There are three honest ways to answer this: read your utility's published report, run a test at home, or send a sample to a laboratory. Each gives a number on the same scale, and that number is the single most useful figure in the house because it sets softener sizing, salt use and how fast scale returns.
What hardness is, and how the scale reads
The USGS defines hardness as the amount of dissolved calcium and magnesium in the water, and classifies it as soft at 0 to 60 mg/L as calcium carbonate, moderately hard at 61 to 120, hard at 121 to 180, and very hard above 180. Those two minerals are what leave white crust on a kettle element, cloud the shower glass, curdle soap into scum and quietly line the inside of a water heater. Hardness is not a health contaminant and it is not dangerous to drink. It is an equipment and household nuisance, and the higher the number, the faster every one of those effects arrives.
The three ways to get your number
If you are on a community supply, the quickest route is your utility's annual water quality report, since most publish hardness in it and the EPA maintains a finder for locating yours. Call the utility if the figure is not printed, because they know it. If you are on a well, or you want the number at your own tap rather than at the plant, a titration drop kit gives a reliable reading in a couple of minutes: you add drops to a measured sample and count until the color turns. Test strips are quicker and less precise, which is fine for sorting soft from very hard. A laboratory panel is the third route and is worth it when you are also testing for iron, manganese and pH before buying equipment.
Two units, one number
Hardness is reported either in milligrams per liter as calcium carbonate, which is what the USGS scale above uses, or in grains per gallon, which is what softener dealers size equipment in. They describe the same water. A softener label quoting a grain capacity is telling you how much hardness the resin bed can remove between regenerations, so a dealer needs your hardness in grains and your household water use to size it. If your report gives one unit and your installer wants the other, convert rather than guess, and be sure you are comparing hardness as calcium carbonate rather than a calcium reading on its own. Ask which unit any quoted figure uses before you accept a sizing.
Add iron before you size anything
Sizing a softener on hardness alone is the classic error, because dissolved iron consumes exchange capacity as well. Installers add a compensation allowance for iron to the hardness figure, which is why an iron reading belongs on the same test. Very hard water with iron in it needs a larger unit, more salt and more frequent regeneration than the hardness number alone would suggest, and skipping the iron reading is how an undersized unit ends up letting hard water through at the end of every cycle. If you are on a well, take pH too, since it decides which iron treatments will work. Retest after any change in the well or the supply, since hardness is not fixed forever.
Questions people ask about what is my water hardness
Is hard water bad for my health?
No. Calcium and magnesium are not health contaminants at drinking water levels. Hardness is an appliance, plumbing and cleaning nuisance rather than a safety issue.
What counts as hard water?
The USGS scale calls 0 to 60 mg/L soft, 61 to 120 moderately hard, 121 to 180 hard, and anything above 180 mg/L very hard, all measured as calcium carbonate.
Can I find my hardness without buying a test?
Usually yes, if you are on a community supply. The utility's annual water quality report often prints hardness, and the utility will tell you if it does not.
Does hardness change during the year?
It can, especially on wells and on systems blending sources seasonally, which is why a softener is best sized from a recent test rather than an old one.