Guide · Intervals

Treadmill intervals by distance: how they're measured, what incline to use and how to pace them

By Luis Carvajal · Updated 1 October 2026

In short: a treadmill doesn't measure the distance you run. It calculates it from belt speed and time, and it can be a few percent off. Setting a 1% incline is a convention that comes from a 1996 study and works as an approximation, not an exact rule. The most reliable way to pace your intervals isn't the screen. It's your breathing.

How the treadmill knows how far you've gone

Outside, distance comes from GPS or a measured track. On a treadmill you don't go anywhere, so the machine does a sum: belt speed times time. If it shows 12 km/h (7.5 mph) for five minutes, it logs a kilometre.

The catch is that the real belt speed isn't always what the screen says. One study used cameras to measure the belts of four different treadmills while people walked and ran. Average speed ranged from 98% to 103% of the set speed, depending on the model. On top of that, the belt slows slightly with every footstrike, and more so the heavier the runner (Willwacher et al., 2021).

In plain terms: your 1,000 m rep might really be 980 or 1,030. For training, it doesn't matter. For comparing your treadmill times with your track times, it does a bit. And between the gym treadmill and the one at home, better not to compare at all.

The 1% incline rule: where it comes from

On a treadmill there's no headwind, not even the air you push aside as you move forward. So at the same speed, treadmill running usually costs a little less than running outside. The idea of a slight incline is to make up for that.

The 1% comes from a study by Andrew Jones and Jonathan Doust. Nine trained runners who were used to treadmills ran at six speeds, from 10.5 to 18 km/h (6.5 to 11.2 mph), with the treadmill at 0%, 1%, 2% and 3%, and also on a flat road. The researchers measured how much oxygen they used in each case (Jones and Doust, 1996).

At the two slowest speeds, 10.5 and 12 km/h (6.5 and 7.5 mph), the road couldn't be told apart from 0% or 1%.
At 13.5 km/h (8.4 mph), only 1% matched the road.
Faster than that, the road fell between 1% and 2%.

That's where the 1% advice comes from. But it's worth reading with some nuance. A 2026 review looks back at thirty years of studies and concludes that the equivalence depends on speed, conditions and the individual, and that a fixed 1% doesn't always reflect the cost of running outside (Bottura and Fletcher, 2026). Another recent study, with twelve runners between 8 and 15 km/h (5 and 9.3 mph), found that running on a track still cost more than on a treadmill at 1% (Shahidi et al., 2026).

The practical version: 1% is a good starting point, especially if your intervals are fast. At easy paces it makes little difference. And if you pace by feel, as you'll see below, incline matters less, because you're adjusting to the effort, not to a number.

Three sample sessions

All three start the same way: 10 minutes of very easy jogging, building up gradually. They finish with 5 to 10 minutes of easy jogging or walking. Recoveries are slow jogging or walking, never stepping off the belt.

To start: 6 × 400 m

Six short reps with 90 seconds of recovery. This is the session for learning to find your speed. If it's your first time doing intervals, start with four.

The classic: 5 × 800 m

Five reps with 2 minutes of recovery. Long enough for the effort to settle in, short enough that your mind doesn't get bored staring at the wall.

Longer: 4 × 1,000 m

Four reps with 2 to 3 minutes of recovery. The first one takes the most patience: go out hard and the third will feel endless.

If you'd rather mix it up without thinking too much, fartlek works on a treadmill too: you change speed when you feel like it, with no set distances.

How to pace by feel, not by speed

On a treadmill it's tempting to pick a speed and hold it no matter what. It works better the other way round: decide on the effort and let the speed follow.

Use the first rep to test the water. Pick a speed where, by the end, you can say a few words but not a full sentence. That's the talk test, validated against the ventilatory and lactate thresholds.
Adjust half a step at a time. If the first rep felt comfortable, go up 0.5 km/h (about 0.3 mph) on the next. If you only just made it, go down 0.5. Small changes.
The last rep should feel like the first. If you have to drop the speed a lot on the last one, you started too hard. Next time, start easier.
Use an effort scale. On a scale from 1 to 10, 400 m to 1,000 m reps usually sit at a high but controlled effort, not all out. The rating of perceived exertion is a recognised way to measure intensity without any gadgets.

On a hot day, after a bad night or on a different treadmill, the same speed will cost you more. That's fine. Effort is what you're training, not the number on the screen.

Treadmill safety

A treadmill looks harmless, and it isn't quite. Consumer Reports' safety tips are simple:

Clip the safety key to your clothing. If you fall or drift backwards, the key pulls out and the belt stops.
Start with your feet on the side rails, not on the belt, and step on once it's moving slowly.
Don't step off while the belt is moving. Wait for it to stop completely. During recoveries, lower the speed rather than jumping onto the side rails.
Look ahead, not at your feet. Looking down makes it easier to lose your balance.
Leave clear space: at least 2 feet (60 cm) on each side and 6 feet (about 2 m) behind.
At home, keep the key away from the treadmill and out of children's reach.

If you feel dizzy, get chest pain or a shortness of breath out of proportion to the effort, stop the treadmill calmly and step off once it has stopped.

Coached Run

Coached Run has distance intervals of 400, 600, 800, 1,000, 1,200 and 1,600 m, and the coach counts in metres, tells you when you're halfway, when you're into the last hundred and when it's time to jog. On a treadmill, distance is calculated from the speeds in your profile, and the app tells you it's an estimate; if you prefer, the same session can run on time. With an Apple Watch, a tap on the wrist marks each change, in case the treadmill noise drowns out the voice. You'll never see your pace on screen. Three full sessions free, then a one-time purchase, no subscription.

Frequently asked questions

Should I set the treadmill to 1% for intervals?

It's a good reference, especially at higher speeds, but it isn't exact for everyone. If you pace by effort, there's no need to obsess over incline. What matters is always using the same one so you can compare yourself.

Is the distance on the treadmill accurate?

It's an estimate based on belt speed and time. In a study of four treadmills, the real speed ranged from 98% to 103% of the set speed. Good enough for training, not for comparing with the track to the second.

How much rest should I take between treadmill intervals?

As a guide, from 90 seconds for 400 m reps to 2 or 3 minutes for 1,000 m reps. If you still can't talk in short sentences after the recovery, make it longer.

Can I jump onto the side rails to recover?

Better not. Stepping on and off a moving belt is one of the easiest ways to fall. Lower the speed and recover jogging or walking.