4 MPH to KPH

Four miles per hour is 6.437376 kilometers per hour

Four miles per hour is 6.437376 kilometers per hour, which rounds to 6.44 km/h and is 1.78816 meters per second. The figure is exact at six decimals because a mile is 1609.344 meters and an hour is 3600 seconds, so 4 x 1.609344 = 6.437376 with no rounding anywhere in the chain.

The factor comes from one division. 1609.344 / 3600 = 0.44704, which is the meters per second in one mile per hour, and 0.44704 x 3.6 = 1.609344, the kilometers per hour in the same. NIST SP 811, Appendix B.9 prints the mile to meter row as 1.609 344 E+03 in boldface, the mark it uses for exact values, and the hour to second row as 3.6 E+03. The BIPM SI brochure fixes the second and the metre those rows are built on.

Speed is a ratio, so it converts by the distance factor alone. A mile per hour is 1.609344 kilometers per hour for the same reason a mile is 1.609344 kilometers: the hour sits on both sides and cancels. That is why 4 mph and 4 miles give the same multiplier, and why a page that answers a speed question with a distance has swapped one unit for another.

Four is a small number, so the answer stays below 7 km/h. A reader who expects a two digit figure after converting mph to kph is thinking of highway speeds, where 60 mph reaches 96.56064 km/h and 70 mph reaches 112.65408 km/h.

Multiply by 1.609344, divide to come back

Forward takes one multiplication: miles per hour x 1.609344. Backward takes one division, and the division is the same as multiplying by 0.6213711922, which is 1 / 1.609344. A 6.437376 km/h reading divided by 1.609344 returns exactly 4.

Half a mile per hour is 0.804672 km/h, so the table moves in steps of about 0.8 km/h at the slow end. That makes 4 mph sit between the 6 km/h and 7 km/h marks on a metric dial, closer to 6.5 than to either whole number.

The 1.6 shortcut that works on a highway is nearly harmless here. Four times 1.6 is 6.4, which is 0.037376 km/h below the exact 6.437376, an error of 0.58 percent. At 4 mph the shortcut is fine for a sign and useless for a stopwatch.

Fifteen minutes a mile, nine minutes nineteen a kilometer

Speed and pace are reciprocals, and at 4 mph the reciprocal is easy. Sixty minutes divided by 4 gives 15 minutes per mile. The same hour spread over 6.437376 km gives 9 minutes 19.2 seconds per kilometer, which is 559.234 seconds, or 55.92 seconds per 100 meters.

A treadmill carries both units and often both labels. On a US machine the belt set to 4.0 is running 4 mph, which is 6.44 km/h. On a metric machine the belt set to 6.4 km/h is running 3.9768 mph, a shade under the 4 mph mark. The two settings are 0.0232 mph apart, which is why a workout log kept in one unit drifts from a log kept in the other.

One hour at 4 mph covers 4 miles, and 4 miles is 6.437376 km. A walk that takes 45 minutes at the same pace covers 3 miles, or 4.828032 km, because 4 x 0.75 = 3. The distance scales with time while the speed stays put.

Pace tables in running shops are written per mile and per kilometer side by side for this reason. A 15 minute mile is 9:19.2 per kilometer, and a 9:30 per kilometer pace is 6.3158 km/h, which is 3.9244 mph. Neither figure is friendlier than the other; each matches the markings on a different track.

Metric signs read back: 4, 6, 8 and 10 km/h

The return trip divides by 1.609344. A 4 km/h sign is 2.4855 mph, a 6 km/h sign is 3.7282 mph, and an 8 km/h sign is 4.971 mph. Only the 8 km/h limit is faster than 4 mph, and it is faster by 0.971 mph.

A 6 km/h street is slower than 4 mph, which is the part that surprises drivers. Six km/h is 3.7282 mph, so a car holding 4 mph on that street is 0.2718 mph over the limit, and that margin is 0.437376 km/h.

The 10 km/h row is the other useful one. Ten km/h is 6.2137 mph, so a walking pace of 4 mph is 35.6 percent slower than that sign allows. Reading a metric sign as roughly two thirds of its face value in mph works between 6 and 15 km/h, which covers most pedestrian zones.

Where 4 mph is the legal limit: class 2 mobility scooters

Britain writes 4 mph into law for one class of vehicle. GOV.UK's rules for mobility scooters and powered wheelchairs put a scooter in class 2 when its top speed is 4 mph or less, and in class 3 when the top speed is 8 mph. In metric terms those ceilings are 6.437376 km/h and 12.874752 km/h.

The class decides where the vehicle may go. Class 2 and class 3 machines can use pavements, other pedestrian areas and cycle tracks, while only class 3 machines may use the road, and those have to be registered with DVLA. A machine that goes faster than 8 mph, is wider than 0.85 meters or weighs more than 150 kg falls outside both classes and may only be used on the road.

That makes 4 mph a number with a legal edge rather than a round figure on a dial. A class 2 scooter running at 6.5 km/h is 0.062624 km/h over its ceiling, and a scooter showing 7 mph is 11.265408 km/h, well past the 6.437376 km/h mark the class allows.

The same 8 mph ceiling lands at 12.874752 km/h, which is a useful number for anyone comparing a British scooter with a European one sold in metric units. A machine advertised at 12 km/h is 7.4565 mph, so it fits class 3 with 0.5435 mph to spare.

Knots, feet per second and the units that come along

Boats and aircraft log speed in knots, and 4 mph converts to 3.4759 knots. One knot is one nautical mile per hour, which is 1.852 km/h exactly, so a knot is 1.150779448 mph. Divide 4 by 1.150779448 and you land on 3.4759.

The reverse is worth knowing on the water. A 4 knot boat is doing 4.6031 mph, or 7.408 km/h, which is why a river current reported as 4 knots is stronger than a 4 mph current by a wide margin. A 2 knot current is 2.3016 mph and 3.704 km/h.

Feet per second is the unit in physics classrooms and in some US engineering codes. Four mph is 5.8666667 ft/s, because 1 mph is 1.4666667 ft/s. Meters per second is the SI form at 1.78816 m/s, and the two agree: 1.78816 x 3.280839895 = 5.8666667 ft.

One more relation ties the set together. Four mph covers 100 meters in 55.92 seconds, since 100 / 1.78816 = 55.923. A pedestrian timing light that allows 7 seconds for 100 meters is assuming a pace near 32 mph, which no walker reaches.

Speeds near four miles per hour

Four mph sits at the top of walking and the bottom of cycling. The table puts it next to its neighbours in both units, with the pace that each speed implies.

A brisk walk runs 3 to 4 mph, which is 4.83 to 6.44 km/h. A jog starts near 5 mph, or 8.05 km/h, and a commuter bike holds 12 to 15 mph, which is 19.31 to 24.14 km/h. Read down the table and every step of 1 mph adds 1.609344 km/h, so the gap between columns stays constant.

The pace column is the one that changes shape. Going from 4 to 8 mph halves the minutes per mile from 15:00 to 7:30, while the km/h figure doubles from 6.437376 to 12.874752. Doubling speed halves pace, and that relation holds in either unit.

A 4 mph average over a 10 mile ride takes 2.5 hours, which is 150 minutes. At 8 mph the same ride takes 1.25 hours, or 75 minutes, and the distance is 16.09344 km either way.

Rounding 6.437376 to 6.4, 6.44 or 6

The rounded answer depends on the display. One decimal gives 6.4 km/h, which is 0.58 percent below the exact 6.437376. Two decimals give 6.44 km/h, which is 0.04 percent above it. A whole number gives 6 km/h, which is 6.79 percent below and the only rounding worth arguing about.

Six km/h is 3.7282 mph, so whole number rounding turns a 4 mph reading into something nearer 3.73 mph. The gap shows up over distance. Ten miles at 4 mph take 150 minutes, and the same ten miles at 6 km/h take 160.9 minutes, which is 10.9 minutes longer for a rounding error of one digit.

For a sign, 6.44 is the right figure to print, since a driver reading 6.4 is 0.6 percent slow and no speedometer is that accurate. For a physics answer, keep 6.437376 and the unit m/s at 1.78816, because those digits survive a square or a division by time without picking up visible error.

The head check is simple. Four miles is about 6.4 km, so 4 mph is about 6.4 km/h, and half of that is 3.2 km/h for 2 mph. That pair covers the slow speeds on this page and keeps a mental conversion within a few percent of the exact figures.