Work out speed from distance and time in metric or imperial-friendly travel math.
This tool provides estimates for informational purposes only. It is not a substitute for professional advice. Individual results vary based on your inputs and assumptions, so review important decisions with a qualified professional.
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Speed, distance, and time are three of the most practically useful calculations in everyday life — from working out how long a road trip will take to calculating your running pace, from understanding how vehicle stopping distances relate to speed to converting between mph and km/h when driving abroad. This guide explains the speed-distance-time triangle in full, covers unit conversions between mph, km/h, m/s, and knots, explains why both the US and UK use mph for road speeds, provides UK Highway Code stopping distances, and covers pace calculations for runners and cyclists.
The fundamental relationship between speed, distance, and time:
The "SDT triangle" (or DST triangle) is a memory aid: cover the quantity you want to find and the formula is shown by the remaining two variables.
| From | To | Multiply by | Example |
|---|---|---|---|
| mph | km/h | 1.60934 | 60 mph × 1.609 = 96.6 km/h |
| km/h | mph | 0.62137 | 100 km/h × 0.621 = 62.1 mph |
| mph | m/s | 0.44704 | 60 mph × 0.447 = 26.8 m/s |
| m/s | mph | 2.23694 | 10 m/s × 2.237 = 22.4 mph |
| m/s | km/h | 3.6 | 10 m/s × 3.6 = 36 km/h |
| knots | mph | 1.15078 | 100 knots × 1.151 = 115.1 mph |
| knots | km/h | 1.852 | 100 knots × 1.852 = 185.2 km/h |
One of the most notable speed measurement facts about the US and UK is that both countries use miles per hour (mph) for road speed limits. This means a UK driver visiting the US will find speed limits in a familiar unit — though US speed limits are often lower than UK limits in urban areas and higher on interstates vs UK motorways.
| Road Type | US Speed Limit | UK Speed Limit |
|---|---|---|
| Urban/built-up area | 25–35 mph (varies by state) | 30 mph (20 mph in some zones) |
| Single carriageway rural | 55–65 mph (varies) | 60 mph |
| Dual carriageway | 65–70 mph | 70 mph |
| Motorway / Interstate | 65–80 mph (some states 85 mph) | 70 mph |
Most of Europe uses km/h for road speeds. When UK and US drivers visit France, Germany, Italy, or Spain, they must remember to interpret km/h signs — 130 km/h on a French autoroute is approximately 81 mph. The quick conversion: km/h ÷ 1.6 ≈ mph.
The UK Highway Code provides standard stopping distances that combine thinking distance (reaction time × speed) and braking distance. These figures assume a reaction time of approximately 0.67 seconds and good braking on a dry road:
| Speed | Thinking Distance | Braking Distance | Total Stopping Distance |
|---|---|---|---|
| 20 mph | 6 m | 6 m | 12 m (about 3 car lengths) |
| 30 mph | 9 m | 14 m | 23 m (about 6 car lengths) |
| 50 mph | 15 m | 38 m | 53 m (about 13 car lengths) |
| 70 mph | 21 m | 75 m | 96 m (about 24 car lengths) |
Braking distance scales with the square of speed — double the speed and braking distance quadruples. At 70 mph, braking distance is more than 5 times greater than at 30 mph. Wet roads can double or triple braking distances. The UK Highway Code advises maintaining a 2-second gap in dry conditions and a 4-second gap in wet conditions.
The average human reaction time is 0.2–0.5 seconds under normal conditions. Distractions (phone use, fatigue, alcohol) increase this significantly. The thinking distance formula:
Thinking distance (m) = speed (mph) × 0.3 (Highway Code approximation)
At 30 mph: 30 × 0.3 = 9 m. At 70 mph: 70 × 0.3 = 21 m.
More accurately in SI units: Thinking distance (m) = speed (m/s) × reaction time (s)
At 70 mph (31.3 m/s) with 0.67 second reaction time: 31.3 × 0.67 = 20.97 m ≈ 21 m.
For a 120-mile journey taking 2 hours 30 minutes (2.5 hours): Average speed = 120 ÷ 2.5 = 48 mph. This is the average even if the speedometer showed 70 mph on motorways and 0 mph at stops.
UK average speed cameras (SPECS) measure average speed over a distance, not instantaneous speed — you cannot speed between cameras and slow down for the second one. The calculation used: average speed = distance between cameras ÷ time taken.
Runners and cyclists often work in pace (time per unit distance) rather than speed (distance per unit time):
| Pace (min/mile) | Pace (min/km) | Speed (mph) | Level |
|---|---|---|---|
| 12:00 | 7:27 | 5.0 | Beginner jogger |
| 10:00 | 6:12 | 6.0 | Easy recreational runner |
| 9:00 | 5:35 | 6.7 | Moderate runner |
| 8:00 | 4:58 | 7.5 | Comfortable pace, half marathon target |
| 6:00 | 3:44 | 10.0 | Strong club runner |
| 4:36 | 2:51 | 13.0 | Sub-2-hour half marathon pace |
Wind speed is reported differently in different contexts. UK and US weather forecasts typically use mph for wind speed. Aviation uses knots. Scientific contexts use m/s. The Beaufort scale provides a descriptive classification:
| Beaufort | Description | Wind Speed (mph) | Wind Speed (km/h) |
|---|---|---|---|
| 0 | Calm | <1 | <1 |
| 3 | Gentle breeze | 8–12 | 12–19 |
| 6 | Strong breeze | 25–31 | 40–50 |
| 8 | Gale | 39–46 | 62–74 |
| 12 | Hurricane force | ≥73 | ≥117 |
Multiply mph by 1.60934 to get km/h. So 60 mph × 1.609 = 96.6 km/h. For a quick mental estimate, multiply by 1.6 (close enough for most purposes). To convert km/h to mph, multiply by 0.621. So 100 km/h × 0.621 = 62.1 mph.
Speed = Distance ÷ Time (s = d/t). The SDT triangle gives all three rearrangements: Distance = Speed × Time; Time = Distance ÷ Speed. Units must be consistent: if speed is in mph and distance in miles, time is in hours.
Average speed = Total distance ÷ Total time. For a 150-mile drive that took 2 hours 45 minutes (2.75 hours): average speed = 150 ÷ 2.75 = 54.5 mph. Average speed includes all stops and slow sections — it is always lower than your maximum speed during the journey.
At 30 mph: 23 m total (9 m thinking + 14 m braking). At 50 mph: 53 m (15 m + 38 m). At 70 mph: 96 m (21 m + 75 m). These are for dry roads. Wet roads can double the braking distance. Braking distance scales with speed squared, so doubling speed quadruples braking distance.
Multiply m/s by 2.23694 to get mph. So 10 m/s × 2.237 = 22.4 mph. Alternatively, multiply by 2.237 or use the approximation ×2.25 for quick estimates. To convert mph to m/s, multiply by 0.44704.
Yes. Both the United States and the United Kingdom use miles per hour (mph) for road speed limits and vehicle speedometers. This is unusual globally — virtually every other country uses km/h for road speeds. The shared use of mph means UK drivers visiting the US (and vice versa) can immediately understand posted speed limits without conversion.
Speed is a scalar quantity — it describes how fast an object is moving with no reference to direction. Velocity is a vector quantity — it describes both speed and direction. So a car driving 60 mph north and a car driving 60 mph south have the same speed (60 mph) but different velocities. In everyday use, speed and velocity are often used interchangeably.
Convert km/h to mph (× 0.621), then calculate pace in min/mile = 60 ÷ speed (mph). For example, running at 10 km/h: 10 × 0.621 = 6.21 mph. Pace = 60 ÷ 6.21 = 9.66 minutes per mile ≈ 9:40 min/mile. Alternatively, pace in min/km = 60 ÷ speed (km/h), so at 10 km/h: pace = 6:00 min/km.
Disclaimer: Speed limit information is for general reference only. Always observe current posted speed limits and follow local traffic laws. Stopping distances are Highway Code estimates — actual stopping distances vary with road condition, tyre condition, vehicle type, and driver reaction time.