Run Off The Bike Calculator

Almost nobody blows up on the swim. Very few people blow up on the bike. The triathlon run is where plans fall apart, and the reason is that it is the only leg you start already depleted. By the time you leave T2 in a 70.3 you have been working for three and a half hours, burned most of your readily available glycogen, and spent the last several of those hours in a position that keeps your hip flexors shortened and your running muscles largely idle.

The result is that your standalone run pace — the one from your last road 10K or your Tuesday track session — is not a number you will see on race day. This calculator exists to turn that fresh pace into a realistic one by adding a penalty in seconds per kilometre, and it deliberately publishes every component of that penalty so you can argue with it.

Bike intensity is the biggest lever you control. The gap between riding controlled and overbiking is 45 seconds per kilometre. Over an Ironman marathon that is more than half an hour — far more time than you gained by riding hard in the first place. This is the arithmetic behind the oldest advice in the sport: nobody ever won a triathlon on the bike, but plenty of people lost one there.

Heat is the biggest lever you do not control. Hot conditions add 35 seconds per kilometre in this model, and unlike bike intensity you cannot decide to opt out on the morning. What you can do is plan for it in advance: start slower than feels necessary, take cooling at every aid station, and accept the slower target before the race rather than discovering it at 30K.

Nutrition is the lever people ignore. Low fueling confidence adds 28 seconds per kilometre, which puts it on par with a strong bike effort. "Low confidence" means an untested plan, a gut that has rebelled before, or a carbohydrate target you have never actually practised at race intensity. Fixing this one costs nothing but training-day discipline.

The disciplined 70.3. A 5:00/km standalone runner, riding controlled, in mild conditions, with a well-practised fuel plan: 45 base + 0 bike + 6 heat + 0 nutrition = 51 seconds per kilometre. Race pace becomes 5:51/km and the half marathon lands around 2:03. That is a realistic, executable target.

The same athlete, badly planned. Same fitness, but they chase a bike split, the day turns hot, and their nutrition is guesswork: 45 + 45 + 35 + 28 = 153 seconds per kilometre. Race pace collapses to 7:33/km and the half marathon takes 2:39 — thirty-six minutes slower on identical legs. Every one of those three penalties was a decision, not bad luck.

The gap between those two rows is the entire argument for pacing discipline. It is almost never fitness that separates a good race from a bad one at the same event; it is whether the first four hours were run by the plan or by the ego.

These penalties are population averages, and you are not a population. The way to make them yours is a brick run done honestly: ride your intended race intensity for a realistic duration, then run 20-30 minutes at what feels like your goal effort and record the actual pace. Compare it to your standalone pace for the same effort and you have your personal penalty.

Do that two or three times across a training block and you will find your number is stable. Most athletes discover they sit somewhere between the model's controlled and strong settings, and that their real weakness is heat tolerance or fueling rather than bike restraint — or, more often, the reverse.

Once you have a figure you trust, feed the adjusted pace into the split calculator so the full race plan reflects the run you will actually have, not the one your fresh legs suggest.