Estimate trail distance from map scale.
Converting a map measurement to ground distance is a scale multiplication: on a 1:25,000 sheet, one centimetre is 250 metres, so centimetres times the scale denominator divided by 100,000 gives kilometres. Two corrections follow. Measuring with a ruler or a piece of string always understates a trail, because small bends and switchbacks are lost — a 10% to 15% allowance is standard. The effort-equivalent figure then adds roughly 0.8 km per 100 m of ascent, which is the accepted way to compare a steep route against a flat one.
Map distance to ground distance
Kilometres = measured cm x scale denominator / 100,000; trail distance = that x (1 + winding allowance)
Effort-equivalent distance
Effort km = trail km + ascent metres / 100 x 0.8
Ten to fifteen percent for a graded path measured carefully, and twenty to thirty for a mountain trail with switchbacks measured in straight segments. The rougher your measurement method, the larger the allowance needs to be.
Lay a piece of string along the route, then straighten it against the scale bar, or walk a map wheel along it. Straight-line ruler segments are the method that most understates the distance.
Because it makes routes comparable. A 10 km walk with 600 m of climb is a substantially bigger day than a flat 10 km, and expressing the climb as equivalent flat kilometres is how walking guides have long handled it.