Using a point buy 5e calculator instead of simply rolling for stats or taking the standard calculator set does involve a bit more mathematics. Thankfully great resources like the 5e Point Buy Calculator makes it simple for you and your players to build a character using point buy.
You need to simply keep a track of your total stat points spent in the lower right-hand side corner of the calculator and you’re good to go! This calculator, in particular, is especially great as it has all of the 5e published races available for you to select and choose from.
It also has flexible options for adjusting the point buy system if you wish to change the value of some specific stats or increase the maximum or minimum number of stat values.
One minor complaint that you might have with this calculator is that it does allow you to keep adding on stat points even after you’ve gone past the maximum. Regardless of which, it is clearly printed as a fraction so you will be able to note if you have crossed the limit.