Free online boat speed checker and GPS knot meter. See your speed in knots, km/h or mph instantly: no app, no device needed beyond your phone.
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Convert between knots, km/h, mph and m/s.
Traditional boat speedometers use paddle wheels, pitot tubes, or Doppler sonar to measure water speed: the speed of the boat relative to the water. This boat speed checker uses GPS to measure ground speed: your speed relative to the earth's surface. The difference matters when there's a current: a tidal current of 2 knots means GPS speed and water speed can differ significantly.
For general navigation, passage planning, and just satisfying curiosity, GPS ground speed is the most relevant figure. This free boat speedometer tells you how fast you're actually covering distance over the chart, in knots, km/h, or mph.
Marine environments are ideal for GPS. With 360° of open sky and no signal-blocking buildings, your smartphone GPS typically achieves its best accuracy on the water: often within 0.3–0.5 knots of true speed. This makes a phone-based GPS a viable backup or supplementary speedometer on any vessel.
Most smartphones are splash-resistant but not fully waterproof. Keep your phone in a waterproof case or dry bag when on deck. The GPS signal can still penetrate a clear waterproof case without significant degradation.
Traditional boat speedometers measure speed relative to the water, not the seabed. A tidal current or river flow shifts the water mass itself, so a paddle-wheel or pitot-log reading can differ noticeably from your actual progress over the ground. GPS sidesteps this by tracking your position relative to the fixed earth, giving you speed over ground: usually the more useful number for navigation and arrival-time planning. For the full breakdown of why this matters and how big the gap can get, see our guide on knots, mph, and km/h on the water.
Yes. This boat speed checker turns any smartphone into a GPS speedometer, giving accurate speed readings on open water. The open-sky marine environment is ideal for GPS reception, often better than urban road use.
One knot equals one nautical mile per hour: approximately 1.852 km/h or 1.151 mph. Knots are the standard unit for marine and aviation speed worldwide.
Multiply knots by 1.852 to get km/h. The converter above does this automatically for all unit combinations.
Yes. Open water provides excellent sky visibility. Readings are typically within 0.5–1 knot of true ground speed in calm conditions.
A cruising sailboat makes 5–8 knots. Racing yachts can exceed 20–30 knots. Motorboats range from 10 to 50+ knots depending on design and engine.
Speed through water is your speed relative to the water around the hull, measured by a paddle wheel or pitot log. Speed over ground is your speed relative to the seabed, which is what GPS measures. A current or tide can make the two differ by several knots.
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