Speed, distance and time
Time underway at a steady average speed. It does not add stops, currents or weather.
Distance equals speed multiplied by time. Enter any two of the three and the calculator finds the third: the time underway, the distance covered or your average speed. Add a departure time to get an estimated time of arrival (ETA). It works in knots and nautical miles, miles per hour and statute miles, or km/h and kilometers.
Speed, Distance and Time Formulas
Time = distance ÷ speed · Distance = speed × time · Speed = distance ÷ time
Keep the units matched. A knot is one nautical mile per hour, so knots go with nautical miles, mph with statute miles and km/h with kilometers. A nautical mile is exactly 1,852 meters, about 1.15 statute miles. Time goes into the formulas in hours: 2 h 30 min is 2.5 hours and 45 minutes is 0.75 hour.
Worked Examples
- Find the time: 50 nautical miles at 12 knots takes 50 ÷ 12 = 4.17 hours, or 4 h 10 min. Leave at 8:00 AM and your ETA is 12:10 PM.
- Find the distance: 15 knots for 2 h 30 min covers 15 × 2.5 = 37.5 nautical miles.
- Find the speed: 48 nautical miles in 3 hours is 48 ÷ 3 = 16 knots.
- In miles per hour: a 30-mile run at 25 mph takes 1 h 12 min.
- In kilometers: 45 km at 30 km/h takes 1 h 30 min.
How to Find Average Speed
Average speed is the total distance divided by the total time. If you cover 60 nautical miles in 5 hours, your average speed is 12 knots. Count a one-hour stop and the same 60 nautical miles take 6 hours, an average of 10 knots. In mph the math is the same: 30 miles in 1 h 15 min averages 24 mph.
How to Calculate Your ETA
Your ETA is the departure time plus the time underway. The calculator adds the two and marks the arrival with +1 day when it falls after midnight. For example, 120 nautical miles at 8 knots takes 15 h, so a 6:00 PM departure arrives at 9:00 AM +1 day, the next morning. The result assumes a steady average speed with no stops, so add time for fuel stops, slow zones, currents and weather.
The Six-Minute Rule
Six minutes is one-tenth of an hour, so in 6 minutes a boat covers one-tenth of its speed in nautical miles: 1.2 nautical miles at 12 knots, or 1.8 nautical miles at 18 knots. Navigators use it for quick dead reckoning, and the Royal Australian Navy teaches it in its “6-3-1 minute” rule.
A related shortcut gives the minutes per nautical mile: divide 60 by the speed in knots. At 10 knots a mile takes 6 min, at 15 knots 4 min and at 20 knots 3 min. The calculator’s “In 6 minutes” figure applies the six-minute rule to your speed.
Travel Time Table
Time underway for common distances and speeds, with no stops:
| Speed | 10 NM | 50 NM | 100 NM |
|---|---|---|---|
| 6 kn | 1 h 40 min | 8 h 20 min | 16 h 40 min |
| 10 kn | 1 h | 5 h | 10 h |
| 15 kn | 40 min | 3 h 20 min | 6 h 40 min |
| 20 kn | 30 min | 2 h 30 min | 5 h |
| 25 kn | 24 min | 2 h | 4 h |
| 30 kn | 20 min | 1 h 40 min | 3 h 20 min |
This Calculator vs. the Boat Travel Time Calculator
Use this page when you already know two of the three values, such as a distance from a chart or a passage log. The Boat Travel Time Calculator works from a map instead: you choose a departure and a destination, and it finds the sea distance and the time at your cruising speed. To measure a route between ports first, try the Sea Distance Calculator; to change units, use the Knots to MPH Converter or the Nautical Miles to Miles Converter; and to estimate fuel for the trip, use the Boat Fuel Calculator.
Frequently Asked Questions
How do I find average speed?
Divide the total distance by the total time. For example, 48 nautical miles in 3 hours is an average speed of 16 knots, and 30 miles in 1 h 15 min is 24 mph.
How do I calculate an ETA?
Divide the distance by your speed to get the time underway, then add it to your departure time. At 12 knots, 50 nautical miles takes 4 h 10 min, so leaving at 8:00 AM gives an ETA of 12:10 PM.
How far does a boat go in 6 minutes?
One-tenth of its speed in nautical miles, because 6 minutes is one-tenth of an hour. At 18 knots that is 1.8 nautical miles.
How many minutes does a nautical mile take?
Divide 60 by your speed in knots. A nautical mile takes 6 minutes at 10 knots, 4 minutes at 15 knots and 3 minutes at 20 knots.
Can I use mph or km/h instead of knots?
Yes. Choose MPH to work in miles per hour and statute miles, or km/h to work in kilometers. Speed and distance always use matching units, such as knots with nautical miles.
Does the result include stops, currents or weather?
No. It is the time underway at a steady average speed. Add your own allowance for fuel stops, slow zones, currents and weather.