Ironman Bike Split Calculator
BetaPlan your 180km Ironman bike split with physics-based predictions. Upload your race course GPX file and get accurate time estimates based on your FTP, weight, and conditions—specifically designed to help you preserve your legs for the marathon.
Ironman Pacing Rule
Remember: You have a marathon after this. Target an Intensity Factor (IF) of 0.65-0.78 depending on your experience level. Racing the bike at your standalone cycling capacity will destroy your run.
Beta Feature
This tool is in beta. Results are estimates based on physics modeling. Actual race times may vary due to factors like fatigue, nutrition, and road conditions.
Why Bike Pacing Matters for Ironman
The 180km bike leg is where most Ironman races are lost—not won. Push too hard on the bike, and you'll pay the price during the marathon. Our calculator helps you find the optimal balance between a fast bike split and preserving your legs for the run.
The Golden Rule of Ironman Biking
"The race doesn't start until mile 18 of the marathon."
Everything before that point—the swim, the entire 180km bike, the first 12km of the run—is setup. Your goal is to arrive at the marathon feeling fresh enough to actually run.
What IF should I ride in an Ironman?
Ride a full Ironman bike leg at an intensity factor (IF) of roughly 0.65-0.78, which means a normalized power of 65-78% of your FTP. The longer you will be on the bike, the lower the number: first-timers and riders expecting 6-7 hours should sit around 0.65-0.70, most age-groupers at 0.70-0.75, and well-prepared athletes riding under about 5:15 can hold 0.73-0.78. In Joe Friel's Ironman power guidance, a well-prepared age-grouper with a six-hour bike split rides at an IF of 0.67-0.69, while pro winners finish close to 0.80. TrainingPeaks puts a typical full-distance bike leg at about 250-300 TSS, which is a useful upper limit. For a 250 W FTP, an IF of 0.70 is about 175 W. A 70.3 bike is half as long, so targets rise to roughly 0.75-0.85. This calculator paces around the power you enter, so type FTP × your target IF into the FTP field.
Sources: Joe Friel, "How to 'Cheat' by Using a Power Meter in an Ironman" (TrainingPeaks) · Marco Nicoli, "TSS for Ironman Training" (TrainingPeaks) · Hunter Allen & Andrew Coggan, Training and Racing with a Power Meter (70.3 IF ranges). Last updated September 24, 2026.
Example: Predicted 180 km Bike Split by Power and CdA
These splits come from the same physics model the calculator uses, run on a flat 180 km course with no wind at a constant power. Real courses are slower once you add hills, wind, corners and aid stations, so upload your race GPX above for a course-specific prediction.
| Power | CdA 0.23 m² | CdA 0.27 m² | CdA 0.30 m² |
|---|---|---|---|
| 150 W | 5:16 (34.2 km/h) | 5:27 (33 km/h) | 5:34 (32.3 km/h) |
| 175 W | 5:01 (35.9 km/h) | 5:13 (34.5 km/h) | 5:21 (33.7 km/h) |
| 200 W | 4:47 (37.6 km/h) | 5:00 (36 km/h) | 5:09 (35 km/h) |
| 225 W | 4:35 (39.3 km/h) | 4:49 (37.4 km/h) | 4:58 (36.3 km/h) |
| 250 W | 4:23 (41 km/h) | 4:38 (38.9 km/h) | 4:47 (37.6 km/h) |
Assumptions: 75 kg rider + 9 kg bike (84 kg), rolling resistance Crr 0.004, 97% drivetrain efficiency, air density 1.199 kg/m³ (20 °C, sea level, 50% humidity), flat road, no wind, constant power. Example: a 250 W FTP athlete riding at IF 0.70 (175 W) with a CdA of 0.27 m² would finish in about 5:13 on a flat, calm course.
Ironman Bike Intensity Factor (IF) Targets
Unlike standalone cycling events, your Ironman bike should be ridden at a significantly lower intensity. Use these IF targets when entering your FTP into the calculator:
| Experience Level | Target IF | 250W FTP Example | Expected Bike Time |
|---|---|---|---|
| Beginner / First Ironman | 0.65-0.70 | 163-175W avg | 6:00-7:00 |
| Intermediate | 0.70-0.75 | 175-188W avg | 5:15-6:00 |
| Advanced / Competitive AG | 0.73-0.78 | 183-195W avg | 4:45-5:15 |
| Elite / Pro | 0.78-0.82 | 195-205W avg | <4:45 |
CdA Settings for Triathlon Bikes
Your aerodynamic drag coefficient (CdA) varies significantly based on your bike setup and position. Use these reference values when configuring the calculator:
TT/Triathlon Bike (Aero Bars)
- • Good position: 0.22-0.25 m²
- • Average position: 0.25-0.28 m²
- • Upright/relaxed: 0.28-0.32 m²
Road Bike (Clip-on Bars)
- • On bars: 0.28-0.32 m²
- • Drops: 0.30-0.35 m²
- • Hoods: 0.35-0.40 m²
If you don't know your CdA, start with 0.27 m² for a TT bike or 0.32 m² for a road bike. You can refine this based on wind tunnel or velodrome testing.
Typical Ironman Bike Splits by Category
Use these benchmarks to set realistic expectations for your 180km bike split:
Elite / Pro
<4:30
38+ km/h avg
Competitive AG
4:30-5:15
34-40 km/h avg
Age Grouper
5:15-6:00
30-34 km/h avg
Finisher
6:00-7:00
26-30 km/h avg
Survival
7:00+
<26 km/h avg
Where to Get Your Ironman Course GPX
Most Ironman races publish official course GPX files. Here's where to find them:
- Ironman.com: Check your race's athlete guide for official GPX downloads.
- Strava: Search for the course or previous race activities and export as GPX.
- RideWithGPS: Many courses are available with elevation data corrected.
- Komoot: Search for Ironman routes created by other athletes.
Common Ironman Bike Pacing Mistakes
Racing the bike like a time trial
Holding 0.85+ IF because "it feels fine" early on. By km 120, you're cooked. The marathon becomes a death march.
Surging on climbs
Attacking hills with 20-30% power surges feels heroic but burns matches you'll need later. Stay within 5-10W of your flat power on climbs.
Chasing faster cyclists
Getting passed feels bad, but matching their pace destroys YOUR race plan. You'll often pass them on the run—if you stick to your pacing.
Starting too fast
The first 60km should feel "embarrassingly easy." If you're feeling strong, you're going too hard. Save it for the run.
Frequently Asked Questions
How does the Ironman bike split calculator work?
Upload your course GPX, then enter FTP, rider weight, bike weight and CdA. The calculator splits the route into short segments and solves the power balance on each one: aerodynamic drag, rolling resistance, gravity and a 3% drivetrain loss. It holds about 95% of the power you enter on flat roads, adds a little on climbs (capped at 115%) and eases off on descents, then adds up the segment times into a predicted bike split, average speed and normalized power.
Should I enter my real FTP or my Ironman race power?
Enter your planned race power. The calculator builds its pacing around the number in the FTP field, so for an Ironman type your FTP multiplied by your target intensity factor. With a 260 W FTP and a target IF of 0.72, enter about 187 W. Entering your full FTP gives a time-trial prediction that you could not hold for 180 km and still run a marathon.
What CdA should I use if I have never been tested?
Start with 0.27 m² for a triathlon or TT bike in a typical age-group position, or 0.32 m² for a road bike with clip-on bars. A well-fitted, aggressive TT position is around 0.22-0.25 m². If you have power and speed data from a flat, calm ride, the CdA calculator can estimate your own value, which makes the prediction noticeably more accurate.
Can I use it for a 70.3 bike course?
Yes. The calculator accepts any GPX file, so a 90 km 70.3 course works the same way. Because the ride is roughly half as long, raise your race power: most age-groupers ride a 70.3 at an IF of about 0.75-0.85 instead of the 0.65-0.78 used for a full Ironman.
Does the calculator include wind and weather?
Not yet. Predictions assume calm air at about 20 °C, so a windy or hot race day will be slower than the estimate. Treat the result as a baseline and add time for strong wind, rough roads, technical corners and aid-station slow-downs.
Is the Ironman bike split calculator free?
Yes. It is free, runs in your browser and needs no account or email. Your GPX file is processed on your device.