Ask ten age-group triathletes what wattage they plan to ride at Ironman and you will get ten answers, most of them wrong. Some pick a round number. Some copy what a training partner rode. Some ride to feel and hope the marathon works out.
The bike leg of an Ironman is not a time trial. It is a pacing problem where the objective function is total race time, and the binding constraint is the marathon that starts after you rack the bike. Ride the bike 15 watts too hard and you will lose 30 to 60 minutes on the run. That trade is almost never worth it.
This article gives you the numbers. Not "ride easy" — actual wattage targets as a function of your FTP, the course, your age, and your run goal.
The one-line answer (and why it's wrong)
The internet answer is "ride at 70% of FTP". It is not a bad starting point. It is also not good enough to race on.
The problem with a single percentage is that it ignores four things that matter. First, your FTP number itself: is it a fresh 20-minute test or a two-year-old guess? Second, the course. 180km on pancake-flat roads behaves nothing like 180km with 2,500m of climbing. Third, your age and training age. A 28-year-old with five years of Ironman volume handles a higher IF than a 52-year-old first-timer with the same FTP. Fourth, your run. If you are targeting a 3:15 marathon off the bike, your bike IF is lower than if you are walking in a 5:00 marathon.
"70% of FTP" also confuses Average Power with Normalised Power. These are not the same. On a flat course they might be within 3 watts. On a hilly course they diverge by 20 watts or more, and the metabolic cost tracks NP, not AP.
So the one-line answer is a starting heuristic, nothing more. The real answer is a range of Intensity Factors modified by four variables, which is what the rest of this article covers.
Intensity Factor: the real target
Intensity Factor (IF) is Normalised Power divided by FTP. It is the single most useful number in long-course pacing because it is dimensionless: a 0.72 IF means the same physiological stress whether your FTP is 200W or 400W.
The documented IF range for a well-paced Ironman bike leg is 0.65 to 0.78. Below 0.65 you are sandbagging. Above 0.80 you are gambling with the marathon.
Here is how the range breaks down by target finish time and athlete profile:
- Sub-9:30 pros and elite age-groupers: IF 0.76 to 0.82. These athletes have the aerobic depth to metabolise that load for 4:20 to 4:30 of riding.
- 9:30 to 10:30 age-groupers: IF 0.72 to 0.76. Experienced, two or more Ironmans completed, strong run off the bike.
- 10:30 to 12:00 age-groupers: IF 0.68 to 0.72. The bulk of competitive age-groupers sit here.
- 12:00+ and first-timers: IF 0.62 to 0.68. The goal is to finish the marathon running.
Dan Lorang, who has coached Jan Frodeno and Anne Haug to Ironman World titles, has been consistent in interviews that elite athletes under-ride relative to their capacity on purpose — because the penalty for over-pacing compounds through the marathon. That principle scales down. If the best in the world leave watts on the table, age-groupers should too.
Use FTP zones calculator to convert your current FTP into the IF bands above in actual watts.
How your FTP sets the ceiling
Every power target in this article is a percentage of FTP. If your FTP is wrong, every downstream number is wrong.
Two rules for Ironman FTP. First, test within six weeks of race day. FTP drifts with fitness, and a January number is not a July number. Second, test in the position you will race in: your aerobars, not your road bars. Aero FTP is typically 5 to 15 watts lower than road FTP for age-groupers who have not drilled the position.
If your 20-minute test gives 280W and your aero FTP is 265W, your race targets are built on 265, not 280. Athletes routinely skip this and wonder why lap 2 of the bike falls apart.
A useful cross-check: can you hold your stated FTP for 2x20 minutes in aero position with 5 minutes rest? If the second interval drops by more than 5%, your FTP is over-stated. Drop it by the difference and rebuild your targets.
The other FTP-related mistake is testing once a season. Prof. Stephen Seiler's work on training intensity distribution assumes you know where your thresholds are. Retesting every 6 to 8 weeks is not optional for athletes riding 10+ hours a week.
How course profile changes the math
Course profile changes IF targets via Variability Index (VI), the ratio of NP to AP. Low VI (1.00 to 1.05) means steady power. High VI (1.08+) means you rode the bike in pieces.
On a flat course, VI near 1.02 is achievable and the metabolic cost of the ride is close to what NP suggests. You can target the top of your IF band. A 10:00 age-grouper can ride IF 0.74 on Ironman Texas and run a 3:45 marathon.
On a hilly course, VI creeps to 1.08 or 1.10 whether you like it or not, because climbs force power up. The metabolic cost runs roughly 3 to 5% higher than NP implies. The fix is to lower the IF target by 0.03 to 0.05. Ironman Lake Placid or Nice, the same 10:00 age-grouper should target IF 0.69, not 0.74.
Windy courses behave like hilly ones. Headwinds on the return leg spike power unless you discipline yourself to hold IF, not speed. Most age-groupers over-ride headwinds by 15 to 25 watts and pay for it on the run.
Three practical rules. Cap 20-minute rolling NP at 1.05x your IF target, with no surge over that. On climbs longer than 3 minutes, cap instantaneous power at FTP minus 10%. Use descents and tailwinds to recover, not to chase watts. These three rules alone save most age-groupers 5 to 10 minutes on the marathon.
Age-group adjustments
Age matters. Two athletes with a 250W FTP — one 32, one 58 — should not ride the same wattage at Ironman.
The 58-year-old clears lactate more slowly, oxidises fat slightly better, and takes longer to recover from glycogen depletion. The net effect is that the sustainable IF for a multi-hour effort followed by a marathon is roughly 0.02 to 0.03 lower per decade over 40.
A rough adjustment table, starting from a baseline IF appropriate to the athlete's target time:
- Under 40: baseline IF
- 40-49: baseline minus 0.01
- 50-59: baseline minus 0.02 to 0.03
- 60+: baseline minus 0.03 to 0.04
Training age matters as much as chronological age. An athlete in their third Ironman build handles a higher IF than the same athlete in their first, independent of calendar age. The first-timer adjustment is minus 0.03 to 0.05 regardless of how fit the athlete is in other disciplines.
Gender-specific data is limited but female age-groupers tend to hold slightly higher IFs for the same relative run performance, likely due to better fat oxidation at sub-threshold intensities. This is a small effect, 0.01 to 0.02, and individual variation dwarfs it.
This is where coached programming earns its fee. A coach with access to your historical files knows your durability at 4+ hours of riding, which is the real question. Our triathlon bike coaching sets these numbers for every athlete based on long-ride power decay, not a generic percentage.
Worked example: 250W FTP on a flat Ironman
Take a 42-year-old male age-grouper. FTP 250W (aero-tested, fresh). Target 10:15 finish. Course: Ironman Hamburg, essentially flat. Two prior Ironmans completed, best bike split 5:05.
Start with the IF band for a 10:15 target: 0.72 to 0.76. Flat course, so we can sit toward the top of the band. Age 42 subtracts 0.01. That gives a target IF of 0.73 to 0.75.
Convert to watts. 0.73 x 250 = 182W. 0.75 x 250 = 187W. So target NP is 182 to 187W.
Because VI on a flat course should land at 1.02 to 1.03, the target AP is roughly 178 to 183W. That is what the head unit shows as lap average.
Build execution rules from those numbers. First 30km: cap NP at 170W regardless of how good you feel. The bike is won in the last 60km. Middle 90km: settle into 182 to 187W NP. Final 60km: maintain, do not surge. Cap any 5-minute block at 200W (1.07 x target NP).
Nutrition target tracks the power. 182W NP for a 72kg athlete is roughly 720 kcal/hour of work, requiring 90 to 110g of carbohydrate per hour to avoid bonking the marathon. This is where Tim Spector's broader point about individual metabolic response matters: test your gut in training at race watts, not below.
Expected bike split at 187W NP on a flat course with decent aero position: 5:00 to 5:10. Expected marathon off that ride: 3:45 to 4:00. Total: 10:05 to 10:20. On target.
If you cannot state your FTP, your target IF, your target NP, and your three execution rules three weeks out from race day, you are not ready to race. Write those five numbers down today and test them in your next 4-hour ride.
