Triathlon Race Predictor

What the predictor assumes for each distance

These are the actual constants in the model, not a general description of it. Bike intensity is expressed as a fraction of FTP; the run penalty is added to your projected standalone pace; the swim adjustment converts pool pace to open water and is editable.

Source: the site's own pacing model. The bike split is solved from a power balance rather than a speed lookup, so mass and course profile change the result independently of watts.

Community rules of thumb, and what this model gives instead

Triathletes answer the 'how much slower off the bike' question with rules of thumb long before any calculator is involved. On short course this model matches them. On long course it is deliberately harsher, because the rules of thumb describe races that went well and the failure mode people actually post about is the Ironman marathon that fell apart.

Rules of thumb are the recurring answers on triathlon forums; the model column is computed directly from the constants in the table above. If your own brick sessions consistently beat these numbers, trust your data over the default.

How the run projection actually works

Sanity-check the prediction before you trust it

• Use a running race you have raced, not a training run. A parkrun effort and a 5k time trial in a session produce different predictions.
• Use a real FTP number. An inflated FTP - from an auto-estimated figure or a short test - propagates into an optimistic bike split and then hides the run cost of riding at that intensity.
• Raise the open-water adjustment above 10 s/100 m if this is an early open-water race, a non-wetsuit swim, or a crowded rolling start.
• Check the result against the cutoff calculator if you are close to the limits, since the bike gate is the one that ends most days.

Frequently asked questions

How do I predict my triathlon time from a 5k or 10k?

Enter the running race you have actually done and the calculator scales it to the triathlon run distance using Riegel's endurance formula, then adds a run-off-the-bike penalty for the distance you are racing. A 48-minute 10k runner (4:48/km) racing a 70.3 is projected at 5:01/km for a standalone half marathon and 5:46/km after 90 km of riding - a 2:02 run split.

Is a race predictor different from a finish time calculator?

Yes, and the difference is what you type in. This predictor starts from results you already have - a running race, an FTP test, a pool time - and works out what they imply. The finish time calculator starts from the swim, bike and run paces you intend to hold on race day. Use the predictor when you do not yet know what paces are realistic.

How much slower will I run off the bike?

This model adds 18 seconds per kilometre for a sprint, 24 for an Olympic, 45 for a 70.3 and 75 for an Ironman marathon, applied to your projected standalone pace. Over the full distance that is +1.5, +4, +16 and +53 minutes respectively. The long-course figures are deliberately more conservative than the rules of thumb triathletes trade on forums, which put a 70.3 run 5 to 12 minutes down on an open half and an Ironman marathon 20 to 40 minutes down on an open marathon. If you fuel well and ride within yourself you should beat this model; it is built to be the number that still holds on a bad day.

What FTP percentage does the prediction assume?

Intensity factor defaults to 0.88 for a sprint, 0.84 for an Olympic, 0.78 for a 70.3 and 0.70 for an Ironman. Those are conventional age-group race intensities. Riding harder than the assumed intensity does not simply produce a faster prediction - it also means the run penalty you should expect is larger than the default.

Why is my predicted swim slower than my pool time?

Ten seconds per 100 m is added by default to convert pool pace to open water, and you can raise it. Open water removes the walls you push off, the black line you follow and the lane that keeps you straight. Swimmers racing open water for the first time often need 15 to 20 seconds per 100 m rather than 10.

How accurate is a triathlon race predictor?

Treat it as a range, not a time. It cannot know the wind on the bike course, how well you fuel, whether the swim is wetsuit legal, or how much elevation the run has. It is at its most useful for deciding whether a goal is plausible and for seeing which discipline is actually limiting you.

Can I predict an Ironman time from a 70.3 result?

The common rule of thumb is to double the 70.3 and add 30 minutes to an hour, which puts a 6:00 half at roughly 12:30 to 13:00. That rule assumes the full-distance fuelling and long-ride volume are actually there. Entering the underlying fitness numbers here is more informative, because it shows which leg the extra time comes from.

Which discipline should I train to gain the most time?

Change one input at a time and watch the total. For most age-group athletes the bike leg holds the largest absolute gains simply because it is the longest, and the swim the smallest - gaining 5 seconds per 100 m saves under two minutes in a 70.3, while 10 watts of FTP can save several.

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