Three values, one triangle — and most of the planning around any journey, run or flight.
Speed, distance and time form one of the tidiest little triangles in all of math. Cover up the one you want; the other two tell you how to find it. The same formula appears in road trips, marathons, plane connections and physics problems, which is why students worldwide drill it from primary school onwards.
Picture a triangle with D (distance) on top and S (speed) × T (time) along the bottom. Cover up the value you want:
Always make sure units agree: km with km/h and hours; miles with mph and hours; metres with m/s and seconds.
1. Find distance. A car drives at 90 km/h for 2.5 hours. Distance = 90 × 2.5 = 225 km.
2. Find speed. A runner covers 21.1 km in 1 h 45 min (1.75 h). Speed = 21.1 ÷ 1.75 ≈ 12.06 km/h (about 4:59/km pace).
3. Find time. A flight covers 8,500 km at an average ground speed of 850 km/h. Time = 8,500 ÷ 850 = 10 hours in the air.
London to Edinburgh is 660 km. At an average 90 km/h (including stops), expect 7 h 20 min. The honest version usually adds 20% for traffic and breaks → ~8 h 50 min.
Pace is the inverse of speed: minutes per km. A 5:00/km pace is 12 km/h. A sub-4-hour marathon needs to average 5:41/km.
Commercial cruise speed is roughly 880 km/h. Headwinds and routing add 10–20% to scheduled flight time. The east-bound jet stream often shaves an hour off a transatlantic crossing.
Mental shortcuts:
Average speed = total distance ÷ total time. Instantaneous speed = speed at a specific moment. Speed cameras measure instantaneous; average-speed cameras compute total distance between two points divided by elapsed time — much harder to outrun.
Can speed be negative? In physics, velocity has direction; speed is the magnitude (always ≥ 0).
How do I convert pace (min/km) to speed (km/h)? 60 ÷ pace. 6:00/km = 60/6 = 10 km/h.
What's a good running pace? Recreational 5K — 6–7 min/km. Sub-elite — 4–4:30/km.
Use our Scientific Calculator for quick SDT and pace math in any units.