Ironman Power Calculator:
How many watts will it take?

Alan Couzens, M.Sc. (Sports Science)

Aug 6th, 2015

It’s that time of year – race season! For many athletes, the ‘hay is in the barn’ and the emphasis has shifted from building fitness to getting the most possible race day speed from the fitness that you have been able to build. A large part of that comes down to aerodynamics on the bike.

Hopefully by the time we reach that final lead in to the race, the coach & athlete will have a fair idea of a likely range of power output on race day. By using key sessions like metric simulation workouts & other key ‘Big Days’ under race specific conditions, assessing this number is relatively straight forward. However, tying this number in to the speed that the athlete can expect for that power is a little more complicated.

The bulk of Ironman competitors will be travelling to their race destination and won’t have the luxury of training (and benchmarking speed vs power) on the actual race course. Of course, the next best option is to build the closest course that you can in your homeland from what you have available, but even then, your chances of having access to an uninterrupted section of road that directly mimics your race in terms of elevation (& dynamics of elevation change), wind speed (& direction) & altitude is not great. Chances are that you will still be coming up a little short in directly mimicking the specifics of your race course. In this case, you may find the power calculator below useful.

The calculator is based on the power model of Martin et al. (1998) along with the CdA estimation model & equipment testing of Bassett et al. (1999). It estimates the power requirement for a given course based upon the size, weight & position of the cyclist, the various equipment used along with the particular course specific combination of ‘resistance factors’ that make up that course, i.e. elevation changes, wind speed etc. This is a similar method to what the ibike power meter uses, i.e. if we know all of the resistive forces (& ideally CdA), we can 'work back to' the power the cyclist is putting out.

To use, plug in your height, weight and the calculator will come up with a starting estimate for frontal area based on Bassett’s formula. You can fine tune this frontal with the slider depending on if your position is more aggressive (lower trunk angle) than average or less aggressive (greater trunk angle) than average. There will be some trial and error here and that’s the point! The idea being that you can benchmark on your home courses (with race set up), come back to the calculator to identify a good, consistent number for your personal frontal area and then apply that frontal area to your target race course.

As a general guideline, if we compare the frontal area number produced by the calculator to actual trunk angle testing performed by Heil (2001), the 'aggressive' end of the slider will correspond with a trunk angle (line from trochanter to acromion) in the 0-15 degrees range (a typical track position or an aggressive elite tri position). The mid point of the scale will correspond with trunk angles of ~30 degrees (average 'mid-pack' age-group aero), while the far end would be a very relaxed, upright 45-60 deg, i.e. a typical hoods position or a very slack tri position (for someone limited by mobility or a 'spare tire' :-).

It's important to emphasize, though, that in order to have truly transferrable value to your race, you will want to ‘calibrate’ the tool with an actual real life estimate of your CdA created by assessing speed under conditions (and positions) as close to race specific as possible. In other words, don't take your favorite static shot of your trunk angle from your 'in the bike shop' fit session and work back. Don't get your speed from your most Boardman-esque impression on a flat road or velo. Get the numbers from a long race-specific ride over race specific terrain. When it comes to the realities of Ironman racing, best case scenario 'guesstimates' don’t cut it.

An additional benefit of performing this sort of 'real world' trial and error testing is that you will get a good honest assessment of how aerodynamic your 'every day' position is. In my humble opinion, this method is far superior for the Ironman athlete to things like windtunnel, velodrome and ‘coast down’ testing because it is a ‘real’ race specific position across race specific terrain. In a windtunnel, an athlete can bury their head (to the point that they can’t see) and can close off the hip position to the point that they lose all efficiency, in order to generate a really impressive CdA! But how applicable is it to the real world? This is even more true for a coast down test, in which the athlete doesn't even need to pedal(!). Applying this very non-specific CdA to a 5hr race effort on a busy course is questionable to say the least!

Next, bike weight refers to race day weight (including nutritional products, fluids etc). Go to the trouble of actually weighing your loaded bike (don’t go by what your salesman told you when he was selling you the thing! :-)

Next up come the course details…

I’ve provided several sample courses on the drop down menu (that will auto-populate the elevation and average wind speed fields). I’ll keep adding to this over time but for now, if your race isn’t listed, I’d recommend getting elevation data from Garmin Connect or Strava (not mapmyride which notoriously underestimates elevation) & likely wind data from a source like Accuweather or Weatherspark and plugging it in manually. Note: You may find when looking at these files that some races, e.g. Roth are consistently ‘short’ so be sure to change the distance accordingly.

The rider position (aero, drops, hoods) will add the Cd element to CdA, i.e. how ‘round’ vs ‘square’ your position is likely to be.

You can then modify this CdA slightly by changing the component specs – frame, wheel, clothing and helmet type

You can select whether the race is draft-legal or not (or I guess whether you intend to break the rules and cheat anyhow and turn a non-drafting race into a much faster drafting race as so many seem to these days). Additionally, you can select the 12m legal draft option to assess the benefit of riding strategically and holding a *legal* position behind another athlete going your target speed, especially on headwind sections of the course.

Finally, enter a goal race time and click calculate…

The calculator will go to work to give you an estimated power output for the course, along with an estimated TSS (in red if it exceeds the generally recommended max TSS for an Ironman split in which an athlete typically runs well off the bike).

You can then hit back and ‘play around with’ various speed/equipment scenarios (& potentially different race selection) scenarios.

While the algorithm is admittedly a simplified one that doesn’t account for all possible sources of resistance, I’ve found after playing with it (& refining it) a bit that the power that it spits out (best used as NP to account for any zeroes from coasting/soft-pedalling on hillier courses) tends to line up pretty well with the actual Ironman split when used in the way described, i.e. after it is 'calibrated' to the specifics of the cyclist on known courses. Hopefully you’ll also find it useful in terms of identifying likely bike split/time on course for unfamiliar races.

Race smart,


Ironman Power Calculator

Height (m):
Weight (kg):
TT Frontal Area (m^2)Road Frontal Area (m^2)
Aggressive Average Conservative
Bike Weight (kg):
Distance (km):
Altitude (m):
Elevation Gain (m):
Headwind (km/h):
Tailwind (km/h):
Rider Position:
Bike Type:
Wheel Type:
Time (hrs/mins):

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