Speed Calculator
Calculate speed, distance or time — with unit conversions and real-world comparisons
Speed, Distance and Time
The speed-distance-time relationship is one of the most fundamental in physics. Knowing any two values allows you to calculate the third. The key is consistent units — always match the time units.
Formulas
Average vs Instantaneous Speed
This calculator computes average speed — total distance divided by total time. Instantaneous speed is the speed at a specific moment (what a speedometer shows). For non-uniform motion, average speed gives you the overall rate but not the details of the journey.
Speed, Distance, Time: One Triangle, Many Traps
Built and verified by Andrius R. · Updated June 2026
Speed = distance ÷ time is the first physics formula most people learn and the one they use most often — for trips, sport, and sanity-checking claims. The formula is trivial; the traps are units and averages.
The triangle and its three rearrangements
From v = d ÷ t: d = v × t and t = d ÷ v. A 300 km trip at 100 km/h takes 3 hours; 2.5 hours at 80 km/h covers 200 km. The only discipline required: keep units consistent before computing — which is where most errors live.
The conversions worth memorizing
km/h ↔ m/s: divide or multiply by 3.6. 100 km/h = 27.8 m/s; 10 m/s = 36 km/h. (Why 3.6: an hour has 3,600 seconds and a km has 1,000 m.)
mph ↔ km/h: × 1.609. 60 mph = 96.6 km/h; a 70 mph UK motorway limit ≈ 113 km/h.
Knots (nautical miles/hour, sea and air): 1 knot = 1.852 km/h. Reference points: brisk walk ~5 km/h, sprinting Usain Bolt peaked ~44 km/h, sound ~343 m/s (1,235 km/h) in air, light 299,792,458 m/s — exact, because the meter is defined from it.
The average-speed trap (the most failed "easy" question)
Drive somewhere at 30 km/h and return at 60 km/h: the average is 40, not 45. You spent twice as long at the slow speed, and average speed is total distance ÷ total time, which weights by time, not by leg. (For two equal-distance legs the shortcut is the harmonic mean: 2ab ÷ (a+b).) The same trap, stated harder: averaging 30 for the first half of a trip and hoping to average 60 overall requires the return leg at infinite speed — time already spent can't be unspent. Any time you're tempted to average two speeds, average the times instead.
Speed vs velocity vs acceleration
Physics splits hairs for good reason: speed is how fast (a magnitude), velocity is how fast and which way — a car circling a track at a constant 100 km/h has constant speed but constantly changing velocity, which is why it needs a continuous sideways force (and why passengers feel pushed outward). Acceleration is the rate velocity changes: a car doing 0–100 km/h in 8 s averages 27.8 ÷ 8 ≈ 3.5 m/s² — about a third of gravity's 9.81. The force behind that acceleration is the next calculator over: F = ma.
Everyday applications that reward the math
- Trip honesty: GPS averages include stops; your moving average is always flattering by comparison. For a 600 km day, every 10 km/h of realistic average saves ~40 minutes — and pushing 110 to 130 saves less time than people feel it does (50 min on 600 km) at a steep fuel and risk premium.
- Storm distance: sound's 343 m/s means thunder arrives ~3 seconds per kilometer (5 s per mile) after the flash — count and divide.
- Pace vs speed: runners invert the ratio — minutes per km instead of km per hour. 5:00/km = 12 km/h; conversions and race math live in the pace calculator.
From the Blog
// Quick Conversion
To convert km/h to m/s, divide by 3.6. To go the other way, multiply by 3.6. This comes from 1000 m/km ÷ 3600 s/hr.
// Mach Number
Mach 1 = the speed of sound ≈ 1235 km/h (768 mph) at sea level. It varies with temperature — at altitude it's lower.
// Light Speed
Light travels at 299,792,458 m/s. At that speed you could circle the Earth 7.5 times per second.
// Pace vs Speed
Runners use pace (min/km or min/mile) not speed. Pace = 60 ÷ speed (km/h). A 6 min/km pace = 10 km/h.