How we calculate

Every calculator on HomeWaterMath uses the formulas and constants below. Each one links to its source and the date we last checked it.

Water softener size calculator

Grains per day = people × gallons per person per day × hardness in gpg

Grains needed = grains per day × days between regenerations

Water softener cost calculator

10-year cost = upfront + 10 × (yearly salt + yearly service)

Water softener salt calculator

Salt per regeneration = cubic feet of resin × salt setting (lb per cubic foot)

Regenerations per year (metered valve) = 365 × grains per day ÷ softener capacity at that salt setting

Water hardness converter

gpg = mg/L ÷ 17.1

mg/L = gpg × 17.1

mmol/L = mg/L ÷ 100.1

German degrees (°dH) = mg/L ÷ 17.85

French degrees (°fH) = mg/L ÷ 10

Well pressure tank size calculator

Drawdown needed = pump gpm × minimum run time (1 minute up to 3/4 hp, 2 minutes above)

Tank size = drawdown needed ÷ that share

Constants

Constant	Value	Unit	Source	Checked
mg/L per grain per gallon	17.1181	mg/L as CaCO3	NIST Handbook 44 (2026), Appendix C, WQA Ion Exchange Fact Sheet	2026-10-20
mg/L per mmol/L	100.087	mg/L as CaCO3	Sigma-Aldrich, water hardness test (product 37194)	2026-10-20
mg/L per German degree	17.848	mg/L as CaCO3	Sigma-Aldrich, water hardness test (product 37194)	2026-10-20
mg/L per French degree	10	mg/L as CaCO3	Sigma-Aldrich, water hardness test (product 37194)	2026-10-20
Sodium added per gpg removed	7.864	mg/L per gpg	University of Nebraska-Lincoln Extension, NebGuide G1274 (2016), Penn State Extension, Water Softening	2026-10-20
Water use per person per day	75	gallons	WQA, Residential Water Treatment Design (2025), Nelsen Corp., Softener Sizing, CSI Water Treatment CT24/CT32 softener manual	2026-10-20
Hardness added per ppm of iron	4	gpg per ppm	Nelsen Corp., Softener Sizing, CSI Water Treatment CT24/CT32 softener manual	2026-10-20
Maximum iron for a softener	2	ppm	Nelsen Corp., Softener Sizing	2026-10-20
Days between regenerations (common default)	7	days	Penn State Extension, Water Softening, Nelsen Corp., Softener Sizing	2026-10-20
Capacity at 6 lb per cu ft	20,000	grains per cu ft	A.O. Smith C43 Series spec sheet, Water Conditioning & Purification, Guidelines for Designing Water Softeners (2004)	2026-10-20
Capacity at 9 lb per cu ft	25,000	grains per cu ft	A.O. Smith C43 Series spec sheet, Water Conditioning & Purification, Guidelines for Designing Water Softeners (2004)	2026-10-20
Capacity at 12 lb per cu ft	27,500	grains per cu ft	Water Conditioning & Purification, Guidelines for Designing Water Softeners (2004)	2026-10-20
Capacity at 15 lb per cu ft	30,000	grains per cu ft	A.O. Smith C43 Series spec sheet	2026-10-20
NSF/ANSI 44 efficiency threshold	3,350	grains per lb of salt	PNNL Building America Solution Center, Water Softeners	2026-10-20
Salt bag size	40	lb	Lowe's, 40 lb water softener salt	2026-10-20
Salt price per 40 lb bag	7	US dollars	NDSU Extension WQ1031, This Old House, Water Softener System Cost (updated 2026-06-16)	2026-10-20
DIY install kit	175	US dollars	Home Depot, Pentair water softener install kit	2026-10-20
Plumber labor to install a unit you buy	150 to 1,000 (most 500 to 700)	US dollars	This Old House, Water Softener System Cost (updated 2026-06-16), Fixr, Water Softener Installation Cost	2026-10-20
Pro service visit per year	100 to 300	US dollars	HomeGuide, Water Softener Cost, This Old House, Water Softener System Cost (updated 2026-06-16)	2026-10-20
Softener lifespan	10 to 15	years	Culligan, When to Replace Your Water Softener	2026-10-20
Atmospheric pressure (sea level)	14.7	psi	Goulds Water Technology, HYDRO-PRO tank sizing guide (2025)	2026-10-20
Pre-charge below cut-in pressure	2	psi	Goulds Water Technology, HYDRO-PRO tank sizing guide (2025), The Driller, How to Calculate and Control Pressure Tank Drawdown (2006)	2026-10-20
Minimum pump run time	1 (up to 3/4 hp), 2 (above)	minutes	AMTROL Well-X-Trol brochure MC 10188, Goulds Water Technology, HYDRO-PRO tank sizing guide (2025)	2026-10-20

How we round

Grains and dollars are shown as whole numbers, pounds and gallons to 1 decimal. Anything you buy as a whole item, such as bags of salt or tanks, is rounded up.

Assumptions you can change

The calculators start from these defaults, which you can change on each page: 75 gallons of water per person per day, 7 days between regenerations, a 9 lb per cubic foot salt setting, 4 gpg of hardness added per 1 ppm of iron, sea-level air pressure for pressure tanks, and $7 for a 40 lb bag of salt.

Last updated: Oct 20, 2026