Free Calculator · Published September 2026

Acid Neutralizer Sizing Calculator for Well Water

Enter the pH, alkalinity and peak flow to get the media (calcite or a Corosex blend), the tank size, the backwash your well has to supply, how much media dissolves each year and how much hardness it adds.

Built by SubcontractorHub — the software plumbing contractors use to quote, finance, and run every job.

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What Acid Neutralizer Does This Well Need?

pH picks the media. Peak flow picks the tank. The well pump decides whether that tank can actually be backwashed.

From a lab test or a calibrated meter, measured at the well before any treatment.

On most standard well panels. Acidic wells are usually low, often 10–60 mg/L.

If entered, this overrides the estimate from pH and alkalinity.

Divide mg/L by 17.1 to get grains per gallon.

Drives annual gallons at 80 gal per person per day.

Sustained output at pressure-switch cut-in, not the pump’s nameplate maximum.

📋 Important: All calculator results are ballpark estimates

The figures shown are approximate estimates based on carbonate chemistry and typical published tank ratings, which vary by manufacturer. They are not a substitute for a lab water analysis or the equipment spec sheet. Always confirm media choice, tank size and backwash rate with a licensed installer or certified water treatment professional.

How Acid Neutralizer Sizing Works

Why well water is acidic, and why it matters

Rainwater picks up carbon dioxide from the air and the soil, and in areas with granite or sandy geology there is little limestone in the ground to neutralize it. The result is well water carrying carbonic acid, usually somewhere between pH 5.5 and 6.5. The EPA secondary standard for drinking water is pH 6.5 to 8.5.

Acidic water is not usually harmful to drink on its own terms. The problem is what it dissolves. Blue-green staining on fixtures is copper coming out of the pipes, and pinhole leaks in copper lines are the same corrosion further along. Water that dissolves copper can dissolve lead from older solder and brass too, and that leaves no stain. Acidic water is one of the most common reasons a well owner calls a water treatment dealer or a plumbing contractor in the first place.

Step 1: pH picks the media

Raw pHTypical mediaNotes
6.0–6.9100% calciteSelf-limiting, rarely overcorrects
5.6–5.9~80% calcite / 20% MgOCorosex does the heavy lifting
5.0–5.5~70% calcite / 30% MgOWatch for overshoot after the water sits
Below 5.0Soda ash chemical feedMedia tanks struggle to keep up

Ratios are common starting points, not fixed rules, and dealers adjust them after the first follow-up pH test. Magnesium oxide neutralizes about five times as much CO2 per pound as calcite, which is why a fifth of the bed changes the result so much, and also why too much of it pushes the pH past 8.

Step 2: peak flow picks the tank, the pump decides if it works

TankMediaService flow (cont. / peak)Backwash
9 x 481 cu ft3.5 / 5 gpm4 gpm
10 x 541.5 cu ft5 / 7 gpm5 gpm
12 x 522 cu ft7 / 9 gpm7 gpm
13 x 542.5 cu ft8 / 10 gpm9 gpm
14 x 653 cu ft10 / 12 gpm11 gpm

These are typical published ratings and they vary by manufacturer and valve. Calcite needs slow flow for contact time, so the continuous rating is lower than filter tanks of the same size. The backwash figure is the number that gets missed: if the pump cannot hold it for the whole cycle, the bed never lifts, packs solid and starts channeling. On a weak well, an upflow (non-backwashing) neutralizer is the usual workaround.

Step 3: the formula for media use and added hardness

Free CO2 is estimated from pH and alkalinity using the carbonic acid equilibrium, then the calculator solves for how much CO2 has to be consumed to bring the water to pH 7.0:

CO2 (mg/L) = 0.88 × alkalinity × 10^(6.35 − pH)

Calcite: CaCO3 + CO2 + H2O → Ca(HCO3)2  (2.27 ppm hardness per ppm CO2)

MgO: MgO + 2CO2 + H2O → Mg(HCO3)2  (1.14 ppm hardness per ppm CO2)

Media lb/yr = dissolved mg/L × gallons per year × 8.34 ÷ 1,000,000

Hardness added in grains per gallon is the ppm figure divided by 17.1. Top-up timing assumes the bed is refilled once about 30% of the starting charge (roughly 100 lb per cubic foot) has dissolved. Cold groundwater and real contact time both move the answer, so treat it as an estimate and check bed depth at every service.

A worked example

Two-bathroom house, three people, on a well testing pH 6.2, alkalinity 40 mg/L, hardness 5 gpg, some iron. The pump holds 10 gpm.

Media: pH 6.2 is in the calcite range, so 100% calcite

Free CO2 = 0.88 × 40 × 10^0.15 = about 50 ppm

CO2 consumed to reach pH 7.0 = about 29 ppm, dissolving about 66 mg/L of calcite

Hardness added = 66 ÷ 17.1 = about 3.8 gpg, so roughly 8.8 gpg to the softener

Water use = 3 × 80 × 365 = 87,600 gal, so about 48 lb of calcite a year

Tank: 9 gpm peak → 2.0 cu ft in a 12x52, 7 gpm backwash (pump OK), top-up about every 15 months

The detail the homeowner would never guess is the softener. Raw hardness was a borderline 5 gpg; after the neutralizer it is close to 9, so the softener goes after the neutralizer and gets sized on the higher number. The iron filter also sits after the neutralizer, where the raised pH helps it work. See the iron filter for well water guide for choosing that stage and how to test water hardness to confirm the new figure after install.

For dealers: neutralizers are a top-up business

A neutralizer is built to dissolve, which makes it one of the few pieces of water treatment equipment where the customer runs out of the product on a predictable curve. Low pH, busy households and blend beds all shorten that curve, and a bed left to shrink quietly lets the pH drift back down and the copper start moving again.

That turns every install into a scheduled visit: a pH check, a bed depth reading, a bag or two of media and a look at the softener downstream. Keeping those dates across a book of wells is what software for water treatment dealers is for.

Frequently Asked Questions

What size acid neutralizer do I need for well water?

Size a backwashing acid neutralizer on peak flow first, then confirm the well can supply the backwash rate. Typical published ratings are about 5 gpm peak for a 1.0 cubic foot tank in a 9x48 shell, 7 gpm for 1.5 cubic feet in a 10x54, 9 gpm for 2.0 cubic feet in a 12x52, 10 gpm for 2.5 cubic feet in a 13x54 and 12 gpm for 3.0 cubic feet in a 14x65. Backwash needs roughly 4, 5, 7, 9 and 11 gpm respectively. A two-bathroom home peaking near 9 gpm usually lands on a 2.0 cubic foot tank, but only if the pump can deliver 7 gpm for the whole backwash. Ratings vary by manufacturer, so confirm against the spec sheet.

When do you need Corosex instead of calcite?

Calcite on its own is the standard answer for water between about pH 6.0 and 6.9. It is slow to react and self-limiting, so it rarely overcorrects. Below about pH 6.0 calcite cannot dissolve fast enough to finish the job in the contact time a residential tank allows, so a portion of magnesium oxide (sold as Corosex, among other names) is added. A common starting point is around 80/20 calcite to magnesium oxide for pH 5.6 to 5.9 and 70/30 for pH 5.0 to 5.5. Below about pH 5.0 most dealers move to a soda ash chemical feed pump rather than a heavier media blend.

Does an acid neutralizer make water harder?

Yes. The neutralizer works by dissolving calcium carbonate, and magnesium oxide if it is a blend, into the water, and both are hardness minerals. By stoichiometry, each part per million of CO2 neutralized by calcite adds about 2.27 ppm of hardness as calcium carbonate, and each ppm neutralized by magnesium oxide adds about 1.14 ppm. In practice that is often 1 to 5 grains per gallon, more on lower pH water. If a softener is in the system it must go after the neutralizer and be sized to the higher, post-neutralizer hardness.

How often does acid neutralizer media need to be replaced?

Neutralizer media is not replaced on a schedule, it is topped up, because it is designed to dissolve. The rate depends on how much CO2 the water carries and how many gallons go through the tank. Mildly acidic water on a small household can go a year or more between top-ups, while pH 5.5 water on a busy house can use a noticeable share of the bed in a few months. Most dealers check bed depth at every 6 to 12 month service visit and add media when it has dropped by roughly a quarter to a third. Magnesium oxide beds can also cement and need to be broken up or rebedded.

What pH should well water be after a neutralizer?

Aim for about pH 7.0 to 7.5. The EPA secondary standard for drinking water is pH 6.5 to 8.5, and water below 6.5 is where copper and lead plumbing start to corrode. Calcite tends to settle the water near 7.0 because it stops dissolving as the water approaches saturation. Blends with magnesium oxide can overshoot, especially after the water has sat in the tank overnight, so a first draw pH of 8 or higher is a sign the magnesium oxide share is too high for the water.

What do blue-green stains mean on well water fixtures?

Blue-green staining in sinks, tubs and around fittings is copper being dissolved out of the plumbing by acidic, corrosive water. It is the most common visible sign that a well needs a neutralizer. The same water can also pick up lead from older solder and brass fittings, which leaves no stain. The EPA action level for copper at the tap is 1.3 mg/L. If you see the stains, test pH, alkalinity, copper and lead together before choosing equipment, and run the tap before drinking until the water has been treated.

Does the acid neutralizer go before or after the water softener?

Before. The normal order is sediment filter, acid neutralizer, iron filter, water softener, then UV if disinfection is needed. The neutralizer goes early because raising pH helps the iron filter work (many iron media need pH near 6.8 or higher), and it must go ahead of the softener because the neutralizer adds hardness that the softener then removes. A softener placed first would send hard water straight back into the house.

Every Neutralizer Install Is a Media Top-Up on the Calendar

pH checks, media top-ups and the softener service downstream only become recurring water purification revenue if something is tracking them. Quote the full treatment train on site and put every install on a service schedule.

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Related Resources

All calculations are estimates based on historical information and should be verified by the user. This tool is provided as a free service for planning purposes only and is not a substitute for a professional assessment, a supplier quote, or applicable local code requirements. Tank ratings are approximate and vary by manufacturer; confirm media and backwash rates against the equipment spec sheet and a current water analysis.