Make the distance familiar
A 20 km length contains 200 straight 100 m sections. Picture a marked 100 m sprint straight repeated end to end, rather than a whole stadium lap.
Work it out, one step at a time
Turn kilometres into metres
20 × 1,000 = 20,000 m
One kilometre contains 1,000 metres. The length stays the same.
Choose a speed, then find the time
20 km ÷ 5 km/h = 4 h
At a chosen constant 5 km/h, it takes about 4 hours. This is a calculation with no stops; hills, traffic and real journey conditions are not included.
What if the speed doubles?
For the same distance and constant speed, time halves. For example, 10 km takes 2 hours at 5 km/h but 1 hour at 10 km/h.
The distance is this page’s chosen input. The speed is a teaching assumption. Time = distance ÷ speed.
Build a scale modelChoose two references and change the model size.
Build a scale model of any two lengths
Choose from all 71 Macro and 54 Micro references, or enter your own length. Set B’s model length and see A change on the same fixed metre ruler. The bars represent the stated lengths, including their different measurement definitions.
- Length ratio A / B
- Model length B
- Calculated model length A
A · Chosen distance A
20 km
A chosen input length, not a new measurement of a reference.
User / page input
The ratio follows this input assumption.
Definition and evidence reviewOriginal sourceB · Marathon
42.195 km
Official marathon race distance, 42.195 km.
Source claim checked
A standardized course length; course certification and an athlete’s path require separate measurement.
Definition and evidence reviewOriginal sourcemodel A = (A / B) × model B
This calculator uses the existing journey comparison inputs. Review-pending or conceptual entries remain illustrative inputs here; the public reviewed dataset keeps their numeric values withheld. Radius, height, diameter and experimental bound keep their individual definitions.
Average speed comes from total time
Suppose the first 10 km is travelled at 20 km/h and the next 10 km at 60 km/h. The speeds are teaching assumptions. Before calculating, ask whether a simple arithmetic average of the two speeds describes the whole trip.
Each leg takes distance ÷ speed: (10 / 20) × 60 = 30 minutes and (10 / 60) × 60 = 10 minutes. Total time is 40 minutes. Average speed = 20 km / ((40 / 60) hours) = 30 km/h. More time is spent in the slow leg, so its influence is greater than an equal-weight average suggests.
Include a stop in the same calculation
Add a 10-minute stop. The door-to-door average becomes 20 / ((40 + 10) / 60) = 24 km/h. The total distance stays the same. For equal-distance legs the harmonic-mean formula 2 / (1/v₁ + 1/v₂) gives the moving average; for any route, total distance / total elapsed time is the general rule.
Twenty kilometres also falls 1.0975 km short of a half-marathon's 21.0975 km, derived from half the official 42.195 km marathon. That race comparison describes course length; the speed example describes how time accumulates.
Closest ScaleTrip references
Page distance: 20 km
- ManhattanReference measurement: Length · 21.6 km
Ratio (page distance ÷ reference): ≈ 0.93×The north-to-south extent of the long, narrow island.
- Strait of GibraltarReference measurement: Length · 14 km
Ratio (page distance ÷ reference): ≈ 1.43×The narrowest span between Europe and Africa.
- English Channel crossingReference measurement: Length · 34 km
Ratio (page distance ÷ reference): ≈ 0.59×The narrow Dover Strait between England and France.
Sources for these values
- World Athletics — Marathon — official 42.195 km marathon distance · source verified 2026-10-08