Acid neutralizer size calculator: calcite or Corosex tank size from pH and peak flow
Bed area = peak gpm / 5 for calcite (pH 6.0-7.0) or / 3 for a calcite-Corosex blend. A 2.5 gpm home at pH 6.8 needs a 10x54 tank and 98 lb of calcite.
- Verified
- Updated
- 7 min read
- 4 sources
Shop links on this page are affiliate links: a purchase may earn us a commission at no extra cost to you. Disclosure
Short answer
An acid neutralizer tank is sized from peak service flow, not from pH alone: the pH only picks the media. Divide peak gallons per minute (gpm) by the media’s design service-flow rate to get the bed area in square feet, then take the smallest tank whose cross-section meets it. Clack’s Calcite data sheet publishes a service flow of 3-6 gpm per square foot and a pH range of 5.0-7.0; its Corosex sheet publishes 5-6 gpm per square foot and pH 4.5-6.0. This calculator uses 5 gpm per square foot for calcite alone (pH 6.0-7.0) and 3 gpm per square foot for a 50/50 calcite and Corosex blend (pH 4.5-5.9). Worked example: pH 6.8 and 2.5 gpm needs 2.5 / 5 = 0.50 sq ft, which a 10x54 tank (0.545 sq ft) covers, holding 98 lb of calcite at Clack’s 24 in minimum bed depth and 90 lb per cubic foot. Both media raise hardness, per the same sheets.
If your pH reading came from a sample that sat in a bottle for a day, stop here and retest at the tap: a stored sample drifts toward neutral, and a tank sized to it comes out one size too small.
Media, bed area, tank size, pounds of media and backwash gpm from pH and peak flow
Media by pH: calcite alone 6.0-7.0 (Clack Calcite sheet range 5.0-7.0), 50/50 calcite and Corosex 4.5-5.9 (Corosex sheet range 4.5-6.0), below 4.5 out of range for both. Design rates: 5 gpm per sq ft calcite, 3 gpm per sq ft blend. Bed area of a tank = pi x (nominal diameter / 24)^2, a stated assumption, not a Clack figure. Media volume = area x 2 ft; calcite 90 lb per cu ft, Corosex 75 lb per cu ft. Backwash 8-12 gpm per sq ft calcite, 10-12 blend. Tanks from Clack's mineral tank chart, Form 2902.
What is the formula?
Bed area in square feet = peak gpm / design service-flow rate. The tank is the smallest one whose cross-section meets that area; the media fill is area x 2 ft, converted to pounds by bulk density.
| Variable | What it is | Where the figure comes from |
|---|---|---|
| Peak service flow | the most gpm the house draws at once | the whole-house filter flow rate calculator, or a bucket test at the pressure tank |
| Design service-flow rate | gpm each square foot of bed handles in service | Clack Calcite sheet 3-6 gpm per sq ft, Corosex sheet 5-6; this page uses 5 for calcite alone, 3 for the blend |
| Bed area of a tank | pi x (nominal diameter in inches / 24)^2 | geometry; nominal diameter stands in for bed diameter, which Clack’s chart does not print |
| Bed depth | 24 in minimum, 30 in maximum, plus 50% freeboard | Clack Calcite and Corosex sheets |
| Bulk density | calcite 90 lb per cu ft, Corosex 75 lb per cu ft | Clack Calcite and Corosex sheets |
| Backwash rate | calcite 8-12 gpm per sq ft, Corosex 10-12 | Clack Calcite and Corosex sheets |
Service flow is what passes through the bed while a tap runs; backwash is the reverse flow the control valve sends to the drain to lift the bed. Both scale with area, so the wider tank always needs the bigger drain.
Which media does the pH call for?
| pH at the tap | Media | Design rate | Backwash | Why |
|---|---|---|---|---|
| 6.0-7.0 | calcite alone | 5 gpm per sq ft | 8-12 gpm per sq ft | inside Clack’s 5.0-7.0 calcite range with margin |
| 4.5-5.9 | 50/50 calcite and Corosex by volume | 3 gpm per sq ft | 10-12 gpm per sq ft | Clack’s Calcite sheet says blending in Corosex increases the ability to correct low pH; the Corosex sheet requires at least 50-50 with calcite for downflow service unless hardness is under 5 gpg |
| below 4.5 | neither | out of range | out of range | below the Corosex floor of 4.5 and the calcite floor of 5.0; chemical feed is the class |
The blend runs at the low end of calcite’s published range because calcite is the slower-reacting half, per Clack’s Calcite sheet, and contact time is what corrects the pH. Clack publishes no ratio table between 50-50 and 100% Corosex, so this page does not invent one.
What does a worked example look like?
Take pH 6.8 and 2.5 gpm peak flow, the calculator’s default.
- Media: pH 6.8 is inside 6.0-7.0, so calcite alone at 5 gpm per sq ft.
- Bed area: 2.5 / 5 = 0.50 sq ft.
- Tank: a 10x54 gives pi x (10 / 24)^2 = 0.545 sq ft, the smallest that covers 0.50. A 9x48 gives 0.442 and misses.
- Media volume: 0.545 x 2 ft = 1.09 cu ft, well inside the 2.14 cu ft Clack rates the 10x54 at, leaving the 50% freeboard.
- Weight: 1.09 x 90 = 98 lb of calcite, so two 50 lb bags.
- Backwash: 0.545 x 8 to 12 = 4.4-6.5 gpm at the drain.
At pH 5.6 and 2.3 gpm the blend rate applies: 2.3 / 3 = 0.767 sq ft, a 12x52 (0.785 sq ft), 1.57 cu ft of media as 71 lb calcite plus 59 lb Corosex, and 7.9-9.4 gpm of backwash.
Which tank fits each flow?
Capacities and heights are from Clack’s mineral tank chart, Form 2902; the areas and flow ceilings are this page’s arithmetic from nominal diameter.
| Tank | Rated capacity | Bed area | Max gpm, calcite at 5 | Max gpm, blend at 3 | Calcite at 24 in |
|---|---|---|---|---|---|
| 9x48 | 1.54 cu ft | 0.442 sq ft | 2.2 | 1.3 | 80 lb |
| 10x54 | 2.14 cu ft | 0.545 sq ft | 2.7 | 1.6 | 98 lb |
| 12x52 | 3.1 cu ft | 0.785 sq ft | 3.9 | 2.4 | 141 lb |
| 13x54 | 3.7 cu ft | 0.922 sq ft | 4.6 | 2.8 | 166 lb |
| 16x53 | 5.48 cu ft | 1.396 sq ft | 7.0 | 4.2 | 251 lb |
| 18x65 | 9.83 cu ft | 1.767 sq ft | 8.8 | 5.3 | 318 lb |
Clack’s chart lists the tanks as certified to NSF/ANSI 44 and NSF/ANSI/CAN 61 and 372, rated to 150 psi and 120°F. Both media are listed as certified to NSF/ANSI/CAN Standard 60 on their sheets, calcite (Form 2386) and Corosex (Form 2616).
Before you order media
A 10 lb bag of calcite and Corosex media is a top-up for an existing tank, not a fill: the 10x54 example above needs 98 lb, which is two 50 lb bags from a dealer. Buy the small bag only if a tank is already in the basement and the bed has dropped below the 24 in minimum. If you are choosing the tank itself, the acid neutralizer review compares the assembled units against these same sheets.
Who should not use this answer?
- pH below 4.5: outside both media; see the chemical feed section of the blue-green stains guide.
- Municipal supply: EPA’s secondary standard puts pH at 6.5-8.5, and treated city water is normally inside it already.
- Hardness of 5 gpg or more wanting Corosex alone downflow: Clack’s Corosex sheet requires the 50-50 blend, which is what this page sizes; upflow tanks are not sized here.
- Anyone expecting a refill interval: Clack’s sheets say the bed is periodically replenished and publish no consumption rate, so this page does not state one.
Every gallon that leaves a calcite tank is harder than it went in, per both Clack sheets. Retest hardness after the tank is running, then use the water softener size calculator if the number warrants one. Backwash and drain rules for iron media differ, and the iron filter backwash calculator covers them.
Where do the numbers come from?
- pH ranges, service flow, backwash rate, bed depth, freeboard, bulk density, the hardness increase and the bag sizes: Clack’s Calcite sheet (Form 2386) and Corosex sheet (Form 2616).
- Tank capacities, heights, pressure and temperature ratings and NSF listings: Clack’s Mineral Tanks sheet (Form 2902).
- pH 6.5-8.5: EPA’s secondary drinking water standards.
- Bed area from nominal diameter, the 5 and 3 gpm per sq ft design points and the flat 50/50 blend: this page’s stated assumptions, not manufacturer figures.
Frequently asked questions
What pH range does calcite alone handle?
Clack's Calcite data sheet publishes a water pH range of 5.0 to 7.0, a service flow of 3 to 6 gpm per square foot and a backwash rate of 8 to 12 gpm per square foot. This calculator uses calcite alone from pH 6.0 up and switches to a calcite and Corosex blend below that, because Clack says blending increases the ability to correct low pH.
When do I need to blend in Corosex?
Clack's Corosex data sheet gives Corosex a pH range of 4.5 to 6.0, and the Calcite sheet says combining the two adds Corosex's high-flow neutralizing to calcite's slower low-flow reaction. The Corosex sheet also says downflow service needs hardness under 5 grains per gallon or a blend of at least 50-50 with calcite, which is the ratio this page sizes.
Will a calcite neutralizer make my water harder?
Yes. Both Clack data sheets carry the same sentence: the media will increase hardness and a softener may become necessary after the neutralizing filter. Calcite is calcium carbonate and Corosex is magnesium oxide, so every point of pH gained is dissolved mineral added. Retest hardness after installation and size any softener with the water softener size calculator on this site.
How much backwash flow does the tank need?
Bed area times the media's backwash rate. Clack publishes 8 to 12 gpm per square foot for calcite and 10 to 12 for Corosex, so a 10x54 tank (0.545 sq ft) of calcite needs 4.4 to 6.5 gpm at the drain, and a 12x52 blend tank (0.785 sq ft) needs 7.9 to 9.4 gpm. Clack publishes no drain pipe size table, so check the drain line's rating against local code.
Can I use calcite below pH 4.5?
No. Clack's Corosex floor is pH 4.5 and its Calcite floor is 5.0, so a reading below 4.5 is outside both published ranges and this calculator stops rather than extrapolating. The treatment class for that water is a chemical feed system dosing soda ash, which the blue-green stains guide on this site covers alongside the extension-service context.
Fact sheet and how to cite this page
- Answer
- A home drawing 2.5 gpm at pH 6.8 needs 2.5 / 5 = 0.50 sq ft of calcite bed, which a 10x54 tank (0.545 sq ft) covers with 98 lb of calcite at Clack’s 24 in minimum bed depth and 90 lb per cubic foot.
- Formula
bed area (sq ft) = peak gpm / design service-flow rate (5 gpm per sq ft calcite, 3 gpm per sq ft 50/50 blend); media (cu ft) = bed area x 2 ft; calcite lb = cu ft x 90, Corosex lb = cu ft x 75- Inputs
- pH: 6.8
- Peak service flow: 2.5 gpm
- Media: calcite only (pH 6.0-7.0)
- Output
- 0.50 sq ft required; 10x54 tank (0.545 sq ft); 98 lb calcite; 4.4-6.5 gpm backwash
- Assumptions
- Nominal tank diameter stands in for bed diameter; Clack’s tank chart gives capacity, not internal bed diameter, so the true area is slightly smaller and the sizing carries a small margin
- Calcite alone at 5 gpm per sq ft sits inside Clack’s published 3-6 range; the 50/50 blend uses 3 gpm per sq ft, the low end, because calcite is the slower-reacting half
- A flat 50/50 blend by volume is used from pH 4.5 to 5.9; Clack publishes no ratio table beyond the 50-50 downflow floor on the Corosex sheet
- Backwash gpm is bed area x 8-12 for calcite and x 10-12 for a blend, Clack’s published rates
- Method
- The calculator picks the media from pH against Clack’s published ranges, divides peak gpm by that media’s design service-flow rate for the bed area, selects the smallest Clack mineral tank whose cross-section meets it, multiplies the area by the 24 in minimum bed depth for media volume, and converts to pounds with the published bulk densities.
- Primary source
- Clack Corporation, Calcite data sheet, Form No. 2386 (PDF) (manufacturer or vendor document)
- Facts verified
- Last updated
- Cite as
- OwnerSpec, "Acid neutralizer size calculator: calcite or Corosex tank size from pH and peak flow", https://ownerspec.com/water/calculators/acid-neutralizer-calcite-size-calculator/, facts verified 14 September 2026.
Every figure above also appears, with its source, in the article. All sources · How OwnerSpec verifies numbers
Sources
- Clack Corporation, Calcite data sheet, Form No. 2386 (PDF) (manufacturer or vendor document)
- Clack Corporation, Corosex data sheet, Form No. 2616 (PDF) (manufacturer or vendor document)
- Clack Corporation, Mineral Tanks spec sheet, Form No. 2902 (PDF) (manufacturer or vendor document)
- US EPA, Secondary Drinking Water Standards: Guidance for Nuisance Chemicals (government or regulatory source)