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How to predict your Ironman time

How long will your first Ironman take? Andrew Hamilton offers a formula for determining when you'll hit the tape

Embarking on an Ironman involves a journey into the unknown. But predicting your first Ironman time is an imprecise science.

You might think that you can just take your steady-state training pace for each discipline and extrapolate from there, but it’s not as simple as that. The extraordinary length of an Ironman means that you’ll almost certainly be entering new territory with muscular fatigue, severe nutritional challenges, and the possibility of cramps, blisters, dehydration and more.

The art of prediction

The physiological demands of an Ironman have always fascinated sports scientists, with a 2010 study looking closer at how a novice triathlete might be able to better predict their race time.

The researchers studied 83 male recreational triathletes participating in Ironman Switzerland in 2009. During the afternoon prior to the race, the researchers took a number of physical measures – height, weight, body mass index and skinfold measurements – to determine body fat percentage.

The athletes had also been keeping comprehensive training diaries, which recorded data for each training session, including the distance, duration and average speed of sessions. They also gathered PBs for an Olympic-distance triathlon and a marathon.

At the end of the race, the times recorded for each triathlete were matched to the data gathered for that triathlete. The results were number crunched to see which variables predicted race times.

What emerged was that the triathletes’ marathon and Olympic-distance personal best times were related to their Ironman race time. Also, average run speed during training was related, though less strongly so. In fact, statistical analysis indicated that these three variables alone explained 64% of the Ironman race time.

This surprised the researchers; they’d expected that, in line with previous studies, body fat percentage, cycling volume and personal best time in an Olympic-distance triathlon would be most related to the Ironman race times.

OD + marathon = IM

The study indicates that either marathon or Olympic-distance triathlon PBs could be useful for predicting your Ironman time. But now a new study on 53 recreational female triathletes suggests it might be possible to combine both of these to yield a more accurate predictive formula.

Researchers found the strongest predictors were previous PBs for the marathon and Olympic-distance triathlon. The data was added to a graph and a line of best fit was calculated. When the data was analysed, the researchers were able to produce a formula to predict long-course race times (all times in mins):

Ironman time prediction = 186.3 + 1.595 × (PB for Olympic-distance triathlon) + 1.318 × (PB for marathon).

For example, if you can run a 3:30hr marathon and have recorded a 3:00hr PB for an Olympic-distance triathlon, your predicted Ironman time would be 12.5hrs.

Of course, this is no guarantee of your Ironman time but for novice triathletes who’ve previously completed a marathon and an Olympic-distance triathlon, it’s a good place to start.

How accurate was this formula for you? Let us know in the comments below!

Profile image of Matt Baird Matt Baird Editor of Cycling Plus magazine


Matt is a regular contributor to 220 Triathlon, having joined the magazine in 2008. He’s raced everything from super-sprint to Ironman, duathlons and off-road triathlons, and can regularly be seen on the roads and trails around Bristol. Matt is the author of Triathlon! from Aurum Press and is now the editor of Cycling Plus magazine.