Naismith’s Rule is a rule of thumb that helps in the planning of a walking or hiking expedition by calculating how long it will take to walk the route, including ascents. The rule was devised by William W. Naismith, a Scottish mountaineer, in 1892. The basic rule is as follows:

Allow 1 hour for every 3 miles (5 km) forward, plus 1 hour for every 2000 feet (600 metres) of ascent.

When walking in groups, calculate for the speed of the slowest person.

The basic rule assumes hikers of reasonable fitness, on typical terrain, under normal conditions. It does not account for delays, such as extended breaks for rest or sight-seeing, or for navigational obstacles. For planning expeditions or walks a party leader may use the rule in putting together a route card.

Alternatively, the rule can be used to determine the equivalent flat distance of a route. This is achieved by recognising that Naismith’s Rule implies an equivalence between distance and climb in time terms: 3 miles (= 15,840 feet) of distance is equivalent in time terms to 2000 feet of climb. That is, 7.92 (= 15840/2000) units of distance are equivalent to 1 unit of climb. For convenience a 8 to 1 rule can be used. So, for example, if a route is 20 km with 1600 metres of climb (as is the case on leg 1 of the Bob Graham Round, Keswick to Threlkeld), the equivalent flat distance of this route is 20+1.6×8 = 32.8 km. Assuming an individual can maintain a speed on the flat of 5 km/h (walking pace), the route will take 6 hours and 34 minutes. The simplicity of this approach is that the time taken can be easily adjusted for an individual’s own (chosen) speed on the flat; at 8 km/h (flat speed) the route will take 4 hours and 6 minutes. The rule has been tested on fell running times and found to be reliable [1].

In practice, the results of Naismith’s Rule are usually considered the minimum time necessary to complete a route. Over the years several "corrections" have been formulated in an attempt to make the rule more accurate. The most common correction is to add 25 or 50% to the time found with Naismith’s Rule. While this may be more accurate for some people or under certain conditions, it does not explicitly account for any additional variables. The accuracy of some corrections is disputed by some[2], in particular the speed at which walkers descend a gentle slope.

Other common corrections are:

When walking on poor terrain, allow 1 hour for every 2.5 miles (4 km) forward, instead of 1 hour per 3 miles.

On a gentle decline (about 5-12°), subtract 10 minutes per 1000 feet of descent. On a steep decline (over 12°), add 10 minutes per 1000 feet of descent.