Pace the bike, save the run
In long-course triathlon the bike leg is where most races are won or lost — not by going fast, but by riding at a power you can sustain while leaving enough in the tank to run well off the bike. This calculator works backwards from the split you want: it finds the average speed your target time demands, estimates the watts needed to ride at that speed, and tells you whether that effort is realistic relative to your threshold power.
How it works
Your target average speed comes straight from distance and time. To turn speed into power the tool uses a standard cycling power model for a flat, calm course:
P = (Crr * m * g * v) + (0.5 * rho * CdA * v^3) (then / drivetrain efficiency)where Crr is rolling-resistance coefficient (0.005), m is rider plus bike mass, g is gravity, rho is air density (1.225), CdA is drag area for an aero road position (0.27), and v is speed in metres per second. The aerodynamic term grows with the cube of speed, which is why small speed gains cost large amounts of power.
The required power is then expressed as an Intensity Factor:
IF = required power / FTPThe tool compares your IF against a duration-appropriate ceiling — about 0.70 for a full Ironman bike and 0.78 for a 70.3 — and flags efforts that are too aggressive to hold without compromising the run.
Example and notes
Suppose your FTP is 250 W and you want a 5-hour Ironman bike split over 180 km. That requires roughly 36 km/h, and on a flat course the model returns a power figure you can check against an IF of 0.70. If the result lands above your sustainable ceiling, the honest options are to ride a slower split, raise your FTP, or accept a weaker run. Remember the model is a flat-course baseline: real terrain, wind, and altitude move the number, so always pace to the power your computer shows on the day, not to a fixed speed.