Quick answer from a licensed plumber: Measure the tank, then use Pentair’s own pairings: a 9×48 tank takes 1.0 cubic foot of resin, a 10×54 takes 1.5, a 12×52 takes 2.0. That is roughly 60 percent of the vessel’s volume (a 9×48 vessel is 1.7 cubic feet), and the rest is room for the bed to expand 50 to 70 percent during backwash, per Purolite. Resin is sold by the cubic foot; a cubic foot weighs about 50 pounds.
Step 1: Measure the Tank
Softener tanks are named by diameter and height in inches, so a “9×48” is 9 inches across and 48 inches tall, not counting the valve. Measure the cylinder’s diameter at the widest point and its height from the floor to the top of the dome, before the valve. Pentair’s Structural spec sheet lists each size with a “TANK HEIGHT” and a taller “OVERALL HEIGHT,” and the overall figure is the one a tape measure gives you: a 9×48 measures about 54 inches overall, a 10×54 about 60 and a 12×48 about 54 (table below).
Step 2: Read the Resin Charge Off the Size
The most direct maker figures come from Pentair’s own 5600SXT softener systems, which it sells in “Three sizes available to meet every customer’s needs – 1.0 ft3, 1.5 ft3, 2.0 ft3.” Its specification table ties each resin charge to a tank:
| Pentair 5600SXT system | Tank | Resin | Pentair’s rating |
|---|---|---|---|
| Item 4005289 (VT-11-01) | 09X48 | 1.0 CF | 11K |
| Item 4005290 (VT-17-01) | 10X54 | 1.5 CF | 17K |
| Item 4005291 (VT-23-01) | 12X52 | 2.0 CF | 23K |
So the rule of thumb the trade uses matches what Pentair ships: a 9×48 tank takes about 1 cubic foot of resin, a 10×54 about 1.5, and a 12×52 about 2. The “K” figures are Pentair’s ratings for those systems as sold; Pentair does not define them on that page, and this page does not convert them to a grains-per-regeneration figure.
Why the Tank Holds More Than the Resin
A resin tank is never filled to the top. Pentair’s Structural sheet gives each vessel’s total volume, and it is well above the resin charge: a 9 x 48 vessel is 12.5 gallons, 1.7 cubic feet; a 10 x 54 is 16.7 gallons, 2.2 cubic feet; a 12 x 48 is 23.4 gallons, 3.1 cubic feet. The gap is working room. Purolite’s data sheet for its C100E softening resin says that during backwash “the resin bed should be expanded in volume between 50 and 70% for at least 10 to 15 minutes,” and warns: “Caution must be taken to avoid loss of resin through the top of the vessel by over expansion of the bed.” Fill a 9×48 with 1.7 cubic feet and the first backwash sends resin down the drain.
| Vessel (Pentair Structural) | Tank height | Overall height | Capacity (gal) | Vessel volume (cu ft) |
|---|---|---|---|---|
| 8 x 35 | 35.0″ | 41.0″ | 7.1 | 0.9 |
| 8 x 44 | 44.0″ | 50.2″ | 9.1 | 1.2 |
| 9 x 40 | 40.0″ | 46.0″ | 10.3 | 1.4 |
| 9 x 48 | 48.0″ | 54.0″ | 12.5 | 1.7 |
| 10 x 44 | 44.0″ | 49.9″ | 13.4 | 1.8 |
| 10 x 47 | 47.0″ | 52.8″ | 14.4 | 1.9 |
| 10 x 54 | 54.0″ | 59.8″ | 16.7 | 2.2 |
| 12 x 48 | 48.0″ | 53.8″ | 23.4 | 3.1 |
| 13 x 54 | 54.0″ | 59.8″ | 27.6 | 3.7 |
Pentair does not list a 12×52 on that sheet; its 5600SXT system uses one, and it sits between the 12×48 (3.1 cubic feet) and 13×54 rows. The pattern holds across the table: the resin charge Pentair ships is roughly 60 to 70 percent of the vessel volume (1.0 of 1.7 cubic feet in a 9×48, 1.5 of 2.2 in a 10×54).
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Step 3: Buy by the Cubic Foot, Not the Pound
Resin is sold by volume. Purolite’s C100E, a “Polystyrenic Gel, Strong Acid Cation Resin, Sodium form, Potable Water Grade,” ships in a “1 ft³ Sack” among other sizes, so a 9×48 refill is one sack and a 10×54 is one and a half. If you are checking a bag by weight instead, Purolite lists the shipping weight at “800 – 840 g/L (50.0 – 52.5 lb/ft³),” so a full cubic foot weighs about 50 pounds and a half-cubic-foot bag about 25.
The number on the resin sheet that matters for softening is capacity. Purolite gives the C100E’s “Total Capacity (min.)” as “1.9 eq/L (41.5 Kgr/ft³) (Na+ form)”; that is the resin’s total exchange capacity, and the working capacity you get in a home softener depends on how much salt the valve uses per regeneration. The sheet’s other figures are the ones to match when you replace resin in an existing tank: bead size “300 – 1200 µm,” and a temperature limit of “120 °C (248.0 °F).”
The Injector Has to Match the Tank Too
If you are changing tank size, the control valve’s brine injector is sized to the tank diameter, not to the resin volume. Pentair’s Fleck 5600SXT service manual lists the injector nozzles and throats by tank: “#000, Brown (6” Tank),” “#00, Violet (7” Tank),” “#0, Red (8” Tank),” “#1, White (9” & 10″ Tank),” “#2, Blue (12” Tank),” “#3, Yellow (13” Tank),” “#4, Green (14” Tank).” A 9×48 and a 10×54 share the #1 injector; a 12-inch tank needs the #2. Pentair’s 5600SXT service manual, in the sources, lists the part numbers, and our Fleck 5600 vs 5600SXT parts guide lays them out by head.
FAQ
How much resin does a 9×48 water softener tank hold?
About 1 cubic foot. Pentair’s own 5600SXT system pairs a 9×48 tank with 1.0 cubic feet of resin; the vessel’s total volume is 1.7 cubic feet, and the rest is room for the bed to expand during backwash.
How much resin does a 10×54 tank hold?
About 1.5 cubic feet. Pentair ships its 10×54 system with 1.5 CF of resin; the vessel itself is 16.7 gallons, or 2.2 cubic feet.
How much resin for a 12×52 tank?
About 2 cubic feet, which is what Pentair puts in its 12×52 5600SXT system.
How much does a cubic foot of softener resin weigh?
Purolite lists the shipping weight of its C100E softening resin at 50.0 to 52.5 pounds per cubic foot, so a 1-cubic-foot sack is about 50 pounds.
Why can’t I fill the resin tank all the way?
Because the bed has to expand during backwash. Purolite says the resin bed should expand 50 to 70 percent in volume during backwash and warns against losing resin out of the top of the vessel. Pentair’s systems fill a tank to roughly 60 to 70 percent of its volume.
Why Trust Plumbing Picks
Plumbing Picks is written by a licensed Georgia journeyman plumber. Every part number, model list, size, setting and fit on this page comes from the manufacturer pages and documents listed below, which we opened and read; nothing here comes from hands-on testing. Nobody pays to be included here. More on who writes this and how we pick.
Sources
- Pentair: 5600SXT Series Softener (system sizes)
- Pentair: Structural FRP Vessels spec sheet (41422)
- Purolite: C100E strong acid cation resin, sodium form, potable water grade
- Pentair: Fleck 5600SXT Downflow Service Manual (42684)
Bottom Line
Diameter and height give you the tank; the tank gives you the charge: 1.0 cubic foot for a 9×48, 1.5 for a 10×54, 2.0 for a 12×52. Leave the top of the vessel empty for backwash, buy by the cubic foot, and match the injector to the tank diameter if the tank size changes.