Speed Distance Time Calculator
Calculate speed, distance, or time using the SDT formula. Supports km/h and mph.
Select what to calculate, enter the other two values, and get your answer instantly.
The Speed-Distance-Time Triangle
Three formulas, one relationship. If you know any two, you can calculate the third:
- Speed = Distance / Time — how fast you're going
- Distance = Speed x Time — how far you'll travel
- Time = Distance / Speed — how long the journey takes
The classic memory trick is the SDT triangle: draw a triangle with Distance on top, Speed bottom-left, Time bottom-right. Cover what you want to find. If the remaining two are side by side, multiply them. If one is above the other, divide top by bottom.
These formulas assume constant speed. In reality, your average speed accounts for acceleration, braking, and stops — which is why sat-nav journey times are usually more realistic than dividing distance by speed limit.
Real-World Average Speeds
| Road Type | Speed Limit | Realistic Average | Time for 100 Miles |
|---|---|---|---|
| Motorway (M roads) | 70 mph | 55-65 mph | 1h 32m - 1h 49m |
| Dual carriageway (A roads) | 70 mph | 45-55 mph | 1h 49m - 2h 13m |
| Single carriageway | 60 mph | 35-45 mph | 2h 13m - 2h 51m |
| Urban / city roads | 30 mph | 15-22 mph | 4h 33m - 6h 40m |
| Country lanes | 60 mph | 25-35 mph | 2h 51m - 4h |
What this means for you: Your real average speed is always lower than the speed limit. Traffic, junctions, roundabouts, and speed cameras all reduce it. For planning purposes, assume 55 mph average on motorways and 35 mph on A-roads through towns.
Common Speed Conversions
| mph | km/h | m/s | Context |
|---|---|---|---|
| 20 | 32 | 8.9 | School zones |
| 30 | 48 | 13.4 | Urban speed limit |
| 50 | 80 | 22.4 | Motorway roadworks |
| 60 | 97 | 26.8 | Single carriageway limit |
| 70 | 113 | 31.3 | UK motorway limit |
| 100 | 161 | 44.7 | European motorways |
| 130 | 209 | 58.1 | German Autobahn advisory |
What this means for you: The quick conversion: multiply mph by 1.6 to get km/h, or divide km/h by 1.6 for mph. For rough mental maths, mph x 1.5 + 10% gets you close enough.
Practical Uses for This Calculator
Planning arrival times
Got 180 miles to drive at an average 50 mph? That's 3 hours 36 minutes. Add 15-20 minutes for fuel and comfort stops on long journeys.
Running and cycling pace
A 5K in 25 minutes means you averaged 12 km/h (7.5 mph). Use the calculator to set target paces for training runs and races.
Speeding fine maths
Doing 80 in a 70 zone saves just 6 minutes over 60 miles — but risks a £100 fine and 3 points. The time saving from speeding is almost always trivial.
Delivery and logistics
Multi-stop routes need average speed estimates per leg. Urban legs at 20 mph, motorway at 55 mph, with 5-10 minutes per stop for loading/unloading.
Average Speeds You Can Expect
| Mode of Travel | Average Speed | Planning Notes |
|---|---|---|
| Walking | 3 mph / 5 km/h | Google Maps uses 3.1 mph for walking directions |
| Cycling (city) | 10-12 mph / 16-19 km/h | Include time for traffic lights and junctions |
| Running (casual) | 6 mph / 10 km/h | Roughly a 10-minute mile pace |
| UK motorway | 55-60 mph average | 70 mph limit, but traffic and junctions reduce average |
| UK A-roads | 35-45 mph average | Varies hugely by region and time of day |
| City driving (London) | 7-12 mph | Average London traffic speed is ~11 mph |
The biggest mistake in journey planning: using the speed limit as your average speed. On a 200-mile UK motorway trip, budget for 55 mph average (3h 40m) not 70 mph (2h 50m). Add 15 minutes for fuel and rest stops on trips over 2 hours.
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Common uses
- Estimating journey times for road trips and commutes
- Working out average speed from distance and time
- Planning running, cycling, or walking pace targets
- Calculating delivery route times for logistics
- Converting between mph and km/h for travel abroad
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