Calculators
Sizing calculators for home water treatment: grain capacity, flow rate, micron rating and more, with the formula shown.
Inputs you already have, one formula, one answer. Every calculator shows its working and names the source the formula came from, so you can check it rather than trust it.
Water softeners 5 pages
Size
Softened water sodium calculator: mg per glass
Sodium added = 8 mg/L per gpg of hardness removed (NDSU). 15 gpg softened water carries 120 mg/L: 28 mg per 8 oz glass, 240 mg in 2 L a day.
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Water softener salt usage calculator: bags per year
Salt per regeneration = resin cu ft x dose (6, 10 or 15 lb/cu ft). A 1 cu ft unit at 10 lb, 4 people, 10 gpg uses about 406 lb a year: ten 40 lb bags.
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Water hardness unit converter: gpg, ppm, dH, mmol/L
1 gpg = 17.118 mg/L as CaCO3, 1 dH = 17.848, 1 fH = 10, 1 Clark = 14.254, 1 mmol/L = 100.09. Convert any hardness unit to the others and read the USGS band.
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Softener resin volume to grain capacity calculator
Usable grains = resin cu ft x grains per cu ft at the salt dose. 1.0 cu ft of 8% resin gives 20,000 grains at 6 lb, 24,000 at 10 lb. Calculator and tank table.
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Water softener size calculator: grain capacity
Grains = people x 75 gal/day x (gpg + 5 per mg/L iron) x days between regenerations. 4 people at 10 gpg for 7 days is 21,000 grains. Calculator and the size table.
Well water 5 pages
Size
Well pump size calculator: GPM by fixture count
Pump gpm = fixtures x 1 gpm (Water Systems Council, Red Lion): an 8-fixture house takes an 8 gpm pump, never above the well yield, submersible past 90 ft.
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How much bleach to shock a well: calculator
Casing gallons = 0.041 x diameter squared x feet of water. A 6-inch well with 100 feet holds 148 gal and takes about 4 cups of 6% bleach at 100 ppm.
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Iron filter size calculator: tank vs pump GPM
Largest tank = pump GPM divided by the media backwash rate. A 10 GPM pump backwashes a 12-inch Birm tank (9.4 GPM needed) and stalls on 13-inch (11.1 GPM).
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Acid neutralizer size calculator: calcite tank
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.
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UV water purifier size calculator: GPM by dose
Peak gpm = bathrooms x 2.5 + 2.2; a 2-bath house pulls 7.2 gpm. Size to the 40 mJ/cm2 Class A rating: a VIQUA VH410 sold as 18 GPM is 14 gpm at that dose.
Filters and cartridges 2 pages
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Water filter life calculator: gallons and days
Days = rated gallons / gallons per day, capped by the maker's months. A 40-gallon, 2-month Brita Standard at 0.5 gal/day lasts 60 days, not 80.
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Micron rating picker: which sediment filter micron
Coarse to fine: a 700 to 15 micron screen, then a 5 micron cartridge (NDSU: 10 to 15 micron for a drilled well, 2 for a dug well), absolute 1 micron for cysts.
Drinking water 1 pages
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Water distiller cost per gallon calculator: kWh
kWh per gallon = watts x hours per batch / gallons. An 800 W distiller at 4 hours a gallon uses 3.2 kWh, about 59 cents at EIA's July 2026 18.31 cents per kWh.
Reverse osmosis 1 pages
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Reverse osmosis GPD and tank size calculator
Effective GPD = rated GPD x (psi / 60) x (1 + 0.015 x (F - 77)). A 75 GPD unit at 50 psi, 60 F makes about 47 GPD from a 3.2 gal tank. Calculator.
Whole-house systems 1 pages
Size
Whole-house filter flow rate calculator: GPM
Peak gpm = showers running at once x 2.5 + one faucet at 2.2, or WQA fixture units x 0.4 + 2. A 2-bath home pulls about 7 gpm; a 20 x 4.5 in housing is rated to 15.