Few numbers matter more at sea than the ETA. Agents, charterers, pilots, and the next port all plan around it — and a wrong one causes real trouble. The good news: ETA calculation is simple arithmetic. The skill is in the adjustments.
The three formulas
Everything starts from one relationship between distance, speed, and time. Work in nautical miles and knots (nautical miles per hour), and memorise all three forms:
- Time = Distance ÷ Speed (gives hours)
- Speed = Distance ÷ Time
- Distance = Speed × Time
Example: 1,200 nautical miles at 13.5 knots. Time = 1,200 ÷ 13.5 = 88.9 hours. That is 3 days and 16.9 hours — roughly 3 days 17 hours of steaming. Add that to your departure time and you have the ETA.
Converting decimal hours to clock time
The decimal part trips people up. Multiply it by 60 to get minutes: 0.9 hours × 60 = 54 minutes. So 88.9 hours is 88 hours 54 minutes, or 3 days, 16 hours, 54 minutes. If you depart at 06:00 on the 10th, your ETA is about 22:54 on the 13th — before any adjustments.
The adjustments that make or break an ETA
A raw distance-over-speed number is only a starting point. A reliable ETA accounts for:
- Current (set and drift): a 1-knot favourable current over 3 days saves you 72 miles — nearly 5.5 hours at 13 knots. An adverse current costs you the same. Use the pilot charts and your own observed set and drift.
- Weather margin: add 5% to the steaming time in fair seasons, more in heavy-weather areas and seasons. Owners would rather you arrive early than explain a missed laycan.
- Speed limits and ECAs: emission control areas, traffic separation schemes, and coastal speed restrictions all force slower speeds for part of the passage — calculate each leg separately.
- Tidal windows and pilot stations: your ETA is not when you arrive off the port — it is when the pilot boards, the tide serves, and the berth is free. Work backwards from the required arrival time.
- Slow steaming orders: if the charterer orders eco speed, the fuel saving is real but so is the extra day. Recompute and report the new ETA the moment the order changes.
Recompute every day
An ETA is a living number. Take the noon position each day, measure the distance remaining, divide by your realistic speed of advance (not the log speed — the speed made good over the ground), and update. If the new ETA moves by more than a couple of hours, tell the office and the agent early. Nobody minds an updated ETA; everybody minds a surprise.
Always keep two ETAs: the "required" ETA (what the charter party or agent needs) and the "realistic" ETA (what your numbers say). When they disagree, that gap is a decision for the master — and it should be made days out, not hours.
Run the numbers in seconds with SMART SEAMEN's free ETA and speed calculator — get started.
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