Simple, straightforward, works. – Offline IV Calc for Pokemon GO Review

Upload a screenshot, input your trainer lvl, select correct level based on the arc, select the Pokemon, input CP, input HP... boom. After a couple uses, you'll realize that the app spits out a CP range and HP range that is possible based on the Pokemon level, so you can guess where your Pokemon falls on the IV scale without hitting the calculate button.
Review by Whizzpopping on Offline IV Calc for Pokemon GO.