You can use this app if you are following a 5x5 barbell workout routine. This app will compute the warmup sets for you given an exercise.

Why warmup?

Warming up increases how much you can lift while decreasing the risk of injury. The warmup sets raise the temperature of your muscles and lubricate your joints. They also give you form practice before lifting heavy. And they prepare you mentally for the heavy weights to come.

This app is open-source:
https://bitbucket.org/ron_ang/kmmbarbellwarmup/src