Why flat pace fails on hills — and how to use Naismith's Rule on Garmin

Normal pace answers a simple question: how long did each horizontal kilometre take? On a flat road that is useful. On a steep trail it can make a controlled effort look disastrously slow, because it ignores the vertical work that caused the slowdown.

What Naismith's Rule adds

The classic rule starts with time for horizontal distance, then adds time for ascent. A common form allows one hour for every 5 km forward plus one hour for every 600 m climbed. The result is not a promise of your finish time. It is a terrain-aware baseline that makes a hilly route comparable with a flatter one.

See the adjustment during the activity

HillFactor is a free Garmin data field that applies this idea during a hike or trail run. It shows a Naismith-adjusted pace, estimated time and current grade beside the raw terrain values, so you can tell whether lost speed comes from the climb or from fading effort.

• Add the field to a Hiking, Walking or Trail Running data screen.
• Start the activity normally and allow the altitude and distance streams to settle.
• Use the adjusted number as a pacing reference, not a deadline.

When the number is most useful

The estimate is best for comparing your own effort across rolling routes, checking whether a long climb is progressing roughly as expected, and avoiding the temptation to chase a flat-road pace uphill. For race cutoffs or remote trips, always use the organizer's rules, route profile, weather and your own margin.