It has some functionality without paying, but it is heavily restricted. I wound up paying, figuring that $8 per year (less than $1 per month) was a small price to pay for an app that I use nearly every day to track my pool water parameters. I’m sure there are other apps out there with similar features, and probably some free ones at that, but I don’t think you’ll regret paying for poolmath. If nothing else, think of it as a donation going toward maintaining the app and this website.