Water Hardness Calculator
This water hardness calculator turns your calcium and magnesium readings into a total hardness number you can act on, and sorts it into the USGS soft / moderately hard / hard / very hard category. It reports that number in mg/L as CaCO3 (the same as ppm), grains per gallon for US softener specs, and German and French degrees for European appliance manuals. High hardness means more dissolved calcium and magnesium, which drives soap lather, pipe scale, and appliance wear. A second mode sizes a water softener — resin grain capacity and salt per regeneration — from your household water use.
From a water test reading to your hardness category and softener size
The hardness mode runs one chain: multiply calcium and magnesium (in mg/L) by their calcium-carbonate factors, add them, then read that total in whatever unit you need. A sample with 40 mg/L calcium and 12 mg/L magnesium is 2.497 × 40 + 4.118 × 12 = 99.9 + 49.4 = 149.3 mg/L as CaCO3 — about 8.72 grains per gallon, which the USGS scale calls hard. The table and sections below are that same chain applied to the unit conversions, the four USGS categories, the temporary/permanent split, and softener sizing.
For hardness, 1 mg/L as CaCO3 equals 1 ppm — your water report and this tool use the identical number.
US softener specs are in grains per gallon (gpg). Divide mg/L by 17.118 to convert — the tool shows both.
Type the raw calcium and magnesium mg/L from your report. Entering them already expressed as CaCO3 overstates hardness by roughly 2.5 times.
Soft is 0-60, moderately hard 61-120, hard 121-180, and very hard above 180 mg/L as CaCO3.
Example: a very hard well, then sizing a softener
A well sample reads 72 mg/L calcium and 18 mg/L magnesium. Total hardness is 2.497 × 72 + 4.118 × 18 = 179.8 + 74.1 = 253.9 mg/L as CaCO3 — 14.83 grains per gallon, well into the very-hard band. Add an alkalinity of 140 mg/L as CaCO3 and the tool splits that total into 140 mg/L temporary (carbonate) hardness and 113.9 mg/L permanent (non-carbonate) hardness.
Now switch to softener-sizing mode for a hard-water household of three using 80 gallons per person per day at 20 gpg. The daily hardness load is 20 × 80 × 3 = 4,800 grains per day; with the tool's 1.5x buffer that is 7,200 grains of resin capacity and about 2.4 lb of salt per regeneration.
mg/L, ppm, grains, and degrees: which hardness unit is which
Total hardness is the same physical quantity in every unit — only the scale changes. Milligrams per liter as CaCO3 is identical to parts per million (ppm) in water, so a report reading '150 ppm' and this tool reading '150 mg/L as CaCO3' agree. US water softeners are rated in grains per gallon: divide mg/L by 17.118 (the NIST grain-per-gallon identity). European appliance manuals use German degrees (°dH, divide by 17.848) and French degrees (°fH, divide by 10). This tool prints all four from a single calcium-plus-magnesium entry, so you never have to convert by hand.
Every row below is produced by this calculator, so the chart and the tool always agree. The 60, 120, and 180 mg/L rows are the exact USGS band edges.
| Total hardness (mg/L as CaCO3 = ppm) | Grains per gallon (÷ 17.118) | German degrees °dH (÷ 17.848) | USGS category |
|---|---|---|---|
| 17 (about 1 gpg) | 1.00 | 0.96 | Soft |
| 60 | 3.51 | 3.36 | Soft (top of band) |
| 120 | 7.01 | 6.72 | Moderately hard (top of band) |
| 180 | 10.52 | 10.09 | Hard (top of band) |
| 250 | 14.60 | 14.01 | Very hard |
What soft, hard, and very hard actually mean
The USGS Water Science School sorts total hardness into four bands, and this tool uses the same breakpoints, inclusive at each upper edge: soft at 0-60 mg/L (0-3.5 gpg), moderately hard at 61-120 mg/L (3.6-7.0 gpg), hard at 121-180 mg/L (7.1-10.5 gpg), and very hard above 180 mg/L (over 10.5 gpg). A total that lands exactly on 60, 120, or 180 stays in the lower band.
What the category means in practice: soft and moderately hard water lathers easily and leaves little scale, so treatment is rarely worth it. Hard water starts to leave scale on fixtures, glassware, and water heaters and dulls soap. Very hard water is where scale builds fast and a softener is most often worth evaluating — the USGS notes hardness is an aesthetic and nuisance issue, not a health limit (US EPA lists it as a secondary standard), so the decision to soften is about scale and comfort, not safety.
Sizing a softener: grains, salt, and the conventions behind them
Switch to softener-sizing mode when you already know your hardness in grains per gallon. The tool multiplies hardness (gpg) × daily use per person (gallons) × household size to get a daily hardness load in grains, then multiplies by a 1.5x buffer to set the required resin grain capacity, and divides that capacity by 3,000 grains per pound of salt to estimate salt per regeneration. For a household of four at 10 gpg using 75 gallons each per day, the daily load is 10 × 75 × 4 = 3,000 grains per day, so the tool reports 4,500 grains of resin capacity and about 1.5 lb of salt per regeneration.
The 1.5x buffer and 3,000 grains-per-pound figures are field conventions, not fixed constants — real ion-exchange efficiency runs roughly 3,000 to 4,000+ grains per pound depending on the salt dose, and softeners are usually specified by a fixed bed capacity (a 32,000-grain unit, for example) rather than a one-day load. Treat these numbers as a conservative planning estimate and confirm against your unit's manual. One honest caveat: the tool also shows a days-between-regenerations figure, but because it sizes capacity to exactly one day of load times the buffer, that figure is always the 1.5-day buffer rather than a real refill interval — a true interval depends on your softener's fixed grain capacity, which this quick estimate does not ask for.
Temporary vs permanent hardness, and what the split needs
Hardness comes in two forms. Temporary (carbonate) hardness is the part paired with bicarbonate alkalinity; it precipitates out when water is boiled, which is the scale you see in a kettle. Permanent (non-carbonate) hardness is paired with sulfate and chloride and stays dissolved no matter how long you boil — only softening removes it. The optional alkalinity field turns this split on: the tool estimates temporary hardness as the smaller of your alkalinity and your total hardness, and calls the remainder permanent. In the well example above, a total of 253.9 mg/L with 140 mg/L alkalinity splits into 140 mg/L temporary and 113.9 mg/L permanent. Leave alkalinity blank and the tool simply reports total hardness — the split needs that second measurement from your report.
Before you trust this number
A few habits keep this estimate honest. Enter calcium and magnesium as the raw ion concentrations your lab reports, in mg/L — not values already expressed as CaCO3, which would overstate hardness by about 2.5 times. Remember that mg/L and ppm are the same unit here, but grains per gallon is not: mixing gpg into an mg/L field shifts the result about 17-fold and lands you in the wrong band. The tool assumes one well-mixed water source and a stable, one-time reading; surface-fed supplies can shift with the season, so a single test is a snapshot.
Hardness also is not the whole water story: this tool covers calcium and magnesium only and ignores iron, manganese, pH-driven scaling, and other minerals that affect staining, taste, and softener life. The softener figures are conservative conventions, not a treatment design. For anything you will spend money on — a softener purchase, a plumbing scale problem, or a health question — start from a certified lab water analysis rather than a single calculated number.