Every hiking time estimate you’ve ever seen — the guidebook’s “4–5 hours,” the trailhead sign, the app’s prediction — descends from one sentence written by a Scottish mountaineer in 1892. Learn the sentence and its three modern corrections, and you can out-plan most of them with a pencil.

The rule itself

William W. Naismith’s original formulation, converted to modern units:

Allow 1 hour for every 5 km (3 miles) of distance, plus 1 hour for every 600 m (2,000 ft) of ascent.

That’s the whole thing. A 12 km hike with 900 m of climbing: 12 ÷ 5 = 2.4 hours, plus 900 ÷ 600 = 1.5 hours → 3 hours 54 minutes.

Two properties make it durable. It separates the two costs of hiking — covering ground and gaining height — so neither hides the other. And its constants have held up: a century of route cards, and modern GPS datasets, keep landing near Naismith’s numbers for fit walkers on good trails.

Correction one: your actual speed

Naismith walked fast. His 5 km/h baseline suits a conditioned hiker with purpose; it flatters most of us. Practical speed bands:

  • New to hiking: ~3.2 km/h (2 mph)
  • Average trail fitness: ~4 km/h (2.5 mph)
  • Fit and experienced: ~4.8 km/h (3 mph) — Naismith’s own pace
  • Mountain athlete: 5.6+ km/h

Note the ascent term stays fixed — climbing 600 m costs everyone about an hour; fitness mostly changes how fast you cover the flat.

Correction two: terrain

Eric Langmuir’s Mountaincraft and Leadership — the UK’s standard mountain-leader text — added what Naismith’s Scottish paths didn’t need: surface multipliers. Rough mountain trail costs about 20% more moving time; talus and scree 35%; unbroken snow 60% or worse. Skip this correction on rocky ground and your neat estimate quietly loses an hour.

Correction three: breaks

The rule predicts moving time. Real days contain lunch, photos, map checks, and boot adjustments — typically 10–25% of moving time, climbing toward the high end in groups and on photogenic routes. Professional route cards add it as a percentage; so should you.

The worked example, corrected

That same 12 km / 900 m route, for an average hiker on a rough mountain path with 15% breaks:

  • Distance: 12 ÷ 4 = 3 h
  • Ascent: 900 ÷ 600 = 1.5 h
  • Terrain: (3 + 1.5) × 1.2 = 5.4 h moving
  • Breaks: 5.4 × 1.15 = 6 h 13 min

Naismith’s raw 3 h 54 min was a fit hiker moving briskly and never stopping. The corrected figure is a real person having a good day. Both are “right” — only one should set your turnaround time.

Our hiking time calculator runs all three corrections from four inputs, and the route planner measures distance and ascent off the map so you don’t have to trust a sign. The formula is 130 years old; there’s no reason your use of it should feel that way.