7:45:46 a.m. The mat at the swim exit beeps. The race is 45 minutes and 46 seconds old, there are 69.1 miles still to go, and a half Ironman has just turned into a question of hours rather than minutes.
How long does a half Ironman take? There is no single answer, but there is a way to see where every hour of it goes. Take one example plan: a swim at 2:10 per 100 yards, 17 mph on the bike, 10:30 miles on the run, and transitions of 6 and 4 minutes. Put it into the Triathlon Calculator and the answer is 6:31:04. Start the clock at 7:00 a.m., as an example, and follow the day from the water to the finish line.
The 70.3 day, hour by hour
7:00 a.m. The swim start
An Ironman 70.3 swim is 1.2 miles, or 2,112 yards. At 2:10 per 100 yards that takes 45:46. In a rolling start the clock is your own: it starts when you cross the mat into the water, and so do the cutoffs.
The first of them is 1:10 for the swim. This plan reaches the exit with 24:14 to spare, which is a comfortable margin, and it is also the tightest margin of the whole day. That surprises people. The swim is the shortest leg in time, just 11.7% of the race, but it is the one place where a slow day can end the race outright.
7:45:46 a.m. Out of the water, into T1
Wetsuit off, helmet on, shoes, then a run with the bike to the mount line. Six minutes in this plan. The calculator treats transitions as plain minutes because that is what they are on the result sheet: two separate times, T1 and T2, between the legs. Ten minutes in all, 2.6% of the day.
Ten minutes sounds small next to six and a half hours. But it is time that costs no fitness to win back, only practice: the same layout of shoes, helmet and bike every time, in training as on race day.
7:51:46 a.m. Bike out
Now the long part. 56 miles at 17 mph take 3:17:39, and that is 50.5% of the whole race. Half the day is spent on the bike.
Because the bike is that big, its speed moves the finish more than anything else. One mph more, 56 miles at 18 mph, takes 3:06:40, which is 10:59 faster. One mph less, 56 miles at 16 mph, takes 3:30:00. For comparison, swimming 10 seconds per 100 yards faster across the whole swim would save about three and a half minutes.
Around mile 20. The rider ahead, and why you stay 12 m back
Somewhere on the course there is always a rider just ahead at almost your speed. Sitting on that wheel would be very cheap. The Cycling Drafting Calculator puts a number on it: at 17 mph, with rider and bike at 180 lb on the hoods, riding alone takes 118 W, while the second wheel needs only 77 W. That is 41 W, or 35%, less for the same speed. The same 118 W would carry you at 20.3 mph.
Over the 56 miles, that would turn a 3:17:39 bike split into 2:45:31, and the finish into 5:58:56. Which is exactly why it is against the rules. In IRONMAN and 70.3 age-group racing you have to stay out of a draft zone of 12 m behind the rider ahead, about six bike lengths, and a pass has to be completed within 25 seconds. The zone is there because the advantage is this large.
There is a legal way to buy part of it back: position. The same calculator gives 99 W alone at 17 mph on aero bars instead of 118 W on the hoods, about half of what a wheel would give, and nobody can take it away from you.
11:09:25 a.m. Into T2
Swim, T1 and bike add up to 4:09:25. The second cutoff, 5:30 for everything up to the end of the bike, is 1:20:35 away. Four more minutes in T2: rack the bike, change shoes, out.
11:13:25 a.m. Run out, straight into the wind
The last leg is 13.1 miles, a half marathon on legs that have just done three hours on the bike. At 10:30 per mile it takes 2:17:39, and if the morning is calm the race ends at 1:31:04 p.m.
Now suppose the wind has picked up by late morning. Say the forecast gives 15 mph and the run course is out and back on open ground. The Running Headwind Calculator first brings the forecast down to runner height, where it is about 11.4 mph, then finds the pace that costs the same effort as 10:30 on a calm day. On the way out, into the wind, that is 11:33.7 per mile. On the way back, with the wind behind, 10:13.4.
The way back does not pay for the way out. Into the wind you lose 1:03.7 per mile; with it behind you get back only 16.6 s. On average the run slows by 23.5 s per mile, and over the 13.11 miles of a half marathon that is 5:08. The run becomes 2:22:47 instead of 2:17:39. The reason is the square in the drag equation: a headwind raises the air speed against you a lot more than the same wind behind you lowers it.
Where the course runs matters as much as the wind. Among houses and trees much less of the forecast reaches a runner, and the same 15 mph costs only 15.6 s per mile on average. That is the difference between a lakefront and a suburban loop.
1:31:04 p.m. or 1:36:12 p.m. The finish
Calm, the finish comes at 1:31:04 p.m., 1:58:56 inside the 8:30 limit. With the 15 mph wind it comes at 1:36:12 p.m. The plan was the same. Only the air was different.
The whole day in one table
Here is the calm version of the day from the first table of the calculator, with the clock added and the margin left to each standard cutoff.
| Clock | Leg | Time | Elapsed | Margin to cutoff |
|---|---|---|---|---|
| 7:00:00 a.m. | Swim start | 0:00 | ||
| 7:45:46 a.m. | Swim, 2,112 yd | 45:46 | 45:46 | 24:14 under 1:10 |
| 7:51:46 a.m. | T1 | 6:00 | 51:46 | |
| 11:09:25 a.m. | Bike, 56 mi | 3:17:39 | 4:09:25 | 1:20:35 under 5:30 |
| 11:13:25 a.m. | T2 | 4:00 | 4:13:25 | |
| 1:31:04 p.m. | Run, 13.1 mi | 2:17:39 | 6:31:04 | 1:58:56 under 8:30 |
Read it from the right-hand column. The finish limit is far away; the swim limit is the closest. For a slower swimmer the picture changes completely, and the calculator shows the first limit that fails rather than only the last one.
The same day, played differently
What would have changed the clock most? Each row changes one or two things and keeps the rest of the plan.
| Version of the day | Bike split | Run | Finish |
|---|---|---|---|
| The plan, calm air | 3:17:39 | 2:17:39 | 6:31:04 |
| 15 mph wind on the run (out and back) | 3:17:39 | 2:22:47 | 6:36:12 |
| 20 mph wind on the run (out and back) | 3:17:39 | 2:25:00 | 6:38:25 |
| 18 mph on the bike, calm | 3:06:40 | 2:17:39 | 6:20:05 |
| 18 mph on the bike, 15 mph wind on the run | 3:06:40 | 2:22:47 | 6:25:13 |
| Same watts on a wheel, 20.3 mph (not allowed) | 2:45:31 | 2:17:39 | 5:58:56 |
One mph on the bike is worth about eleven minutes. A 15 mph wind on the run takes about five of them back. And the last row, the one the rules forbid, is worth more than half an hour. If a training block has to choose, the bike is where the hours of a 70.3 live.
Cheat sheet: bike splits and wind on the run
The first table is the 56-mile bike split at each speed and the finish it gives with the same swim, run and transitions as above. Every row opens the calculator with that speed already filled in.
| Bike speed | 56-mile split | 70.3 finish |
|---|---|---|
| 15 mph | 3:44:00 | 6:57:25 |
| 16 mph | 3:30:00 | 6:43:25 |
| 17 mph | 3:17:39 | 6:31:04 |
| 18 mph | 3:06:40 | 6:20:05 |
| 19 mph | 2:56:51 | 6:10:16 |
| 20 mph | 2:48:00 | 6:01:25 |
| 21 mph | 2:40:00 | 5:53:25 |
| 22 mph | 2:32:44 | 5:46:09 |
The second is the run of this plan, 10:30 per mile on a calm day for a 160 lb runner, in other forecast winds on open ground. The last column is the whole half marathon on an out-and-back course.
| Forecast wind | Headwind, per mile | Tailwind, per mile | Out and back, per mile | Over 13.11 mi |
|---|---|---|---|---|
| 5 mph | +14.5 s | -7.5 s | +3.5 s | +45.9 s |
| 10 mph | +35.8 s | -9.4 s | +13.2 s | +2:53.1 |
| 15 mph | +1:03.7 | -16.6 s | +23.5 s | +5:08.1 |
| 20 mph | +1:38 | -30.8 s | +33.6 s | +7:20.5 |
| 25 mph | +2:18.7 | -51.6 s | +43.6 s | +9:31.6 |
| 30 mph | +3:05.9 | -1:18.3 | +53.8 s | +11:45.3 |
At 25 and 30 mph the calculator adds a warning for this runner, and it is worth reading: the study behind its effort model held runners back with forces of up to 8 N, and a strong headwind pushes harder than that, so the bigger numbers are a trend rather than a promise.
Planning a different distance? The same example paces give a quick answer to how long a super sprint triathlon takes, how long a sprint triathlon takes, how long an Olympic triathlon takes, how long a half Ironman takes and how long a full Ironman takes. For the full distance, the 112-mile bike split at 14, 15, 16, 17, 18, 19 and 20 mph comes with a check against the 10:30 bike cutoff.
Tools discussed in this article
More sports tools
- Swim Pace Calculator: pace per 100 yd or 100 m and CSS for the swim leg
- Swimming Heart Rate Calculator: heart rate bands that fit the water
- Cycling Power Calculator: watts for a speed, with wind, grade and position
- FTP Calculator: functional threshold power from a 20-minute, ramp, 2 x 8-minute or hour test
- Power Zones Calculator: seven training zones in watts from your FTP
- Cycling Cadence Calculator: the rpm for a speed in one gear
- Bike Gear Calculator: every gear of a drivetrain in gear inches and speed
- Half Marathon Splits Calculator: mile splits for the run of a 70.3
- VDOT Calculator: training paces from a running race
- Tanda Marathon Predictor: a marathon time from eight weeks of training
- Running Cadence Calculator: steps per minute from pace and step length
- Treadmill Calculator: treadmill speed and incline as an outdoor pace
- Beep Test Calculator: VO2max from the 20 m shuttle run
- Sweat Rate Calculator: how much to drink over a six-hour race
- Climbing Grade Converter: route and boulder grades between scales
- Weather Pace Adjustment Calculator: heat, humidity, wind and surface in one pace correction
- Running Pace Calculator: pace, time and distance for any run
- Calories Burned Calculator: energy used on a long training day