Water softener and whole-house filter sizing calculator

Enter your test numbers and this returns the softener grain capacity the published sizing conventions produce, the peak flow rate your fixtures can actually demand, and the specific NSF/ANSI certifications to ask a seller for in writing. Where the published conventions disagree with each other — and on two points here they do — you get both numbers rather than a tidy single answer.

Disclosure and scope. This page contains affiliate links; we may earn a commission on qualifying purchases at no cost to you. This tool reproduces published sizing conventions, federal fixture flow figures and certification scopes so you can check them yourself. It is a starting point for a conversation with a licensed plumber or a certified water treatment professional, not a water treatment specification, and it has no way of knowing what is in your water. Only a laboratory test does.

Your water

This changes what you are entitled to be told, and what nobody will tell you unless you test.
Utility reports almost always use mg/L. Softener manuals almost always use gpg. 1 gpg = 17.1 mg/L.

Household

Peak demand — how many of these could run at the same moment

What you are actually trying to fix

Tick only what a test or your utility's report has shown, or what you can see and taste. Every box maps to a specific certification you should ask for in writing.

Where these numbers come from

Nothing on this page is our own engineering determination. Every figure below is reproduced from a published source and linked so you can read it in context. Where sources disagree, the tool shows both and says which is which. Where a figure is our own arithmetic on top of published inputs, it says that too.

  1. Hardness classification and the unit conversion. USGS Water Science School: 0 to 60 mg/L as calcium carbonate is soft, 61 to 120 moderately hard, 121 to 180 hard, more than 180 very hard. The same page notes its national hardness map draws on 1975 NASQAN sampling, and that national-scale data of this kind should not be relied on for an individual household — it points you at your local health agency or water utility instead. That is why this tool asks you for a number rather than guessing one from your ZIP code. USGS, Hardness of Water
  2. Household water volume, measured. The Water Research Foundation's Residential End Uses of Water, Version 2 (2016) metered single-family homes across 23 utilities in the United States and Canada: indoor use averaged 138 gallons per household per day and 58.6 gallons per capita per day, at an average household size of 2.65 people. Toilets were the largest indoor use, then faucets, showers, clothes washers and leaks. WRF project 4309, Residential End Uses of Water Version 2
  3. Household water volume, corroborating. EPA WaterSense states the average American family uses more than 300 gallons of water per day at home, and that roughly 70 percent of that use occurs indoors — which lands near the metered figure above rather than near the trade convention of 75. EPA WaterSense, How We Use Water
  4. Fixture flow figures. EPA WaterSense: a labeled showerhead uses no more than 2.0 gpm against a standard showerhead's 2.5 gpm; the labeled lavatory faucet maximum is 1.5 gpm at 60 psi, against an average of 2.2 gpm for lavatory faucets sold in the United States; a labeled toilet flushes at no more than 1.28 gallons. EPA WaterSense, Showerheads and the WaterSense lavatory faucet specification (PDF)
  5. Softener salt and water efficiency. The Department of Energy's Building America Solution Center, reproducing NSF/ANSI 44: an efficiency-rated softener must "have a rated salt efficiency of not less than 3,350 grains of total hardness exchange per pound of salt" and must "not generate more than 5.0 gallons of water per 1,000 grains of hardness removed during the service cycle"; it must use a flow meter or hardness sensor to initiate regeneration, and time-clock regeneration does not qualify. Building America Solution Center, Water Softeners
  6. Iron, manganese and TDS thresholds. EPA's National Secondary Drinking Water Standards set guideline levels of 0.3 mg/L for iron, 0.05 mg/L for manganese and 500 mg/L for total dissolved solids. EPA states that it does not enforce these secondary maximum contaminant levels and that they exist to help systems manage aesthetic considerations such as taste, color and odor. EPA, Secondary Drinking Water Standards
  7. What your utility owes you. Every community water system must deliver a Consumer Confidence Report to its customers by July 1 each year, based on the previous calendar year's data, covering the source of the water, monitoring results for detected contaminants and any rule violations. EPA, CCR Information for Consumers
  8. What nobody owes a well owner. EPA states that private domestic wells are not regulated by the federal government under the Safe Drinking Water Act, that owners are responsible for making sure their drinking water is safe, and recommends testing annually for total coliform bacteria, nitrates, total dissolved solids and pH. EPA, Protect Your Home's Water
  9. Certification scopes. CDC advises looking for a certification mark on the filter label and looking the specific product up in the certifier's database to see what it is certified to remove, and points to filters certified to NSF/ANSI 53 or NSF/ANSI 58 for PFAS reduction. CDC, About Choosing Home Water Filters
  10. The iron compensation factor. Manufacturer guidance published for residential softeners does not agree with itself. Some documentation instructs adding 4 gpg to the hardness setting for every 1 ppm of ferrous iron; other documentation instructs adding 5. We have not found the two reconciled anywhere, so the tool shows both — the difference is large enough to change the tank you buy.
  11. Pipe sizing tables. Deliberately not reproduced here. Plumbing codes size supply piping with fixture-unit tables, the adopted edition varies by jurisdiction, and those documents are not free to read. The peak-flow figure this tool produces answers a different question, and the results section says which one.

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