Dry Acid Calculator: Sodium Bisulfate Dose
To lower total alkalinity from 110 to 100 ppm in a 15,000-gallon pool, add about 3.2 lb of 93.2% dry acid (sodium bisulfate). The same job takes about 38 fl oz of 31.45% muriatic acid, so pick by handling and cost, not by strength.
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)
Dry acid is the granular acid sold as "pH down", "pH decreaser", or "alkalinity decreaser". Read the active ingredient on the bag: if it says sodium bisulfate (also called sodium hydrogen sulfate), this calculator fits it. Enter your gallons and readings, choose whether you want to lower alkalinity or pH, and the calculator gives the weight to add.
How much dry acid per 10 ppm of alkalinity
The table shows the amount that lowers total alkalinity (TA) by 10 ppm, next to the muriatic acid that does the same job. In 10,000 gallons that is about 2.1 lb of dry acid; in 1,000 gallons, about 3.4 oz. The amounts are rounded down, so you never add more than the math says.
| Pool size | Dry acid (sodium bisulfate) 93.2% | Muriatic acid 31.45% |
|---|---|---|
| 500 gal | 1.7 oz | 1.2 fl oz |
| 1,000 gal | 3.4 oz | 2.5 fl oz |
| 2,000 gal | 6.8 oz | 5.1 fl oz |
| 3,000 gal | 10.3 oz | 7.6 fl oz |
| 4,000 gal | 13.7 oz | 10 fl oz |
| 5,000 gal | 1.05 lb | 12 fl oz |
| 6,000 gal | 1.25 lb | 15 fl oz |
| 7,000 gal | 1.5 lb | 17 fl oz |
| 8,000 gal | 1.7 lb | 20 fl oz |
| 10,000 gal | 2.1 lb | 25 fl oz |
| 12,000 gal | 2.55 lb | 30 fl oz |
| 15,000 gal | 3.2 lb | 38 fl oz |
| 20,000 gal | 4.25 lb | 51 fl oz |
| 25,000 gal | 5.35 lb | 64 fl oz |
| 30,000 gal | 6.4 lb | 76 fl oz |
| 40,000 gal | 8.55 lb | 102 fl oz |
Each size links to its full dose chart.
Acid lowers pH and TA together. To bring TA down without leaving pH too low, add one dose, keep pH at 7.2 or above, then aerate (run jets, fountains, or a spillover) to push pH back up. Aeration raises pH without raising TA. Repeat every day or two until TA is where you want it. The full cycle is on how to lower alkalinity in a pool.
Lowering pH with dry acid
Switch the calculator's first field to pH. The pH dose depends on more than gallons: high alkalinity resists the change, and stabilizer (cyanuric acid, CYA) changes the math a little. That is why the calculator asks for TA and CYA. Lower pH by no more than about 0.4 in one step, then retest after the water has mixed. For the pH targets and the reasons pH keeps rising, see how to lower pH in a pool.
If you prefer a liquid, the muriatic acid calculator uses the same water model. Both give the same result in the water. Always follow the product label.
Dry acid vs muriatic acid
| Question | Dry acid (sodium bisulfate, 93.2%) | Muriatic acid (hydrochloric, 31.45%) |
|---|---|---|
| Amount to lower TA by 10 ppm in 15,000 gal | 3.2 lb | 38 fl oz |
| Form | Granules, measured by weight | Liquid, measured by volume |
| Fumes | No fumes (its maker says so); dust when poured | Strong fumes that corrode nearby metal |
| What it leaves in the water | Sulfate | Chloride (the same ion as in salt) |
| Storage | Keep dry, sealed, and below 120 °F (per its safety data sheet) | Keep sealed, in a ventilated area, and away from metal |
| Common use | Hot tubs and small pools | Large pools |
Per job, muriatic acid usually costs less, but compare your own prices: multiply each product's price per pound or per fluid ounce by the amount the table says you need. Dry acid is easier to store, measure, and carry, which is why it is the usual pH decreaser for hot tubs. If you have a hot tub with high pH, see hot tub pH too high.
Sulfate: the long-term trade-off
Each pound of dry acid leaves sulfate behind. Sulfate does not evaporate or get used up; it leaves only when water splashes out, drains, or is replaced. Over years, heavy dry-acid use raises total dissolved solids (TDS). Salt pools are the main concern: Hayward's AquaRite salt chlorinator manual warns that dry acid can leave by-products that damage the chlorinator cell, and discourages it especially in dry climates where water is rarely replaced. We could not confirm a published sulfate limit from a cell or plaster maker; the numbers that circulate online come from forums.
For a pool that uses a little acid each season, this rarely matters. For a pool that needs acid every week, and for any salt pool, muriatic acid avoids the sulfate build-up. Hot tubs are drained and refilled every few months, so sulfate resets with each refill.
How to add dry acid, step by step
- Test pH and TA, and enter both in the calculator with your gallons.
- Weigh the dose on a kitchen or postal scale. Scoops and cups vary.
- Run the pump. Follow your label for how to add it: broadcast over the water, or dissolved first in a plastic bucket of pool water. If you dissolve it, add the acid to the water, not the water to the acid.
- Pour slowly into the deep end or in front of a return jet, away from the skimmer, and close to the water surface so dust does not blow back.
- Let it circulate for at least a few hours, then retest pH and TA.
- Wait until the dose has mixed before anyone swims, and follow your product's label for swim timing.
Safety
Never mix dry acid with chlorine of any kind, in a bucket or in a feeder: the two release chlorine gas. Store acid and chlorine apart, not on the same shelf. Add acid to water, never water to acid. Wear gloves and eye protection, and pour low and with the wind at your back so dust stays out of your face. Keep the bag closed and dry. If acid gets in eyes or on skin, rinse with plenty of water and call Poison Control at 1-800-222-1222. Always follow the product label. More in our pool chemical safety guide.
FAQ
How much dry acid lowers alkalinity by 10 ppm?
About 2.1 lb of 93.2% sodium bisulfate per 10,000 gallons, or 3.4 oz per 1,000 gallons. The calculator scales it to your pool. Always follow the product label.
Is dry acid the same as pH down?
Usually, yes. Most granular "pH down" and "pH decreaser" products are sodium bisulfate. Check the active ingredient on the label; if it lists a different acid, the doses here do not apply.
Is dry acid stronger than muriatic acid?
Not in any way that matters to the pool. You need more pounds of dry acid than fluid ounces of muriatic acid, but both lower pH and TA by the same amount when you add the doses shown here.
Does dry acid raise TDS?
Yes, slowly. It leaves sulfate, which stays until water is replaced. For a pool that needs acid often, muriatic acid avoids that build-up.
Sources
- Doses and the comparison table: our engine's acid model (sodium bisulfate 93.2%, 120.06 g/mol); see methodology.
- CDC, Home pool and hot tub water treatment and testing (pH 7.0–7.8): cdc.gov
- HASA, Dry acid safety data sheet No. 207 (93% sodium bisulfate, storage, dust): PDF
- Jones-Hamilton, sodium bisulfate for pools and spas (no fumes): jones-hamilton.com
- Hayward, AquaRite salt chlorination system owner's manual (dry acid and the cell): PDF
- CDC, pool chemical injuries: archive.cdc.gov