Guides · Half marathon on a treadmill
Running a half marathon on a treadmill: is 13.1 really 13.1?
The 13 miler in your half plan lands on a day the weather wins, so it goes on the belt. More than two hours later the display rolls past 13.1, you hit stop, and you have a half marathon and a time. Then the doubt creeps in. You never had a way to check that 13.1. The display said it, and you stopped.
Yes, you can run a half marathon on a treadmill. Whether the belt carried you 13.1 miles is a separate question, because the display gives you no way to check its count against the belt’s own moving surface. Measure the belt at the speed you will hold, before the run.
A half marathon ends at 13.1094 miles
A half marathon is 13.1094 miles, or 21.0975 kilometers. That fourth decimal matters on a display that shows two. If you stop the moment 13.10 appears, you are at least 15 meters short of a half, and that is before the belt gets a say. Hold on until the display reads 13.12. That is past a half whether your display rounds or cuts off its last digit. It does nothing about whether the count itself is right.
The display gives you a count, not a check
The 13.1 on the display is a count to two decimals, and it looks exact. Even so, nothing on the display lets you check that count against the belt that carried you.
Think of an estimated electric bill. Some months nobody reads the meter, so the utility bills you from an estimate. The estimate is still printed to the cent. The cents make it look careful, but they are not a reading. Your 13.1 carries two decimals the same way, and decimals alone do not make it a reading. The belt under your shoes is the meter, and the display gives you no way to check it yourself.
A road half is the opposite. Its 13.1 was measured before anyone ran it, as the guide on how a road course gets its distance explains.
What a belt off by 2 percent does to your half
Say you plan the run by the clock. You set 5.5 mph, which the display calls a 10:55 mile, and you hold it for 2:23:01, the time a half takes at that pace. The ground you covered is the belt’s true speed multiplied by that time.
A 5.5 mph setting held for 2:23:01 on three belts
Every row was set to a 10:55 mile. The belts are invented, and I did not measure any of them.
| Belt | True mph | True pace | Ground covered |
|---|---|---|---|
| Exactly right | 5.50 | 10:55 | 13.11 miles |
| 2 percent slow | 5.39 | 11:08 | 12.85 miles |
| 2 percent fast | 5.61 | 10:42 | 13.37 miles |
Paces are per mile. The two off belts sit inside 98 to 103 percent of the setting, the range a lab study found for average belt speed on four treadmills, one of them documented as worn, with people walking and running on them.
On the 2 percent slow belt, you stopped about a quarter of a mile short of a half marathon. The time in your log belongs to a run that ended before the distance did. On the 2 percent fast belt, you had covered a full half marathon more than two and a half minutes before you stopped, so the log undersells the half you really ran.
Whether the display shows any of that gap is another matter. In my own runs, the display has shown more distance than the belt carried me. The guide on what the same gap does to a whole block of long runs goes further.
Your watch cannot vouch for the 13.1
Indoors, your watch works out distance from the motion of your wrist. Some watches also ask you, after a treadmill run, to type in the distance the treadmill showed, so a watch that agrees at 13.1 may simply be repeating it. The guide on what your watch was counting for those two hours, and why it cannot catch a short belt has the full story.
Does a half marathon on a treadmill count?
A half marathon on a treadmill counts as training in full. It counts as your half marathon if the belt really carried you 13.1 miles. A race sets its own rules, and a virtual race may take a treadmill result or refuse it. The guide on running a virtual race on a treadmill covers those rules.
Know the distance before the run, not after it
Measure the belt at the setting you will hold, on the treadmill you will run the half on. From then on, the display is not what tells you the distance. Your time multiplied by the belt’s true speed is.
Say your belt measures 5.39 at the 5.5 setting. A true half at that speed takes 2:25:56, so you keep going 2 minutes 55 seconds past the time the setting’s pace promised. If it measures 5.61, a true half takes 2:20:12. If you plan to change speeds during the run, each stretch is its own multiplication, so measure every setting you will use.
A belt that measures off is a job for your manual or a technician. You can still run a true half on it, because the clock and the measured speed already tell you where 13.1 is. If the half is already behind you, measure the belt at the setting you held and do the same multiplication with the time in your log.
Measure the belt before your half
Treadwell reads your belt’s true speed through an iPhone Pro’s camera and LiDAR sensor. The day before your half, stand on the floor beside the deck and bring the empty belt up to the speed you plan to hold. Aim the phone down at it as it turns, and your clock has the number it needs.
Make your 13.1 a real 13.1
Learn what your belt truly runs at your half marathon setting, so the clock can tell you when the half is done. Treadwell runs on iPhone Pro models, because the reading depends on their LiDAR sensor.
Scan with your iPhone Pro to get the appTested: in our tachometer tests the app read within about 2 percent of true belt speed.
Quick answers
Can you run a half marathon on a treadmill? Yes, you can run a half marathon on a treadmill. Whether the belt carried you 13.1 miles is a separate question, because the display gives you no way to check its count against the belt’s own moving surface. Measure the belt at the speed you will hold, before the run.
When can you stop a half marathon on a treadmill? You can stop once the display reads 13.12 if the machine’s count is right, because a half marathon is 13.1094 miles and stopping the moment 13.10 appears leaves you at least 15 meters short. On any machine, your running time multiplied by the belt’s measured speed is what tells you whether you covered 13.1.
Does a half marathon on a treadmill count? Yes. It counts as your half marathon if the belt really carried you 13.1 miles, and it counts as training either way. Whether it counts for a race is the race’s call, so read the entry page before you run, because some virtual races take treadmill results and some refuse them.
Sources.
- The length of a half marathon: World Athletics, Half Marathon discipline page, which gives 13.1094 miles or 21.0975 km. The 15 meters is my arithmetic on that figure.
- Estimated electric bills: PG&E Electric Rule No. 9, a filed California utility tariff. Section C and the bi-monthly reading provision say that when a meter is not read, the utility bills estimated use and trues up the bill after the next reading. It backs the everyday idea of an estimated bill and nothing more.
- Belt speed within 98 to 103 percent of the setting: Willwacher et al., Scientific Reports 11:2244, 2021. Its seven participants walked and ran on four laboratory treadmills, one of them documented as worn, and average belt speed stayed within that range of the set speed.
- How a watch counts indoor distance: for indoor runs, Apple’s watch relies on its accelerometer (Apple Support article 105002). Garmin’s owner’s manuals for the Forerunner 165, 255, 265, 955, 965 and 970 each include a Calibrating the Treadmill Distance section, where you enter the distance the treadmill display shows.
- Virtual race rules: the RunSignUp Thankful Turkey 5K, 10K and Half accepts treadmill runs, and the NYRR Virtual Turkey Trot 5K does not accept them. The virtual race guide linked above has the detail.
- On my own runs, the display showed more distance than the belt carried. On treadmill #6 the display read 2.3 miles against a true 2.0 on June 13, 2026, and 5.6 miles against a true 5.0 on June 14, 2026. Both were graded against that machine’s standing tachometer table, and the Apple Watch I wore echoed the display on both. On a third run the display read 5 miles against a true 4, and that run’s machine and date were not recorded.
- The 5.5 mph runner, the three belts, the measured 5.39 and 5.61, and the electric bill are invented examples. I measured none of those belts.