About Speed Converter — mph, km/h, m/s, Knots & ft/s
The speed converter translates a speed between the units that share one project: meters per second from SI calculations and wind-load equations, kilometers per hour and miles per hour from vehicle and code contexts, feet per second from US hydraulics and ballistics, feet per minute from duct air-velocity and conveyor specs, and knots from marine and aviation work. Every conversion runs through the SI unit — the meter per second — using the factors published in NIST SP 811 Appendix B, so a chain like mph → km/h → mph returns exactly where it started.
The anchors worth memorizing: 1 mph = 0.44704 m/s exactly (via the 1959 international foot), so 60 mph = 26.8224 m/s ≈ 96.56 km/h; 1 m/s = 3.6 km/h exactly; and 1 knot = one nautical mile (1,852 m exactly) per hour ≈ 0.5144 m/s ≈ 1.151 mph. Every factor in this set is exact — the only rounding you will ever see is the 6-significant-digit display.
How It Works
- Enter the speed value, pick the unit it is currently in ("from"), and pick the unit you need ("to"). The headline shows the converted value; the table below it shows the same speed in all six units at once, so you can lift whichever one the next document needs.
- Every conversion is value × (meters per second per from-unit) ÷ (meters per second per to-unit). The factors are the NIST SP 811 Appendix B values, and all of them are exact: the mile/foot family through the international foot (0.3048 m), km/h as the ratio 1/3.6, and the knot through the 1,852 m nautical mile.
- Displayed values are rounded to 6 significant digits. That is display rounding only — the underlying conversion is done in full double precision, which is why round-trips return your original number.
- Watch the speed-vs-velocity distinction: this tool converts the magnitude. Direction (a heading, a slope component) is bookkeeping that belongs to your vector, not to the unit — which is also why negative inputs are rejected rather than silently accepted.
Worked Example
A US highway design speed of 60 mph needs to enter an SI stopping-distance formula. Through the meter per second: 60 mph × 0.44704 m/s-per-mph = 26.8224 m/s exactly — the factor is exact, so there is no rounding loss. The same speed is 96.5606 km/h (× 3.6) and 52.1386 knots; and since 60 mph is a mile a minute, it is also exactly 88 ft/s — a number traffic engineers keep in their heads.
mph to km/h conversion chart (with m/s and knots)
Common speed limits and reference speeds converted from miles per hour with the exact NIST SP 811 factor (1 mph = 0.44704 m/s), shown to 6 significant digits — exactly what the converter outputs for the same inputs.
| Speed | km/h | m/s | knots |
|---|---|---|---|
| 25 mph (school zone) | 40.2336 | 11.176 | 21.7244 |
| 55 mph | 88.5139 | 24.5872 | 47.7937 |
| 60 mph | 96.5606 | 26.8224 | 52.1386 |
| 70 mph (US interstate) | 112.654 | 31.2928 | 60.8283 |
| 100 mph | 160.934 | 44.704 | 86.8976 |
Formulas
- Conversion through the base unit
x = value × f_from ÷ f_to- Meters per second per unit (the factor table)
mph 0.44704 · ft/s 0.3048 · ft/min 0.00508 · km/h 1/3.6 · knot 1,852/3,600- The everyday anchors
60 mph = 26.8224 m/s = 88 ft/s ≈ 96.56 km/h; 1 knot ≈ 1.15078 mph; 1 m/s = 3.6 km/h
Standards & References
- NIST SP 811, Guide for the Use of the International System of Units, Appendix B.9 (Velocity) — mile per hour 4.4704×10⁻¹ m/s (exact); foot per second 3.048×10⁻¹ m/s (exact); foot per minute 5.08×10⁻³ m/s (exact); knot 5.144444×10⁻¹ m/s (nautical mile = 1,852 m exact)
- International yard and pound agreement (1959) — 1 ft = 0.3048 m exactly, the anchor of the mph/ft-s family
- Displayed to 6 significant digits; conversions computed in full double precision
Frequently Asked Questions
How do I convert mph to km/h?
Multiply mph by 1.609344 (the exact international mile in kilometers). So 60 mph ≈ 96.56 km/h and 100 mph ≈ 160.93 km/h. Going the other way, multiply km/h by 0.621371 to get mph.
How fast is a knot?
One nautical mile per hour, where the nautical mile is exactly 1,852 m — so 1 knot = 0.514444 m/s ≈ 1.15078 mph ≈ 1.852 km/h. A 20-knot wind is about 23 mph. And it is just "knots": the unit already contains the per-hour, so "knots per hour" would be an acceleration.
Why do engineers use m/s instead of km/h?
Because the SI formulas want it: kinetic energy (½mv²), stagnation pressure (½ρv²), stopping distance, and every wind-load code equation take speed in meters per second, and feeding km/h in without dividing by 3.6 inflates the result by 3.6² ≈ 13×. Convert first, then substitute — that factor-of-13 error is one of the classic wind-pressure mistakes.
Is 60 mph really exactly 88 ft/s?
Yes, exactly. A mile is 5,280 ft and an hour is 3,600 s, so 60 mph = 60 × 5,280/3,600 = 88 ft/s with no rounding at all. It is a handy sanity anchor: every 15 mph adds exactly 22 ft/s.
Where is ft/min used?
Mostly in HVAC and material handling: duct air velocities are specified in fpm (a typical supply duct runs 600–1,200 fpm), and conveyor and elevator speeds are quoted the same way. 1 ft/s = 60 ft/min exactly, and 100 fpm = 0.508 m/s exactly, so the conversions are clean.