NauticTools2Coastal navigation
Distance, speed and time – the rule of three at sea
How far will you get in two hours? How fast were you really going? When will you arrive? All three questions come down to one relationship: distance = speed × time. Know two values and you get the third – and NauticTools2 also works out your estimated time of arrival (ETA).
- Distance in nm
- Speed in kn
- Time in hh:mm
- ETA
- Duration from start and end
The three quantities
- Distance – in nautical miles (1 nm = 1852 m).
- Speed – in knots. One knot is one nautical mile per hour, about 0.51 m/s.
- Time – the passage time in hours and minutes.
When do you need it?
- Planning your arrival: When will you reach the harbour, the lock or the bar that can only be crossed with enough water?
- Dead reckoning: How far have you come since the last fix? The distance is laid off on the chart from the last position.
- Finding your speed: Two fixes and the time between them give your actual speed over the ground – a good check against the log.
Formulas and working
Solve for distance
D = S × T
Solve for speed
S = D / T
Solve for time
T = D / S, and optionally ETA = departure + T
Six-minute rule: Six minutes are a tenth of an hour, so in six minutes you cover a tenth of your speed in nautical miles – at 7 kn that is 0.7 nm. Handy for dead reckoning in your head and for radar plotting.
Worked examples
You have been sailing for 2 hours 15 minutes at 5.3 kn. How far have you come?
T = 2 h 15 min = 2 + 15/60 = 2.25 h
D = 5.3 kn × 2.25 h ≈ 11.93 nm
A yacht covers 17.4 nm in 3 hours 45 minutes. What is her speed?
T = 3 h 45 min = 3.75 h
S = 17.4 nm / 3.75 h = 4.64 kn
The destination is still 17.8 nm away and your speed is 6 kn. How long will it take, and when will you arrive if you leave on 7 July 2026 at 18:05?
T = 17.8 nm / 6 kn = 2.967 h = 2 h 58 min
ETA = 07.07.2026 18:05 + 2:58 = 07.07.2026 21:03
Things to watch out for
- Minutes are not decimals: 2 h 15 min is 2.25 h, not 2.15 h – the most common slip in this calculation. That is why NauticTools2 takes time directly as hh:mm.
- Speed through the water or over the ground? The log measures speed through the water. For arrival times, though, speed over the ground is what counts – against a foul tide the difference can be large.
- Past midnight: If a passage runs past midnight, the date changes. NauticTools2 therefore always shows the ETA with its date.
How it works in NauticTools2
Choose what to solve for
Distance, speed or time – the other two become the input fields.
Enter the values
Time as hh:mm, for example 2:15. The colon can be left out: 215 works just as well. Long passages such as 101:23 are fine too.
Calculate the ETA
When solving for time, the departure date and time give the arrival time. The current time is filled in.
Duration from start and end
When solving for distance or speed, the time can also be calculated from a start and an end time – handy for passages past midnight.
Frequently asked questions
How do you convert knots to km/h or mph?
Multiply by 1.852 for km/h or by 1.151 for mph: 6 kn are about 11.1 km/h or 6.9 mph.
What is the six-minute rule?
In six minutes you cover a tenth of your speed in nautical miles. At 5 kn that is 0.5 nm, at 12 kn 1.2 nm.
How do I find my actual speed?
Take two fixes, measure the distance between them on the chart and divide by the time elapsed. That gives your speed over the ground.
Related tools
- Rail Log – measure your speed without a log
- Current Triangle – speed over the ground in a tidal stream
- Dead Reckoning – position from course and distance
NauticTools2 is free and runs on Windows 10 and 11. An Android version is in preparation.
Get it from the Microsoft Store