Measure · 5 units
Speed
Speed is length over time. The knot is one nautical mile per hour, exactly 1852/3600 m/s, which is why it is the natural unit wherever positions are given in degrees and minutes of latitude.
Convert speed
Then and now
The name comes from the log line: a line knotted at regular intervals was paid out astern and the knots counted against a sand-glass.
All speed units
“Exact” means the unit is defined as an exact multiple of the SI unit; the decimal is printed in full when it ends. Rows marked historical or obsolete are kept for reading old documents.
| Unit | Defined as | In SI | Status | Source |
|---|---|---|---|---|
| SI | ||||
| metre per second m/s | 1 m in 1 s | 1 m/s | exact | BIPM SI Brochure |
| SI (accepted) | ||||
| kilometre per hour km/h | 1000 m in 3600 s | 0.277 777 777 778… m/s | exact | BIPM SI Brochure |
| Imperial / US | ||||
| foot per second ft/s | 1 foot in 1 s | 0.304 8 m/s | exact | NIST SP 811 |
| mile per hour mph | 1 statute mile in 1 hour | 0.447 04 m/s | exact | NIST Handbook 44 |
| Nautical | ||||
| knot kn | 1 nautical mile per hour | 0.514 444 444 444… m/s | exact A speed, never a distance: “knots per hour” is wrong. | IHO 1929 / BIPM |