To calculate weight from plates: enter the total number of plates including both sides. So if there's a single 45 on each side that's 2 total.
To calculate plates from weight: enter the weight. Optionally, you can enter your 1 rep max and desired percent, and it will calculate the weight for you.
If you add or remove plates while you have data entered, the app remembers the weight/desired weight and recalculates the plates needed to reach that weight.