Speed Calculator

The three formulas

Results are shown to six decimal places in the units implied by your inputs. A time result of 4.415385 hours is 4 hours plus 0.415385 × 60 = 24.9 minutes, so about 4 h 25 min.

Minutes to decimal hours

Convert minutes to hours by dividing by 60, and seconds by 3,600.

Entering 4.40 for 4 h 40 min understates the time by 16 minutes; for a 287-mile trip, that inflates the result from 61.5 mph to 65.2 mph.

Speed unit conversions

NIST Special Publication 811 lists these factors to meters per second; the mile-per-hour and foot-per-second factors are exact.

Two handy consequences: 60 mph is exactly 88 ft/s, so at highway speed a car covers 88 feet every second; and 100 km/h is about 62.1 mph. Knots (nautical miles per hour) are used in aviation and at sea, and are about 15% faster than the same number in mph.

Average speed vs instantaneous speed

This calculator gives average speed: total distance divided by total elapsed time. OpenStax College Physics defines it the same way and distinguishes it from instantaneous speed, the value a speedometer shows at one moment. Stops for gas, traffic and slow city sections are folded into the average, so a road trip's door-to-door average is always lower than its highway cruising speed.

Physics also separates speed from velocity. Velocity uses displacement, the straight-line change in position, and has a direction. OpenStax's example: drive 3 km to a store and 3 km back in 30 minutes, and your average speed is 12 km/h, but your average velocity is zero, because you ended where you started.

Why you can't just average two speeds

If you drive 60 miles at 30 mph and 60 miles back at 60 mph, your average is not 45 mph. The first leg takes 2 hours and the second 1 hour, so 120 miles ÷ 3 hours = 40 mph. You spend more time at the slower speed, so it weighs more. For equal distances, the correct average is the harmonic mean: 2 × v₁ × v₂ ÷ (v₁ + v₂) = 2 × 30 × 60 ÷ 90 = 40 mph. The safe method is always the one this calculator uses: add up all the distance, add up all the time, then divide.

Running and cycling pace

Runners usually think in pace (minutes per mile) rather than speed. Pace and speed are reciprocals: pace in minutes per mile = 60 ÷ speed in mph. A 4-hour marathon (26.2 miles) averages 26.2 ÷ 4 = 6.55 mph, which is a pace of 60 ÷ 6.55 = 9.16 minutes, or about 9:10 per mile. A 10:00 mile is 6 mph; a 7:30 mile is 8 mph.

Common mistakes

• Writing minutes as decimals. 1 h 30 min is 1.5 h, not 1.3 h.
• Mixing miles and kilometers. If the distance is in km and you want mph, convert the distance first (1 mile = 1.609344 km exactly).
• Using time in minutes with a per-hour speed. 120 miles at 60 mph is 2, which is hours, not minutes.
• Averaging speeds instead of using total distance and total time. See the round-trip example above.

How far you travel while reacting

Distance = speed × time also explains why following distance matters. At 60 mph you cover 88 ft every second, so in a 1.5-second reaction (an illustrative figure; actual reaction times vary by person and situation) the car travels 132 ft before the brakes are even applied. At 30 mph it is 66 ft in the same 1.5 seconds. Use "Distance" mode with speed in ft/s and time in seconds to run your own numbers: 88 ft/s × 2 s = 176 ft, which is why a two-second gap is a common rule of thumb for following distance. Braking distance is not linear in speed, so it can't be found with this formula alone.

Speed, distance and time beyond the road

The same relation works at any scale as long as the units match. Light travels at exactly 299,792,458 m/s, the value NIST uses to define the meter; at that speed, light covers the roughly 1.5 × 10¹¹ m from the Sun to Earth in about 500 seconds, a little over 8 minutes. For walking, a 3 mph pace covers a mile in 20 minutes, so a 2.4-mile loop takes 2.4 ÷ 3 = 0.8 h, or 48 minutes. For flights and shipping, enter nautical miles and knots together; mixing nautical miles with statute miles per hour gives an answer that is off by about 15%.