Marine ETA Calculator
The marine ETA calculator computes your estimated time of arrival by dividing passage distance in nautical miles by your speed over ground in knots to find travel time, then adding that to your departure time. Use speed over ground (SOG) from your GPS rather than speed through water to account for current effects. Enter your departure time in 24-hour format, the distance to go, and your expected average speed over ground. The calculator returns travel time in hours and minutes and the estimated arrival time.
Marine ETA formula
Travel time (hours) = Distance (nm) / SOG (knots)
ETA = Departure time + Travel time
Convert decimal hours to h:mm:
Hours = floor(T); Minutes = round((T - Hours) * 60)
ETA wraps through midnight: an ETA past 24:00 indicates arrival the next day.
ETA planning considerations
- Use GPS SOG averaged over several minutes for a stable estimate rather than instantaneous readings.
- Plan to arrive at tidal inlets at favorable tide and during daylight hours.
- Check marina office hours; many close by 18:00-20:00 and may not monitor VHF after hours.
- Update ETA regularly as SOG changes due to wind and current variations through the passage.
Marine ETA: frequently asked questions
How is marine ETA calculated?
ETA = Departure time + Travel time, where Travel time (hours) = Distance (nm) / Speed over ground (knots). For example, departing at 06:00 for 90 nm at 9 knots takes 10 hours, giving an ETA of 16:00.
Should I use speed through water or speed over ground for ETA?
Always use speed over ground (SOG) from your GPS for ETA calculations. SOG accounts for all effects including current and leeway, so it reflects your actual progress along the ground track. Speed through water (STW) ignores current and will give an incorrect ETA when current is significant.
How do I account for variable speed in ETA planning?
For passages where you expect different speeds for different legs (e.g., motoring then sailing, or passing through a favorable tide), calculate each leg separately and sum the travel times. This gives a more accurate total ETA than using a single average speed.
What safety margin should I add to an ETA?
Standard practice is to plan to arrive in daylight, well before predicted tidal windows close, and with fuel reserves intact. Add at least 20-30% buffer to your calculated travel time for offshore passages to account for weather, reduced speed, and unexpected stops.
How do I handle time zone changes on a passage?
Use UTC (Coordinated Universal Time) for all navigation calculations, then convert to local time at destination. This avoids confusion from daylight saving time changes or time zone crossings during the passage. Log all times in UTC.
Official sources
- National Geospatial-Intelligence Agency: Bowditch American Practical Navigator (Pub. 9).
- US Coast Guard: USCG Navigation Center.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.