.6 lb |
Borax and muriatic acid instead
lb of borax = (target − current) ÷ 10 × 7.36 × gallons ÷ 10,000 · muriatic acid ≈ 8 fl oz per lb of borax
Borax (sodium tetraborate decahydrate, the laundry booster) is 11.3 % boron, so it takes 7.36 lb for 10 ppm in 10,000 gallons: 36.8 lb for 0 to 50 ppm. Borax raises pH, and the acid turns it into boric acid in the pool: about 8 fl oz of 31.45 % muriatic acid per pound of borax, from the reaction. Add the acid in portions, test pH between them and use the pool pH calculator for the last step.
Borates buffer pH so it drifts up more slowly; they do not replace chlorine or stabilizer. Keep free chlorine where the pool chlorine calculator puts it, and check overall water balance with the pool LSI calculator.
Boron content from the formulas (H3BO3 17.48 % B, Na2B4O7·10H2O 11.34 % B); 23.9 lb boric acid for 0 → 50 ppm in 10,000 gal matches the beatbot.com and thepoolnerd.com borate calculators and Water Shapes "Understanding Borate" (76 oz per 10 ppm). Read October 23, 2026.
Frequently asked questions
How much boric acid do I need for my pool?
About 4.8 lb of boric acid raises borates by 10 ppm in 10,000 gallons. From 0 to 50 ppm that is 23.9 lb for 10,000 gallons and 47.7 lb for 20,000 gallons. Add it in portions over the deep end with the pump running and test after a day.
What level of borates should a pool have?
30 to 50 ppm. Below 30 ppm the pH buffering effect is small; above 50 ppm adds little and borates are not recommended past that. Borates leave only with water you drain or splash out, so a pool started at 50 ppm usually sits above 30 by the end of the season.
Can I use borax instead of boric acid?
Yes, but borax raises pH, so you add muriatic acid with it. About 7.4 lb of borax gives 10 ppm in 10,000 gallons, and roughly 8 fl oz of 31.45 % muriatic acid per pound of borax brings the pH back. Add the acid in portions and test between them.
Does boric acid lower pH?
Only slightly. Boric acid is a weak acid, so even a full dose lowers pH just a little. Test pH a few hours after adding it and correct with soda ash if it fell below 7.4. Borax does the opposite and raises pH.
Do borates replace stabilizer or chlorine?
No. Borates buffer pH so it drifts up more slowly, and many owners say the water feels softer. You still need free chlorine at the level your stabilizer calls for, and stabilizer protects that chlorine from sunlight. Test borates with borate test strips, since a normal kit does not show them.