Pool pH Calculator
How much acid lowers your pool's pH, or soda ash raises it, sized to your alkalinity, and how much to add before you retest.
Why our numbers are more realistic
Many pH calculators use one amount for every 0.1 of pH. The acid a pool really needs depends on its alkalinity, and the popular rules come out about two to three and a half times higher than the chemistry and the product labels we checked, at the usual 80 to 150 ppm alkalinity.
| Typical calculator | This calculator | Why it matters |
|---|---|---|
| A fixed amount per 0.1 pH, with no alkalinity input | Asks your alkalinity and works from your starting pH | Alkalinity soaks up acid, so the dose rises with it, and each 0.1 takes less acid at higher pH. For 8.0 to 7.4 in 10,000 gallons at 80 ppm alkalinity, popular calculators say 24 to 38 fl oz of 31.45% acid; the chemistry says about 13. Haviland's pH Down label lands near the chemistry. At 38 fl oz, the chemistry puts that pool near pH 6.9. |
| Gives the whole dose at once | Starts with three-quarters, never more than a day's label limit, then retest | Stabilizer and borates make real doses vary, and too much acid drops pH and alkalinity, which then makes pH swing. One muriatic acid maker's label, as retailers summarize it, allows at most 1 quart per 20,000 gallons in 24 hours. |
| Assumes 31.45% acid | Offers 14.5% and dry acid too | 14.5% acid needs about 2.35 times the volume of 31.45%. |
| Offers dry acid with no warning | Switches salt pools to muriatic acid | Hayward's manual says dry acid can damage the chlorinator cell. Dry acid also adds sulfate, which only leaves when water is replaced. |
| Treats soda ash like acid in reverse | Shows how much it raises alkalinity, caps one addition, and suggests aerating first | Soda ash raises alkalinity about 11 ppm per pound per 10,000 gallons, and above 120 ppm it can cloud the water. Aeration raises pH without raising alkalinity. |
| Retest in 4 to 6 hours, or no time given | Retest after 30 minutes to 2 hours with the pump on, then the next day | Trouble Free Pool says at least 30 minutes with the pump running; Haviland's label says 2 hours. pH creeps back up for a day afterward, so a next-day test shows where it settled. |
How this works
- Acid: worked out from the standard carbonate chemistry, using your pH now, the target, and your alkalinity. It gives about 9 fl oz of 31.45% acid to take 10,000 gallons from 8.0 to 7.5 at 80 ppm alkalinity, and about 14 fl oz at 120 ppm.
- Other acids: 14.5% muriatic takes about 2.35 fl oz for every fl oz of 31.45%. Dry acid (93% sodium bisulfate) takes about 1.35 oz by weight.
- First addition: three-quarters of the estimate, and never more than 16 fl oz of 31.45% acid (or the same strength of another acid) per 10,000 gallons in a day, a muriatic acid label's limit.
- Soda ash: product labels' amounts, about 12 oz per 10,000 gallons to raise pH from 7.0 to 7.4, scaled to your change (our estimate). We start with three-quarters and never more than 1 lb per 10,000 gallons in one addition.
- Range: we leave 7.2–7.8 alone, the range labels direct. Outside it, we aim for 7.5. Enter a target if your pool maker gives one.
- Approximate by nature: the chemistry leaves out stabilizer and borates, which resist pH changes, so a real pool can need more. Starting low and retesting is the safe direction.
Common questions
How much muriatic acid to lower pH?
It depends on your alkalinity. At 80 ppm, taking 10,000 gallons from 8.0 to 7.5 is about 9 fl oz of 31.45% acid; at 120 ppm it's about 14. We'd start with three-quarters of that and retest.
Why does my pool's pH keep going up?
Water gives off carbon dioxide, and that pushes pH up, faster with fountains, spillovers and swimmers splashing. High alkalinity makes it climb faster, so if you add acid every few days, check that alkalinity isn't high. The alkalinity calculator's questions cover lowering it. Salt generators make it climb faster too, because the cell's bubbles stir carbon dioxide out of the water; adding acid every week is normal in a salt pool, and keeping alkalinity at the low end of your range slows it.
How long after adding acid can you swim?
Add it with no one in the pool. Swim once it has mixed with the pump running and a retest reads between 7.2 and 7.8, or as your product's label directs.
Muriatic acid or dry acid?
Muriatic acid for most pools. Dry acid is easier to store and handle, but it adds sulfate, which builds up and only leaves when water is replaced. Don't use it in a salt water pool: a generator maker warns it can damage the cell.
Does pH change how well chlorine works?
Yes, but less than you might think if your pool has stabilizer. The chlorine calculator's page explains how much.
Safety
- Follow the acid's label: wear goggles and gloves, pour it slowly into the deep end with the pump running, and store it away from chlorine. Swim once a retest reads between 7.2 and 7.8.
- Add chemicals to water, never water to chemicals. Never mix pool chemicals with each other.
- Follow the product label. It overrides any calculator.
After you add it: order, waits and test limits
Order. Adjusting more than one thing? Do alkalinity first, then pH, then chlorine, with the pump running and at least 20 minutes between, retesting as you go. Keep calcium several hours apart from baking soda or soda ash, since together they can cloud the water. Never pour acid and chlorine in together.
| What you added | Retest after | Good to know |
|---|---|---|
| Liquid chlorine or bleach | About 30 minutes, pump on | Swim once a retest reads 4 ppm or below. |
| Cal-hypo shock | About 30 minutes for chlorine | Wait 24 hours before adjusting pH or alkalinity. Swim once a retest reads 4 ppm or below. |
| Acid (muriatic or dry) | 30 minutes to 2 hours, then the next day | pH creeps back up for a day as the water gives off carbon dioxide. Swim once a retest reads between 7.2 and 7.8. |
| Soda ash (pH up) | About 2 hours | Raises alkalinity too. |
| Baking soda | 6 to 24 hours | Run the pump at least 6 hours first. |
| Calcium chloride | 4 to 8 hours of pump time | Recheck chlorine too. |
| Salt | 24 hours, pump on | Brush the bottom so it dissolves. |
| Stabilizer (CYA) | 3 to 5 days | Don’t backwash for 48 to 72 hours. |
The short waits need the pump running; retesting the next day catches slow drift.
What your test can’t tell you.
- Most pH tests read only 6.8 to 8.2, so a reading at either end may be further out.
- Chlorine above about 10 ppm throws off the pH test and can bleach a chlorine drop test to a false low. Color-match DPD chlorine tests read only to about 5 ppm; an FAS-DPD drop test reads higher.
- Many kits can’t read stabilizer below about 20 ppm.
- Salt generator displays can be off by about 500 ppm; check with a salt test.
- Test strips read calcium only roughly; use a drop kit.
After heavy rain or an overflow, test free chlorine, pH, alkalinity and stabilizer before adding anything. Rain dilutes the water and can move pH either way, so test rather than guess. Strong sun and heat burn chlorine off faster, so in a hot, sunny climate test chlorine daily.
Filling from a well? Test the fill water at the spigot for metals first. Above about 0.2 ppm copper or 0.3 ppm iron, chlorine can turn the metals into stains. Add a metal sequestrant before you chlorinate, wait 3 to 4 days before shocking, and avoid copper-based algaecides.
From: Swim University on chemical order, Trouble Free Pool on adding chemicals, Trouble Free Pool on pH, Taylor Technologies FAQ, Swim University on metals and Trouble Free Pool on metals, plus the product labels and manuals in Sources.
What this doesn't cover
- Pools with borates, which resist pH changes, so expect to need more than this estimate.
- Lowering alkalinity on purpose, which takes repeated acid-and-aerate rounds.
- Spas and hot tubs, which run hotter and have their own ranges.
- Commercial and public pools, which follow your state's health code.
Sources
- Trouble Free Pool, acid: acid strengths sold; dry acid adds sulfate; swim after acid has dispersed.
- Trouble Free Pool calculator: pH estimates are approximate and depend on alkalinity and borates.
- Haviland pH Down label: 12 oz per 10,000 gallons at pH 7.7–8.0; circulate 2 hours and retest.
- Haviland pH Up label: 1 lb per 10,000 gallons at pH 6.8–7.1; circulate 2 hours and retest.
- Clorox Pool&Spa pH Down safety data sheet: 93.2% sodium bisulfate.
- The Pool Nerd acid calculator and pH calculator: 6.4 fl oz per 0.1 pH per 10,000 gallons; 12 oz soda ash for 7.0 to 7.4, at most 1 lb per dose.
- Swim University, lowering pH: a fixed chart by pH band.
- Poolonomics, lowering pH: add about 75% of the dose first, then test.
- Hayward AquaRite manual (Rev B): dry acid can damage the chlorinator cell.
Numbers last reviewed: October 9, 2026.