What ETA and passage time mean
ETA (Estimated Time of Arrival) is the expected arrival time at a destination — waypoint, harbour, anchorage.
Passage time is the duration from departure to arrival. Both depend on distance and speed — at sea usually in nautical miles (nm) and knots (kn).
The basic formula is simple:
Passage time (h) = Distance (nm) ÷ Speed (kn)
ETA = Departure time + Passage time
Minimum inputs for a usable ETA
| Quantity | Typical source |
|---|---|
| Distance | Chart, waypoints, GPS track |
| Speed | Motor/sail experience, polar diagram, Dutchman’s log |
| Departure time | Plan or “now” |
Without a realistic speed, any ETA is wishful thinking.
Include current and wind honestly
The formula above assumes constant speed. At sea that changes:
- Current with or against you → different SOG over the ground
- Wind → different speed through the water
- Seas → engine RPM no longer delivers planned kn
For first-pass planning, an experienced SOG (e.g. 5 kn) is often enough. For fine planning, course correction and current correction are worth the effort.
Dutchman’s log as a speed check
Classically you measure speed with a Dutchman’s log (boat length, time bow to stern) and get knots. That complements GPS or an engine RPM table.
→ Dutchman’s log · Article Passage time vs. ETA
Tides and arrival in harbour
ETA in open water is one thing — berthing is another:
- Tides (HW/LW) for draft and bridges
- Time buffer for harbour manoeuvres
ETA in NauticCalc
The ETA calculator works from:
- Distance (nm)
- Speed (kn)
- Departure time
…to passage duration and ETA — offline, without spreadsheet math on board.
Limit: It does not replace current correction over the whole leg. It gives you the nautical baseline — you know your average speed.
Typical planning mistakes
- Distance entered in km instead of nm
- Speed from your best passage instead of heavy weather
- Time zone forgotten on longer trips
- ETA = berthed, without checking tide windows
Offline at sea
The ETA calculator needs no internet. Together with course, variation, and logbook you stay able to act without a network.
Bottom line
Calculating ETA means bringing distance, realistic kn, and departure together cleanly. If you keep current, wind, and tides in mind, you plan arrival not only to the minute but safely.
Read next: ETA calculator · Course correction · Download