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.
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.