Walk timer: how long will the walk take?

How the estimate works

The timer uses the best known rule of thumb in British walking, usually credited to the Scottish mountaineer William Naismith in the 1890s. In its common metric form it says: allow an hour for every 5 kilometres on the flat, then add one minute for every 10 metres you climb. The timer lets you change the flat pace, because 5 km/h is a brisk speed on a dry road and few people keep it up across ploughed fields or a wet chalk track. The default here is 4.5 km/h, which suits most adults on field paths with gates and stiles.

So the sum is simple:

• time on the flat = distance divided by your pace;
• time for the climb = total ascent in metres divided by 10, in minutes;
• then add the minutes you expect to spend stopped.

A worked example: an 8 km circuit with 120 m of ascent at 4.5 km/h comes to about 107 minutes on the flat and 12 minutes for the climb. Add a quarter of an hour for a flask and a look at a church tower and you have a little over two hours. That is the figure the timer shows when you first open the page.

Where to get the numbers

Distance and climb come from the map. On an OS Explorer sheet, measure the route with a piece of string or a map measurer along the line of the paths, not as the crow flies, and convert using the scale (4 cm to 1 km at 1:25,000). For the climb, count the contour lines you cross going uphill; on Explorer maps they are 5 metres apart in most lowland areas, but check the key on your sheet. On the chalk around Offley, Lilley and King's Walden the climbs are gentle and spread out, so the total ascent of a morning walk is often modest, but it adds up across several dry valleys.

Phone apps will measure a route for you. They are handy, but they tend to underestimate the climb on rolling farmland and they cannot see a closed gate or a ploughed field.

What the rule leaves out

Naismith's rule was made for fit walkers on hill paths in good weather. On these chalk paths a few things slow people down that the formula cannot see:

• Mud. From late autumn to early spring the chalk and clay-with-flints tops turn sticky. Knock ten to twenty per cent off your pace, or more after heavy rain.
• Stiles and gates. A route with a dozen field boundaries can add ten minutes on its own, more with a dog to lift or a pushchair to wrangle.
• Ploughed and cropped fields. Paths across arable land should be reinstated after ploughing, but in practice the line can be rough for a while.
• Short days. In December there are under eight hours of daylight in Hertfordshire. Work back from sunset, not forward from breakfast.
• Company. Children, a camera, or anyone who reads every gravestone will double the stopping time. The stops box is there for honesty.

Adjusting for downhill and fitness

Some walkers use refinements of the rule. Tranter's corrections scale the time for fitness and tiredness, and Langmuir's adds or subtracts time for steep descents. On the gentle gradients of north Hertfordshire the plain rule is usually close enough, which is why the timer keeps it simple and lets you set your own pace instead.

Questions people ask

Is 5 km/h too fast?

For many people, yes, once gates, mud and conversation are counted. Try 4 to 4.5 km/h for field walking and see how your own walks compare.

Should I count the climb back down?

No. The rule adds time only for ascent. Steep descents on wet chalk can be slow, so add a few minutes if a route drops sharply.

Does the timer store anything?

No. The sums run in your browser and nothing you type is sent anywhere.

For route planning on the map, see reading an OS Explorer map on the Chiltern edge, and for a first outing try the circular walk from Great Offley.