Now you can measure your power output without replacing any components on your bicycle! CycleOps' PowerCal gives you real-world and real-time data via a convenient chest strap. It's compact, light, and easy to bring along on every ride and on any bike so you get consistent training metrics. After the initial setup, the PowerCal uses complex scientific algorithms based on your heart rate to send accurate power data to your ANT+ compatible head unit (sold separately) so you can maximize every training session. The included speed sensor attaches to your bike for even more data. And, because the PowerCal is attached to you and not your bike, you can use it to track progress across multisport training disciplines as well.