How to Calculate Speed, Distance, and Time
Speed, distance, and time are three views of a single relationship: speed = distance ÷ time. Know any two of them and you can always find the third — just rearrange the formula. It's the math behind every "what time will I arrive?" and "how fast was I going?" The quickest way to get an answer is the speed, distance, and time calculator, which solves for whichever value you leave blank. Here's how the math works underneath.
The three formulas
They're the same equation, rearranged for the piece you're missing:
| To find… | Formula |
|---|---|
| Speed | distance ÷ time |
| Distance | speed × time |
| Time | distance ÷ speed |
A classic memory aid is the triangle: picture distance on top, with speed and time sitting side by side underneath. Cover the one you want, and the other two show you whether to multiply (side by side) or divide (top over bottom).
Worked examples
Finding speed. You drove 150 miles in 3 hours:
- 150 ÷ 3 = 50 miles per hour
Finding time. You have 200 miles to go at 50 mph:
- 200 ÷ 50 = 4 hours
Finding distance. You drive for 2.5 hours at 60 mph:
- 60 × 2.5 = 150 miles
Keep your units consistent
The single biggest source of wrong answers isn't the formula — it's mismatched units. If speed is in miles per hour, distance has to be in miles and time in hours. Mixing minutes with hours is the usual culprit: 30 minutes is 0.5 hours, not 30, so at 60 mph you cover 30 miles in that half hour, not 1,800. If your speed is metric and your map is imperial, convert first — the kph to mph converter lines them up before you start.
Average speed isn't the average of your speeds
Here's the trap that catches everyone. Drive 30 miles at 30 mph, then 30 miles at 60 mph, and your average speed is not 45 mph. Average speed is total distance ÷ total time. The first leg takes 1 hour, the second takes 0.5 hours, so it's 60 miles ÷ 1.5 hours = 40 mph. You spend more time at the slower speed, so it pulls the average down. Always divide the totals — never average the two speeds.
Where this shows up
Road trips are the obvious one: knowing your average speed turns "how far is it?" into "what time will we get there?" For a specific route, the distance between cities tool gives you the miles to plug in, and the flight time calculator does the same job for air travel, where cruising speed is roughly constant. The same math also runs pace for runners, delivery ETAs, and any "when will it arrive" question. For more everyday-math walkthroughs, see how to calculate a percentage, and our time and date calculators cover the clock-and-calendar side.
Frequently asked questions
What is the formula for speed, distance, and time?
Speed = distance ÷ time. Rearranged, distance = speed × time, and time = distance ÷ speed. All three are the same equation solved for a different variable.
How do I calculate how long a journey will take?
Divide the distance by your speed. At 50 mph, a 200-mile trip takes 200 ÷ 50 = 4 hours. Make sure the distance and speed use the same distance unit.
How do I calculate average speed?
Divide the total distance by the total time — not the average of your individual speeds. If you covered 120 miles in 2.5 hours, your average speed is 48 mph, regardless of how much you sped up or slowed down along the way.
Why does mixing units give the wrong answer?
The formula assumes every value uses matching units. If speed is per hour but you plug in time as minutes, the result is off by a factor of 60. Convert everything to the same units — usually hours and miles, or hours and kilometers — before you calculate.
How do I convert between mph and km/h?
Multiply mph by 1.609 to get km/h, or multiply km/h by 0.621 to get mph. So 60 mph is about 97 km/h. A converter does it instantly if you'd rather not do the arithmetic.
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