Driving uses mph or km/h. But step onto a boat, into a plane, or near any weather report, and a new vocabulary of speed units appears. Knots, Mach numbers, meters per second — each comes from a specific context and serves a specific purpose. Here's what they mean and how to move between them.
The knot: nautical and aeronautical standard
A knot (kt) is one nautical mile per hour. One nautical mile equals 1 minute of latitude — approximately 1.852 kilometers, or 1.151 statute miles.
- 1 knot = 1.852 km/h = 1.151 mph
- 1 mph = 0.869 knots
- 1 km/h = 0.540 knots
Why use nautical miles at sea? Because latitude lines divide naturally into minutes, and a chart in nautical miles aligns perfectly with navigation coordinates. The name "knot" comes from the old sailor's practice of measuring ship speed with a knotted rope towed behind the vessel.
Knots in practice
Sailing: A cruising sailboat moves at 5–8 knots (9–15 km/h, 5.8–9.2 mph). Fast racing sailboats: 15+ knots.
Cargo ships: 15–25 knots cruising.
Aircraft: Cruising airliners: 450–500 knots (835–925 km/h, 520–575 mph). Small planes: 100–150 knots.
Wind speeds: "20 knots" is moderate breeze; "40 knots" is gale force; "64 knots" is hurricane-force minimum.
Meters per second: the scientific standard
Physics, engineering, and weather use meters per second (m/s) as the SI unit of speed. Easy to derive and easy to use in calculations.
- 1 m/s = 3.6 km/h = 2.237 mph = 1.944 knots
- 1 km/h = 0.278 m/s
- 1 mph = 0.447 m/s
A person walking is 1.4 m/s. A runner is 3–4 m/s. A car at highway speed is 25–30 m/s. A jetliner is 250 m/s.
Feet per second: a ballistic unit
US ballistics, physics education, and some engineering contexts use feet per second (ft/s or fps).
- 1 ft/s = 0.682 mph = 0.305 m/s
- Speed of sound in air ≈ 1,125 ft/s at 20°C
- .22 LR rifle round: ~1,300 ft/s muzzle velocity
- AR-15 rifle round: ~3,200 ft/s
- Major League fastball: 140 ft/s (95 mph)
Mach numbers
Mach is a speed expressed as a fraction of the local speed of sound. Mach 1 ≈ 343 m/s = 1,235 km/h = 767 mph at sea level and 20°C. It varies with air temperature.
- Mach 0.85: cruising airliner speed
- Mach 1.0: speed of sound (sonic boom threshold)
- Mach 2.0 (SR-71 retired): 2,470 km/h
- Mach 9 (hypersonic missiles): 11,100 km/h
Unit conversion cheat sheet
| From | → mph | → km/h | → m/s | → knots |
|---|---|---|---|---|
| 1 mph | 1.00 | 1.609 | 0.447 | 0.869 |
| 1 km/h | 0.621 | 1.00 | 0.278 | 0.540 |
| 1 m/s | 2.237 | 3.60 | 1.00 | 1.944 |
| 1 knot | 1.151 | 1.852 | 0.514 | 1.00 |
Wind speed categories
Beaufort wind scale with speed equivalents:
- Force 0 (calm): 0–1 knots, 0–2 km/h
- Force 3 (gentle breeze): 7–10 knots, 12–19 km/h
- Force 5 (fresh breeze): 17–21 knots, 30–39 km/h
- Force 7 (near gale): 28–33 knots, 51–61 km/h
- Force 9 (strong gale): 41–47 knots, 76–87 km/h
- Force 12 (hurricane): 64+ knots, 119+ km/h
Projectile physics
Terminal velocity (free-fall speed where air resistance balances gravity):
- Human skydiver (belly down): ~200 km/h, ~55 m/s, ~124 mph
- Human skydiver (head first): ~320 km/h, ~89 m/s
- Raindrop: ~9 m/s (small) to 30 km/h (large)
- Hailstone: varies widely, can exceed 100 km/h
Space and astronomy
- International Space Station orbital speed: 7.66 km/s (27,580 km/h, 17,150 mph)
- Earth around Sun: 29.8 km/s
- Speed of light: 299,792 km/s = 186,282 mi/s (so fast it takes 1.3 seconds to reach the Moon and 8 minutes to reach Earth from the Sun)
Quick mental conversions
For rough mental math at sea or in the air:
- Knots to mph: add ~15% (20 knots ≈ 23 mph)
- Mph to knots: subtract ~13% (50 mph ≈ 43 knots)
- Km/h to mph: multiply by 0.6 and add a bit (100 km/h ≈ 62 mph)
- Mph to m/s: divide by 2.24 (60 mph ≈ 27 m/s)
- Knots to km/h: double and subtract 15% (20 knots → 40 → 34 km/h, actual 37)
These aren't perfect but good enough for estimating fuel consumption, ETA, or safe separation without pulling out a calculator.
Why pilots and sailors still use knots
In an era of universal GPS and digital navigation, why haven't aviation and maritime moved to a single unit? Three practical reasons:
- Chart compatibility: nautical charts are built around nautical miles. A knot aligns naturally with the distance markings on every navigation chart in active use.
- Latitude and longitude math: 1 nautical mile = 1 arc-minute of latitude, making dead-reckoning calculations trivial without a computer.
- International standardization: switching a global industry is enormously expensive and error-prone — every aircraft instrument, chart, air traffic control procedure, and training manual would need updating. The costs outweigh the benefits of unification.
Russia and China use km/h in some military aviation contexts, but international civil aviation remains firmly on knots for airspeed and nautical miles for distance.
Weather report vocabulary
Media outlets use different speed units depending on audience:
- US hurricane reports: mph (for public), knots (for marine advisories)
- European weather: km/h universally
- Marine forecasts worldwide: knots
- Aviation METAR/TAF: knots for wind, often with gusts in knots
When you hear "sustained winds of 75 mph" on the news, know that's about 65 knots — already Category 1 hurricane territory. The same storm's marine advisory would say "storm force winds of 65 knots."
Just convert it
Our speed converter handles every unit above in one click. Knots to mph for pilots; m/s to km/h for physicists; mph to ft/s for ballistic calculations. One tool, any direction, any magnitude. Speed conversions pop up more than you might think — have the right tool ready.