A 40 lb bag of water softener salt lasts about 6 weeks for a family of four on 10 gpg water with an efficient softener, about 4 weeks on 15 gpg and 3 weeks on 20 gpg. One person on 10 gpg water can make a bag last more than five months. Your hardness, household size and salt setting decide it, so find your row in the tables below.
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Every figure here comes from one sum: the pounds of salt your softener uses per day. Divide the bag by that and you have the days a bag lasts.
The 80 gallons per person per day is the bottom of the USGS range for indoor use, and 3,350 grains per pound of salt is the minimum efficiency for softeners certified to NSF/ANSI 44. The reasoning behind both is in our guide to how much salt a water softener uses.
How long does a 40 lb bag of softener salt last?
| Hardness | 1 person | 2 | 3 | 4 | 5 | 6 |
|---|---|---|---|---|---|---|
| 5 gpg | 335 | 168 | 112 | 84 | 67 | 56 |
| 10 gpg | 168 | 84 | 56 | 42 | 34 | 28 |
| 15 gpg | 112 | 56 | 37 | 28 | 22 | 19 |
| 20 gpg | 84 | 42 | 28 | 21 | 17 | 14 |
Other bag sizes: 50 lb and 25 kg
Salt is also sold in 50 lb bags in the US and in 25 kg bags (55.1 lb) in the UK and Europe. The same households:
| Hardness | 50 lb, 2 people | 50 lb, 4 people | 25 kg, 2 people | 25 kg, 4 people |
|---|---|---|---|---|
| 5 gpg | 209 | 105 | 231 | 115 |
| 10 gpg | 105 | 52 | 115 | 58 |
| 15 gpg | 70 | 35 | 77 | 38 |
| 20 gpg | 52 | 26 | 58 | 29 |
Your bag runs out faster than the table? Check these
The salt setting. The tables assume an efficient setting. A softener on its highest salt dose can use far more. Pentair’s manual for its whole-house models shows the smallest unit removing 4,272 grains per pound at its low setting but only 1,945 at its high one. At 1,945 grains per pound, a family of four on 10 gpg empties a 40 lb bag in 24 days instead of 42.
The hardness setting. If the valve thinks your water is harder than it is, it regenerates early. Our guide to the water softener hardness setting shows that setting 20 on 15 gpg water adds a third to your salt use.
Your water use. 80 gallons a person is a typical figure, not yours. Big gardens on softened water, long showers or a lot of laundry push it up. Your water bill gives the real number: gallons per month ÷ 30 ÷ people.
A timer softener. Older models regenerate on a schedule whether you used the water or not. The Minnesota Pollution Control Agency points out that demand-initiated models, which regenerate after a set volume, use only the salt they need.
Your bag lasts much longer than the table? That’s usually bad news
If the salt level hasn’t moved in weeks and the water is getting hard again, the softener probably isn’t taking salt in. Culligan describes the two usual causes: a salt bridge, a crust that forms across the tank and leaves a hollow underneath so water can’t reach the salt; and “mushing”, where salt turns to sludge at the bottom. Push a broom handle gently through the salt to feel for a hollow, and see water softener not using salt for what to do next.
A small household on soft-ish water is the honest exception. One person on 5 gpg uses a 40 lb bag in about 11 months by our sums; that’s fine.
Three worked examples
Couple in Las Vegas, 16 gpg. 2 × 80 × 16 ÷ 3,350 = 0.76 lb a day. 40 ÷ 0.76 = 52 days per bag, or seven bags a year. Our Las Vegas water hardness page has the table for every household size.
Family of five in Austin, 7.4 gpg. 5 × 80 × 7.4 ÷ 3,350 = 0.88 lb a day. A 40 lb bag lasts 45 days. Austin’s water is softened at the treatment plant, so salt runs out slowly.
Family of four in the UK, 300 mg/L, 25 kg bags. 300 ÷ 17.118 = 17.5 gpg. 4 × 80 × 17.5 ÷ 3,350 = 1.67 lb (0.76 kg) a day. A 25 kg bag lasts 33 days: roughly eleven bags a year. If your metered use is below 80 US gallons (303 litres) a head, it lasts longer.
How often to check and how much to add
Culligan suggests checking the brine tank at least once a month and adding salt when it drops below about half full, keeping the salt above the water level without overfilling. Use the tables to plan: if a bag lasts six weeks, adding one bag each time the tank hits half usually works. If it lasts three weeks, add two at a time so you’re not lifting bags every fortnight.
Work out your own figure, including cost per year, in the calculator:
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Frequently asked questions
How many bags of salt does a water softener use per month?
For a family of four with an efficient softener: about 0.7 bags of 40 lb a month on 10 gpg water, 1.1 on 15 gpg and 1.5 on 20 gpg.
How long does a bag of softener salt last for one person?
About 5.5 months on 10 gpg water and 2.8 months on 20 gpg, for a 40 lb bag.
Is it normal to add salt every two weeks?
For a large household on very hard water, it can be: six people on 20 gpg use a 40 lb bag in about 14 days. For a couple on moderate hardness, it suggests a high salt setting or a hardness setting that’s too high.
Does potassium chloride last as long as salt?
No. Culligan describes potassium chloride as less efficient than sodium chloride, so a bag goes less far.
Does salt go bad in storage?
Softener salt doesn’t spoil, but keep bags off damp floors so they don’t cake into a block that’s hard to pour.
Sources
- PNNL Building America Solution Center: Water Softeners (NSF/ANSI 44 efficiency rating)
- USGS Water Science School: How much water do I use at home each day?
- Pentair: Whole House Water Softening System owner’s manual (PDF)
- Culligan: Why it’s important to add salt to your water softener on time
- Minnesota Pollution Control Agency: Skinny water softeners
