Sailing Dead Reckoning Calculator

The fundamental navigation formula

Dead reckoning at sea relies on the simplest of all navigation calculations:

Distance (nm) = Speed (knots) × Time (hours)

One knot equals one nautical mile per hour. A nautical mile (nm) is 1,852 metres, or approximately 1.15 statute miles. This unit is used universally in marine and aviation navigation because it corresponds directly to one minute of latitude on a chart — enormously useful for distance estimation.

Solving for each variable

Distance = Speed × Time Speed = Distance ÷ Time Time = Distance ÷ Speed

This calculator solves for whichever variable you leave blank. Enter any two values and it calculates the third.

Practical passage planning

For a typical sailing yacht averaging 5–7 knots under sail:

• A 100 nm passage takes approximately 14–20 hours
• An overnight passage of 12 hours covers roughly 60–84 nm
• A 200 nm offshore passage takes approximately 28–40 hours

Accounting for tidal current

Dead reckoning gives your speed through the water. Your actual speed over the ground (SOG) will be faster or slower depending on tidal current. A 2-knot favourable tide adds 2 knots to your progress. A 2-knot contrary tide reduces your effective progress by 2 knots. Always factor tidal streams into passage time estimates for coastal waters.

Leeway and course made good

Wind pressure on sails and hull causes leeway — the vessel is pushed sideways. The course made good over the ground also differs from the heading steered. Dead reckoning is the starting point; layering on tidal and leeway corrections gives the estimated position, used as a cross-check against GPS and visual fixes.