Water Hardness Converter

Water hardness conversions use ppm (mg/L CaCO3) as the base unit.

Unit definitions (all relative to ppm CaCO3):

• 1 ppm = 1 mg/L CaCO3 (these are identical)
• 1 gpg (grains per gallon) = 17.118 ppm
• 1 dGH (German degrees) = 17.848 ppm
• 1 mmol/L = 100.09 ppm (molar mass of CaCO3)
• 1 Clark degree (English degrees) = 14.254 ppm
• 1 French degree (°fH) = 10 ppm

Water hardness scale (ppm):

• Very soft: 0-17 ppm (this is what a softener produces)
• Soft: 18-60 ppm
• Moderately hard: 61-120 ppm
• Hard: 121-180 ppm
• Very hard: 181-300 ppm
• Extremely hard: over 300 ppm

The four middle bands are the US Geological Survey classification, which is what a US water report uses. The very soft and extremely hard bands are added at either end because the official scale stops at “very hard” and lumps 200 ppm together with 600, which behave nothing alike in a kettle.

Common uses:

• Aquariums: dGH is the standard measure
• US water reports: ppm or gpg
• European water reports: dGH or French degrees
• Water softeners: gpg is most common in the US

Water hardness matters because dissolved calcium and magnesium cause the everyday annoyances of scale and poor lathering. Hard water leaves limescale in kettles and pipes, spots on glassware, and makes soap harder to foam, which is why people in hard-water areas reach for more detergent and descaler.

The unit you meet depends on where you are and what you’re buying. US water reports use ppm or grains per gallon, water softeners are almost always rated in grains (which is why converting ppm to gpg is the usual reason to be here), and parts of Europe use German (°dH) or French (°fH) degrees. To size a softener you multiply your hardness in grains by your daily water use, so getting the conversion right directly affects how often the unit regenerates. One reassurance: hard water isn’t a health problem, and the minerals are harmless to drink. It’s a nuisance and an appliance issue, not a safety one.

Sizing a water softener

The arithmetic is short. Daily grain load = people in the house x gallons each per day x hardness in grains per gallon. The usual planning figure is 75 gallons per person per day in the US, or about 150 litres in Europe.

A family of four at 10 gpg works out to 4 x 75 x 10 = 3,000 grains a day. Softeners are sold by grain capacity between regenerations, so a 32,000 grain unit would run about ten days on that, which is right: you want regeneration every 7 to 14 days. More often than that wastes salt and water. Much less often and the resin bed sits saturated, which shortens its life.

If your water carries iron, add 4 gpg to your hardness figure for every 1 ppm of iron before sizing. Iron fouls resin faster than hardness does.

Aquariums

Fish keepers use dGH, and the range that matters is narrower than any of the domestic bands. Soft-water fish such as discus, tetras and most South American species want 2 to 8 dGH. African rift lake cichlids want 10 to 20. Livebearers, guppies and mollies sit at the hard end, 12 to 20, and actually suffer in soft water.

The important point is stability. Fish tolerate a wrong-but-steady hardness far better than a right one that swings, so chasing a target figure with chemicals usually does more harm than leaving the tap water alone.

Brewing

Hardness is not the number a brewer really cares about, but it is where the conversation starts. What matters is which minerals make up that hardness. Calcium from gypsum accentuates hop bitterness and suits a pale ale; magnesium and bicarbonate from chalk suit dark beers, which is why Dublin water made stout and Pilsen’s famously soft water, around 1 to 2 dGH, made the pale lager it is named after.

Total hardness above roughly 200 ppm without matching acidity will push the mash pH too high and give a harsh, astringent finish. Most brewers building water from scratch start with distilled or reverse-osmosis water at zero hardness and add salts back, which is the only way to control the ratio rather than just the total.

Testing at home

A titration kit is a small bottle of reagent you add drop by drop to a measured sample until it changes colour, and the drop count is your hardness. Cheap, accurate to a few ppm, and the method water treatment companies actually use. Test strips are faster and read to the nearest band rather than the nearest number, which is fine for deciding whether you have a problem and useless for sizing equipment.

Whichever you use, test the cold tap that comes straight off the main. A hot tap has been through the cylinder, where some of the calcium has already dropped out as scale, and it reads low.