Is a treadmill easier than running outside?
Air resistance is the whole of it. Indoors you are not pushing a column of air out of the way, which costs more the faster you go. The belt does not pull your legs through for you, which is the usual myth.
Below about 13.5 km/h the difference was too small to measure, so for easy running the incline hardly matters. Above it, 1% closed the gap.
That number comes from nine runners in 1996, and newer work is less tidy: a 2026 track comparison found outdoor running still cost about 4% more than a treadmill at 1%, and a review of the last thirty years concluded that no single gradient fits every speed and every runner.
In practice: leave it flat for easy runs and go by effort, and set 1% when you are trying to hit outdoor paces indoors. The treadmill pace converter does the conversion at any gradient.
What a treadmill genuinely does change is everything around the running. No wind, no hills, no stopping at junctions, and a pace you cannot drift from. That tends to make the session feel harder, not easier.
- Jones, A.M. & Doust, J.H. (1996). A 1% treadmill grade most accurately reflects the energetic cost of outdoor running. J Sports Sci 14(4). Nine trained men at six speeds. At 10.5 and 12 km/h a flat belt already matched the road; from 13.5 km/h upwards it took 1%. This one study is where the convention comes from.
- Shahidi, S.H. et al. (2026). Overground running incurs a higher energetic cost than treadmill running at a 1% grade. PLoS One 21. Twelve trained runners at 8 to 15 km/h: running on a track cost about 4% more oxygen per kilometre than a treadmill at 1%, so the correction fell short.
- Bottura, R. & Fletcher, J. (2026). Beyond the 1% treadmill rule: 30 years of evidence on treadmill and overground running economy. Appl Physiol Nutr Metab 51. Review: the gap depends on speed, conditions, method and the individual, so no fixed gradient fits every case.