How to get hard water off windows?

Use a spray bottle with equal parts of vinegar, lemon juice, and dawn dish soap. Then, scrub with a stiff bristle brush.

CLR is a product just for that you can get it in a grocery or hardware store. I used it to get the calcium build up out of my coffee pot, iron water glasses etc. After I rinse those items really well. If you use it inside I'd wear a mask over my mouth and nose because it's so strong. It's great for taking off the hard water build up on your shower door or the walls. If you use it on the walls just test it first in a small area at the bottom of the shower.

Have your tried lemon juice?

If glass cleaner does not work use a razor scraper.

Vinegar or CLR work well for me. You can dip rags or paper towels in white vinegar, ring them out a bit and then "stick" them to the window. They will stay in place because they are wet. After an hour or two remove the papertowels and wipe the window clean.