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Estimate your finish time and every split separately for an Olympic distance triathlon, a half Ironman 70.3 or an Ironman 140.6. No hand-waved percentages: the swim comes from your CSS and the course conditions, the bike from solving the power equation against the real elevation of the route, and the run from your threshold pace with the penalty of having ridden first.
Don't know it? Work it out in the swim pace and CSS calculator.
Get it from a test in the running pace calculator.
| Split | Time | Pace or power |
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This is the decision that ruins long distance races more than any other. The table recalculates the whole day moving only the percentage of FTP on the bike: you gain minutes riding and hand them back with interest running.
| Bike intensity | Bike | Run | Total | Difference |
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Most triathlon predictors ask for your times in three standalone races and add them up with a blanket penalty. This one does not: it calculates each split from the variable that actually governs it, and carries the effect of one split into the next.
The starting point is your critical swim speed in the pool. On top of it come multiplicative corrections for the distance of the leg, because nobody holds their exact CSS for 3,800 metres, and for the real conditions of the water:
The wetsuit shows up as a fixed correction, but in reality it helps more the worse you float: a swimmer whose legs sink can gain 8 %, while a technically strong one gains barely 3 %.
There are no percentages here. The target power is set as a fraction of your FTP and the calculator searches numerically for the speed at which that power balances the three resistances:
The course is split into two halves, one climbing and one descending at the average gradient implied by the elevation you entered, and each is solved separately. It is a simplification —one long col is not the same as a hundred rollers— but it captures the essential point: elevation costs time even when the profile returns to where it started, because you climb slowly for many minutes and descend quickly for very few.
Run pace starts from your threshold and takes two corrections. The first is distance: nobody runs a marathon at threshold pace. The second is what makes a triathlon a triathlon, the penalty for having ridden first, and it grows very fast with the intensity you rode at:
The cubed exponent is the key, and it is what almost nobody internalises. Riding 10 % above the reference intensity does not increase the penalty by 10 %: it increases it by 33 %. That is the exact mechanism by which people blow up in an Ironman marathon after having had "a great day on the bike".
With good inputs the bike split is the most reliable, around 3-4 % if you know your FTP and your position. The swim sits near 5 %, because open water navigation varies enormously. The run is the worst, easily 8-10 %, because it depends on heat, fuelling and decisions made on the move. Overall, expect a 5 % margin on the finish time: in a five and a half hour 70.3, about fifteen minutes either way.
On the flat in still air, almost all your power goes into pushing air aside. Drag grows with the square of speed, and the power needed to overcome it with the cube.
Ten per cent more speed does not cost 10% more watts, it costs 33%. And the other way round, which is the useful part: easing off 5% on the bike hands you back 14% of your power, and that is the power you still have for the run. Uphill it works differently — there weight and gravity dominate and the relationship gets close to proportional.
Between 76 % and 84 % of FTP for most people, closer to the lower end if it is your first 70.3 or the course is hilly. Above 85 % the bike split improves by a few minutes and the half marathon degrades by considerably more than you gained.
Between 65 % and 75 %. It looks low and feels comfortable for the first few hours, which is precisely the trap: most Ironman marathon disasters are decided in the first two hours on the bike.
In an Olympic distance race, one to three minutes each. In a 70.3, two to four. In an Ironman with bags and a change tent, four to eight minutes per transition, easily adding ten or twelve minutes to your official result.
You need an estimate of your FTP. Without a power meter you can get a reasonable approximation from the climbing watts calculator by doing a long steady effort on a climb, where the physics allows power to be estimated with little error.
Because it should be. Running 10 km after 40 kilometres on the bike is nothing like running a fresh 10 km: the typical penalty runs from 10 to 25 seconds per kilometre in an Olympic distance race and can exceed a minute per kilometre in an Ironman.
No. Those are the three variables most likely to wreck a prediction and all three are highly individual. As a rule, above 28 degrees add 3 to 6 % to the run time, and considerably more if you are not acclimatised.
The model is tuned for Olympic, 70.3 and 140.6. In a sprint the transitions and the flat-out start weigh proportionally far more, so the estimate loses precision.
If this one was useful, these three will be too.
CalculatorWork out how many seconds per kilometre you will lose running in a triathlon against your standalone best, based on distance and bike intensity.
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CalculatorA week by week plan to raise your race carbohydrate tolerance, with the weekly increment, the session to rehearse it in and your product's ceiling.
Open →This calculator is an informational estimate and does not replace a coach or a laboratory test. Nothing you type ever leaves your browser.