Borax in your pool
Borax raises pool pH and adds about half as much alkalinity as soda ash for the same change. In 15,000 gallons, 2.75 lb takes pH from 7.2 to 7.5.
Always follow your product’s label directions. Pool and spa chlorine, bromine, shock and algaecides are EPA-registered pesticides: use products labeled for pool or spa use, and don’t swim or soak until levels are within the limit on your product’s label. If this page and a label disagree, the label wins.
On this page (7 sections)
Soda ash makes the same change with about 1.35 lb.
Calculator: borax for pH or for borates
The calculator above doses borax (sodium tetraborate decahydrate, sold as a laundry booster) for a pH change. It uses your total alkalinity (TA), stabilizer (CYA), and any borates already in the water, because all three change how much base a given pH rise takes. The result also shows how much it raises TA and borates. The default example, 15,000 gallons from pH 7.2 to 7.5 at TA 80 and CYA 30, takes 2.75 lb of borax, raises TA by about 5.8 ppm, and adds about 2.5 ppm of borates.
For a borate buffer, the amounts are much larger. Borates are measured as ppm of boron (B), and borax is 11.34% boron (PHTA):
| Pool size | Borax to 30 ppm | Borax to 50 ppm | Boric acid to 30 ppm | Boric acid to 50 ppm |
|---|---|---|---|---|
| 10,000 gal | 22.1 lb | 36.8 lb | 14.3 lb | 23.9 lb |
| 15,000 gal | 33.1 lb | 55.2 lb | 21.5 lb | 35.8 lb |
| 20,000 gal | 44.2 lb | 73.6 lb | 28.6 lb | 47.7 lb |
| 25,000 gal | 55.2 lb | 92.0 lb | 35.8 lb | 59.7 lb |
Calculated from molar masses (borax decahydrate 381.37 g/mol, 4 boron atoms; boric acid 61.83 g/mol). PHTA's example gives 7.05 lb of borax for +10 ppm in 10,000 gallons; we get 7.36 lb because PHTA counts a gallon of water as 128 oz by weight instead of about 133.5 oz. The 4% gap is within test error.
Adding that much borax raises pH a lot, so PHTA notes you must follow it with muriatic or dry acid. Boric acid, a weak acid, lowers pH only slightly and needs no added acid. That is why many owners use boric acid for the buffer, or a mix of borax and acid. Add it in portions over several days, retest pH between them, and confirm with a borate test strip (they measure boron).
What borates do (pH buffer)
Boric acid and borate form a buffer that resists pH increases, and PHTA describes it as especially effective at the upper end of the pool pH range. In practice that means pH creeps up more slowly in pools with a lot of aeration, salt cells, or low alkalinity. PHTA's bulletin also lists lower chlorine demand and possibly reduced corrosion. A review it cites found support for the corrosion claim but not for claims about water "sparkle" or softer feel.
Borates change how much acid you need, so enter them in the calculator. In 10,000 gallons at TA 80 and CYA 30, bringing pH from 7.8 to 7.5 takes about 7.6 fl oz of 31.45% muriatic acid without borates and about 18 fl oz with 50 ppm borates, because the buffer resists the change in both directions.
Borax vs soda ash
| Change | Borax | TA rise | Soda ash | TA rise |
|---|---|---|---|---|
| pH 7.0 → 7.4 | 3.3 lb | 10.4 ppm | 1.65 lb | 18.9 ppm |
| pH 7.1 → 7.4 | 2.2 lb | 7.0 ppm | 1.1 lb | 12.6 ppm |
| pH 7.2 → 7.5 | 1.8 lb | 5.8 ppm | 14.8 oz | 10.5 ppm |
| pH 7.3 → 7.5 | 1.05 lb | 3.4 ppm | 8.8 oz | 6.2 ppm |
Engine calculation, rounded down. Borax also adds a little boron: about 2.5 ppm for the 7.2 → 7.5 case.
- Choose borax when TA is already in range or high and you only want pH up, or when soda ash has clouded your water before.
- Choose soda ash when TA is also low, or when you want the smallest amount to carry and add.
- Choose neither if pH is only slightly low and you have aeration available: running jets or a fountain raises pH without adding anything. See how to raise pH and, if alkalinity is the real issue, baking soda in a pool.
Either way, raise pH in steps of no more than 0.3 and retest. Target ranges are on pool pH levels.
Algae claims: what's true
PHTA's bulletin lists borates as helping to limit algae growth (an algaestat). The same bulletin reports that a scientific review found no support for the popular explanation that borates "starve algae of CO2", and suggests the effect, if present, works another way. We found no independent controlled study showing borates prevent algae in home pools at 30–50 ppm.
What that means for you: borates may help a little. They are not an algaecide, they don't kill algae that's already growing, and they don't replace free chlorine at the level your CYA requires. A green pool needs chlorine, not borax.
Safety limit: 50 ppm
- Don't exceed 50 ppm boron (PHTA). According to the PHTA bulletin, assessments by the U.S. EPA and NSF International found 30–50 mg/L boron in pools and spas does not pose a significant risk to human health.
- Keep the powder away from children and pets. Borax and boric acid are moderately toxic if swallowed, and small children are the most vulnerable.
- Check for metals before a big borax dose. PHTA notes the pH rise can cause copper or iron to stain. Test metals first if you use well water.
- Borates leave with the water you lose (backwash, splash-out). Retest boron a few times a season and top up when it drops below 30 ppm.
- Spread it slowly over the surface with the pump running, never mix with other chemicals, and follow the product label.
How the calculator handles borates in acid and base doses: methodology. More pH and dosing tools: dosing.
FAQ
Is borax good for pools?
Yes, used for the right job. Small amounts raise pH with less alkalinity rise than soda ash, and at 30–50 ppm boron it keeps pH from rising as fast. It isn't needed in every pool, and it isn't a sanitizer or algaecide.
What does borax do to your pool?
It raises pH, raises total alkalinity slightly, and adds borates. At pH-adjustment doses the borate level stays tiny (a few ppm). It takes tens of pounds per 10,000 gallons to build the 30–50 ppm buffer that slows pH drift.
Does borax kill pool algae?
No. PHTA describes borates as an algaestat, meaning they may help limit growth, but the mechanism isn't settled and borax doesn't kill existing algae. To clear algae, shock with a chlorine product labeled for pools, using the amount on its label, then brush and run the filter. Wait to swim until free chlorine is back at or below the label's swim limit. If the algae is still there after a few days, call a pool professional.
Sources
- PHTA technical bulletin, Boric Acid and Sodium Borates (revised 2018): phta.org (PDF)
- Taylor Technologies, K-1005 instruction manual: PDF
- TroubleFreePool legacy pool calculator (used only to cross-check our borax and acid numbers): legacy.poolcalculator.com
- Orenda, pKa and alkalinity buffering systems: blog.orendatech.com