Now you can measure your power output without replacing any components on your bicycle. PowerTap's PowerCal Bluetooth Smart 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 Bluetooth compatible device (sold separately) so you can maximize every training session. And, because it's attached to you and not your bike, you can use the PowerCal to track progress across multisport training disciplines as well.