Long before anyone runs a test, the appliances in a house have been measuring hardness continuously and recording the result in scale. Reading those signs is a reliable way to know what you are dealing with, and it has the advantage of telling you not just whether the water is hard but how much it is already costing you.
The kettle and the shower head
A kettle element that furs up between descalings is the most direct evidence available, because it heats hard water fast and deposits what comes out of solution. How quickly the crust returns after descaling is a rough measure of how hard the water is: months suggests moderate, weeks suggests hard. A shower head whose jets block or spray unevenly is the same story at lower temperature, and the white crust around the nozzles is the same mineral. Both are the deposits calcium and magnesium leave when water evaporates or is heated.
The water heater, which is the expensive one
A tank heater accumulates scale on the element and in the bottom of the tank, and the symptoms are a rumbling or popping noise as water boils under the deposit layer, longer recovery times, and higher running cost for the same hot water. A tankless unit shows it faster, since it heats water quickly in a narrow passage, and it appears as reduced flow and lukewarm output. Any heater needing descaling more than occasionally is telling you the water is hard enough to be worth treating.
The dishwasher, washing machine and laundry
Spotting and cloudy film on glassware after a dishwasher cycle that used to come out clear is hardness reacting with detergent. Laundry that has become stiff and looks grey over time is the same reaction leaving residue in the fabric. Both usually get worse gradually, so people adapt rather than noticing. If you have been steadily increasing detergent doses or adding rinse aid to get results you used to get by default, that is the appliance reporting the water rather than a fault.
Turning signs into a number
The USGS classifies hardness as dissolved calcium and magnesium: 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. Signs across several appliances usually mean the hard band or above. Confirm with your utility's annual report, which publishes the figure at no cost, or with a test kit on a well. Our hard water shower head guide covers the point of use option if you are not ready to treat the whole house.
Questions people ask about how do you know if you have hard water
What is the clearest household sign of hard water?
Scale returning on the kettle element after descaling, and how quickly it comes back is a rough guide to how hard the water is.
Why is my water heater rumbling?
Water boiling under a layer of scale in the bottom of the tank. It also means longer recovery and higher running cost for the same hot water.
Does spotted glassware mean hard water?
Usually. Spotting and cloudy film after a dishwasher cycle is hardness reacting with detergent and drying on the surface.
How do I get the actual figure?
From your utility's annual report on a community supply, at no cost, or from a hardness test kit or laboratory analysis on a private well.