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Water softener size calculator: how many grains do I need?

Size a water softener from your household, water use and hardness, with the grains between regenerations and the salt you will use.

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Results

Grains between regenerations

31,500

Suggested softener size (grains)

48,000

Grains removed per day
4,500
Salt per month (about 4,000 grains per lb)
34 lb
40-lb salt bags per year
10.3

Grains, hardness and capacity

Hard water contains dissolved calcium and magnesium. In the US, hardness is measured in grains per gallon (gpg); 1 gpg equals 17.1 mg/L (ppm). A softener’s capacity is the number of grains it can remove before it must regenerate with salt.

How to size a softener

  1. Compensated hardness = hardness (gpg) + about 4 × iron (ppm), since iron also uses up capacity.
  2. Grains per day = people × gallons per person per day × compensated hardness.
  3. Grains between regenerations = grains per day × days between regenerations, typically 7.
  4. Softener size: a softener’s rated capacity is measured at its highest salt setting. At an efficient setting you get about three-quarters of it, so choose a rated size of at least the grains needed ÷ 0.75.

Worked example

A family of 4, using 75 gallons each per day, with 15 gpg water and no iron, regenerating every 7 days:

  1. Grains per day: 4 × 75 × 15 = 4,500.
  2. Grains per week: 4,500 × 7 = 31,500.
  3. Needed rating: 31,500 ÷ 0.75 = 42,000, so a 48,000-grain softener.
  4. Salt: about 4,500 × 30 ÷ 4,000 = 34 lb per month, roughly ten 40-lb bags a year.

Hardness levels

Hardness mg/L (ppm) gpg
Soft 0 to 60 0 to 3.5
Moderately hard 61 to 120 3.5 to 7
Hard 121 to 180 7 to 10.5
Very hard over 180 over 10.5

Your water utility publishes hardness in its annual Consumer Confidence Report. For a private well, use a test kit or a lab.

Common sizes

Household Moderately hard water Very hard water
1 to 2 people 24,000 grains 32,000 grains
3 to 4 people 32,000 grains 48,000 grains
5 to 6 people 48,000 grains 64,000 grains

Tips

  • Too small a softener regenerates too often, wasting water and salt and wearing out the resin. Too large a unit can let the resin channel and sit idle.
  • Choose a demand-initiated (metered) control valve: it regenerates based on actual water use, not a timer.
  • Leave a bypass for outdoor faucets; there is no need to soften irrigation water.
  • Softened water contains a little added sodium. People on low-sodium diets can keep an unsoftened kitchen tap or use potassium chloride instead of salt.
  • Some areas restrict salt-based softeners because of brine discharge; check local rules.

Frequently asked questions

What size water softener do I need?

Multiply people × gallons per day × hardness in grains per gallon to get the grains removed daily, then multiply by the days between regenerations. A family of 4 using 75 gallons each with 15 gpg water removes 4,500 grains a day, about 31,500 a week, which suits a 40,000 to 48,000-grain softener.

How do I convert ppm hardness to grains per gallon?

Divide mg/L, or ppm, by 17.1. Water at 257 ppm is 15 grains per gallon.

How hard is my water?

The USGS classes water as soft below 60 mg/L (3.5 gpg), moderately hard from 61 to 120 mg/L, hard from 121 to 180 mg/L and very hard above 180 mg/L (10.5 gpg). Your utility's annual water quality report lists it, or you can use a test kit.

How much salt does a water softener use?

At efficient settings, a softener uses about 1 lb of salt per 4,000 grains removed. A family of 4 with hard water typically uses one 40-lb bag a month.

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Results are estimates provided for information purposes. They do not replace professional advice.

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