Estimate cycling route distance.
Route distance is laps times lap length plus any out-and-back, which is how most training routes are actually constructed. The two derived figures are what make routes comparable. Climb-adjusted distance uses the cycling rule of thumb that every hundred metres of ascent costs roughly the same effort as an extra kilometre of flat riding. Climbing density in metres per kilometre is the standard difficulty measure: under 10 is flat, 10 to 20 is rolling, and above 20 is genuinely hilly regardless of the total distance.
Route distance
Distance = lap distance x laps + detour; miles = km x 0.621371
Climb adjustment and density
Climb-adjusted km = distance + elevation gain / 100; density = elevation gain / distance
Because it makes routes comparable at a glance. A 42 km ride with 320 m of climb rides like about 45 km flat, and the adjustment lets you plan time and nutrition from a single number.
Above 20 metres per kilometre. Grand tour mountain stages commonly exceed 25, and anything under 10 is essentially flat terrain regardless of how it feels into a headwind.
Barometric altimeters are reasonably reliable; GPS-only elevation is not, and can be out by 20% or more on the same route. Where possible, use a barometric device or a corrected figure from mapping software.