Show the formula and working
This free water softener size calculator works out how much hardness your household’s water carries each day and the softener capacity you need between regenerations, for homes in the UK, Ireland and Europe. It accepts hardness in mg/l, Clark, German or French degrees or US grains per gallon, and converts between them.
Quick answer: A water softener size calculator multiplies the litres you use by the hardness of your water. Three people using 140 litres a day each, with hard 300 mg/l water, add 126 g of hardness a day, about 1,944 grains. A softener that regenerates weekly needs a capacity of about 13,600 grains.
What Is a Water Softener Size Calculator?
A water softener size calculator is a tool that estimates the daily hardness load from your water use and hardness, and the capacity a softener needs to treat it between regenerations.
The formula in plain words: convert the hardness to mg/l as calcium carbonate. Multiply by the litres softened per day to get milligrams of hardness a day, and divide by 64.79891 to get grains, because a grain is exactly 64.79891 mg. Multiply the grains per day by the days between regenerations for the capacity you need.
How Do You Use the Water Softener Size Calculator?
- Enter the People in the home and the Water used per person per day.
- Find your hardness from your water supplier, then enter it as the Water hardness and choose its Hardness unit.
- Set the Share of water softened, lower if the garden tap and drinking tap bypass the softener.
- Enter the Days between regenerations and, if you have one in mind, the Softener capacity to check.
- Read the capacity needed and the daily load, then open “Show the formula and working” to see the conversions.
How Hard Is My Water?
The Drinking Water Inspectorate classes water as soft up to 100 mg/l calcium carbonate, slightly hard at 100 to 150, moderately hard at 150 to 200, hard at 200 to 300 and very hard above 300. Your water company publishes the hardness for your postcode, usually in mg/l and often in Clark or German degrees too.
How Do You Convert Water Hardness Units?
One Clark degree equals 14.25 mg/l, one German degree 17.85 mg/l, one French degree 10 mg/l and one US grain per gallon 17.12 mg/l, all as calcium carbonate. The Clark and US figures come from the exact grain of 64.79891 mg divided by the UK or US gallon, as set out on Wikipedia’s hard water page. So 300 mg/l is 21.0 °Clark, 16.8 °dH, 30 °fH or 17.5 gpg.
Can You Drink Softened Water?
The DWI advises keeping an unsoftened tap in the kitchen for drinking and cooking. Most softeners swap the hardness for sodium, which is not suitable for making up infant feeds or for people on a low sodium diet. Have a softener installed by a qualified plumber, who will fit the bypass for the drinking tap and the right check valves.
What Capacity Softener Do I Need?
Choose a softener whose rated capacity per regeneration is at least the grains per day times the days between regenerations. Makers rate capacity in different ways, such as grains, grams of calcium carbonate or litres of water at a given hardness, so convert using the figures in the result. Metered softeners regenerate only when needed, which suits households whose water use varies. Allow a margin of about 20% above the figure here for guests and dry summers. Scale on a kettle element slows heating, so once the water is softened the kettle running cost calculator gives a fair picture of what each boil costs.
Water Hardness Conversions and Daily Load
The daily load is for 420 litres a day (three people at 140 litres each), all softened.
| mg/l CaCO3 (DWI band) | °Clark | °dH | °fH | US gpg | Grains per day |
|---|---|---|---|---|---|
| 100 (soft limit) | 7.0 | 5.6 | 10.0 | 5.8 | 648 |
| 150 (slightly hard limit) | 10.5 | 8.4 | 15.0 | 8.8 | 972 |
| 200 (moderately hard limit) | 14.0 | 11.2 | 20.0 | 11.7 | 1,296 |
| 300 (hard limit) | 21.0 | 16.8 | 30.0 | 17.5 | 1,944 |
| 400 (very hard) | 28.1 | 22.4 | 40.0 | 23.4 | 2,593 |
Worked Example: A Three-Person Home in a Hard Water Area
Example: three people using 140 litres each a day, water at 300 mg/l, everything softened, with weekly regeneration. These are the default inputs.
- Water softened: 3 x 140 = 420 litres a day.
- Hardness: 420 x 300 = 126,000 mg = 126 g a day.
- In grains: 126,000 / 64.79891 = 1,944 grains a day.
- Capacity for 7 days: 1,944 x 7 = 13,611 grains, or 882 g.
If a softener you are considering is rated at 30,000 grains, enter it in Softener capacity to check: it would last about 15.4 days between regenerations. If your supplier quotes 21 °Clark instead, choose Clark degrees in Hardness unit for the same answer. For litres and gallons, the litres to gallons converter helps with older specifications.
Water Softener Size Calculator FAQs
What is a grain of hardness?
A grain is an old unit of weight equal to exactly 64.79891 mg. Softener capacity is often given in grains of hardness removed between regenerations, and hardness in grains per gallon, especially on American style softeners.
Do I need a water softener?
It depends on your hardness and how much limescale bothers you. In soft water areas, under 100 mg/l on the DWI scale, a softener is rarely worth fitting. In hard and very hard areas it reduces scale in kettles, showers, boilers and appliances.
How often should a softener regenerate?
Follow the maker’s instructions. Timed softeners regenerate on a set schedule, while metered ones regenerate after a set volume of water. The capacity must cover the hardness load between regenerations, which this calculator works out for any interval.
Should the outside tap be softened?
Usually not. Garden water does not need softening, and bypassing it saves salt and capacity. Lower the Share of water softened to allow for outside taps and the unsoftened drinking tap.
Is softened water safe for babies?
The DWI notes that the extra sodium in softened water can be a problem for premature babies, and the NHS advises against using artificially softened water to make up infant formula. Use water from the unsoftened kitchen tap, prepared as the NHS advises.
How do I find my water hardness?
Check your water company’s website, which usually has a postcode lookup giving hardness in mg/l and other units. Simple test strips give a quick reading, but the supplier’s figure is the most reliable average.
For more home tools, see the home calculators.
Checked October 2026. Sources: Water hardness (Drinking Water Inspectorate), Hard water (Wikipedia), Grain (unit) (Wikipedia), Save water (Waterwise).