Boat Speed Calculator — Knots, km/h, mph & Hull Speed
Enter the distance covered and the time taken to calculate boat speed in knots, km/h and mph. Optionally enter the waterline length to find the theoretical hull speed — the practical speed limit for displacement hulls.
Distance unit
h
min
Waterline unit
Knots (nautical miles per hour)
- 1
Total time
1 h + 30 ÷ 60 = 1.5 - 2
Speed
10 ÷ 1.5 = 6.67
How does this calculator work?
Boat speed in knots equals distance in nautical miles divided by time in hours. Convert: 1 kn = 1.852 km/h = 1.151 mph. Theoretical hull speed for displacement hulls = 1.34 × √(waterline in feet) knots. Planing hulls exceed hull speed by rising out of the water.
Formula
How this is calculated
Speed is distance divided by time. For boats, the standard output is knots (nautical miles per hour), because nautical miles are tied to the Earth's coordinate system — one nautical mile equals one arc-minute of latitude, making chart navigation straightforward. Enter the distance in nautical miles, kilometres or statute miles together with the elapsed time in hours and minutes, and the calculator converts to a common base to give knots, km/h, mph and m/s simultaneously.
The hull speed formula, 1.34 × √(waterline_ft), estimates the practical top speed of a displacement hull — a vessel that moves through the water rather than over it. At hull speed, the boat sits in the trough between its own bow and stern waves, and additional power causes the hull to drag, not accelerate. Most traditional monohulls sail comfortably at 70–90% of hull speed. Planing hulls (powerboats, some performance sailboats) overcome this limit by climbing on top of the bow wave. The constant 1.34 is empirical and applies to displacement hulls in salt water; it is sometimes quoted as 1.25–1.35 depending on hull form.
Conversion between speed units uses exact nautical definitions: 1 knot = 1.852 km/h exactly, 1 nautical mile = 1.15078 statute miles.
Frequently asked questions
A knot is one nautical mile per hour. One nautical mile equals one arc-minute of latitude (1,852 m exactly), so position, distance and speed all use the same underlying unit on a chart — making navigation by hand far simpler than mixing miles and kilometres.
Hull speed (≈ 1.34 × √waterline_ft in knots) is the speed at which a displacement hull rides in the trough between its own waves; exceeding it requires dramatically more power. Planing hulls (powerboats and some lightweight sailboats) can climb on top of their bow wave and go faster, but the formula no longer applies to them.
The constant ranges from about 1.25 to 1.35 depending on hull fineness and displacement. The value 1.34 is a widely used approximation for moderate-displacement monohulls. Lean, lightweight hulls may achieve slightly higher ratios; full, heavy hulls slightly lower.
TG we-Calculate Editorial Team. (2026). Boat Speed Calculator — Knots, km/h, mph & Hull Speed [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/boat-speed-calculator
TG we-Calculate Editorial Team. "Boat Speed Calculator — Knots, km/h, mph & Hull Speed." TG we-Calculate. 2026. https://we-calculate.com/calculator/boat-speed-calculator.
TG we-Calculate Editorial Team, "Boat Speed Calculator — Knots, km/h, mph & Hull Speed," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/boat-speed-calculator
@misc{wecalculate_boat_speed_calculator, title = {Boat Speed Calculator — Knots, km/h, mph & Hull Speed}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/boat-speed-calculator}}, year = {2026}, note = {TG we-Calculate} }
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