Every voyage estimate comes down to one relationship between distance, speed, and time. Know any two and the third follows. This calculator solves all three cases: voyage time at a planned speed, the speed required to make a deadline, or an arrival timestamp from a departure time.
How it works
The core relationship in nautical units is distance in nautical miles, speed in knots (nautical miles per hour), and time in hours:
time (h) = distance / speed
speed (kn) = distance / time
ETA = departure + time(h)A knot is one nautical mile per hour, so the arithmetic is direct: 240 nautical miles at 12 knots takes 20 hours. The tool converts the hours result into days, hours, and minutes, and in ETA mode adds the steaming time to the departure timestamp you provide.
The three modes explained
Voyage time
You know where you are going and how fast you plan to steam. Enter the distance and speed and the tool tells you how many days, hours, and minutes the passage will take at that speed. Useful for passage planning and advising charterers of expected arrival windows.
Required speed
You have a firm deadline — a berth window, a laycan start, a port tide gate — and you need to know how fast to steam to meet it. Enter the distance and the available time; the tool divides to give you the required speed. Always compare this against the vessel’s safe economic maximum (often around 80–90 percent of full sea speed) and her full-speed limit before committing. Ordering excessive speed to meet a tight laycan costs fuel disproportionately and may exceed classification or charter party limits.
ETA calculation
You are underway and want to project an arrival timestamp. Enter the departure date and time in UTC, the remaining distance to the pilot station or berth, and your speed over ground. The tool adds the steaming time to the departure moment and returns an estimated arrival date and time.
Example: adjusting speed to make a laycan
For example: a vessel is 3,360 nautical miles from the load port. The laycan opens in 9 days (216 hours). Required speed = 3360 / 216 ≈ 15.6 knots. If the vessel’s normal sea speed is 14.5 knots, the master and charterer need to discuss whether the vessel can safely sustain the extra speed or whether the laycan needs to be renegotiated.
At 14.5 knots the passage takes 3360 / 14.5 ≈ 231.7 hours, or about 9 days and 16 hours — roughly 16 hours inside the laycan if it runs 10 days.
Speed over ground vs speed through water
This calculator treats the speed you enter as the effective rate of progress toward the destination. In practice:
- Speed through water is what the log measures and what the engine produces.
- Speed over ground is what actually determines your ETA. It is speed through water plus or minus any current component along your track.
A vessel doing 14 knots through the water into a 1.5-knot head current has only 12.5 knots over ground toward her destination. For ETA purposes always use speed over ground, sourced from GPS, not the ship’s speed log alone.
What the tool does not include
The result is steaming time. Real voyage time also includes:
- Pilot boarding and inbound passage time (typically 2–6 hours for most ports)
- Canal or strait transits (Panama: roughly 8–10 hours; Suez: 12–16 hours)
- Mandatory slow-steaming zones or emission control areas
- Waiting for a berth, tidal window, or ice pilot
- Bunkering stops
Build an appropriate margin into your ETA so you arrive inside the laycan rather than on the wire.