Worked tide example
How to Work Out What Time the Tide Reaches a Given Height
Work out how far the tide has to rise from low water, turn that into twelfths of the range, then count hours of 1, 2, 3, 3, 2, 1 twelfths until you have enough. With low water 1.1 m at 0410 and high water 4.9 m, the tide reaches 3.0 m at about 0710.
The question
Low water is at 0410 (1.1 m) and high water at 1010 (4.9 m). You need 3.0 m of tide to leave your berth. When can you go?
| Item | Value |
|---|---|
| Low water | 0410, 1.1 m |
| High water | 1010, 4.9 m |
| Height needed | 3.0 m |
Working, step by step
- Range = 4.9 m − 1.1 m = 3.8 m; one twelfth = 0.32 m.
- Rise needed = 3.0 m − 1.1 m = 1.9 m, which is 1.9 m ÷ 3.8 m × 12 = 6.0 twelfths.
- In the first 3 hours the tide changes 1 + 2 + 3 = 6 twelfths, exactly what is needed.
- Time = 0410 + 3 h 00 min = 0710.
Answer
There is 3.0 m of tide from about 0710.
The tides in this example are made up for practice. For real passages, use official predictions such as ADMIRALTY EasyTide or a nautical almanac, and the port's own tidal curve.
Try it yourself
Put your own numbers into the free tide calculator, which uses the same Rule of Twelfths and works out under-keel clearance.
Learn the method
More worked tide examples
- Height of tide at a given time
- Depth from charted depth
- Under-keel clearance
- Crossing a drying height
- Bridge clearance
- Anchoring depth and scope
- Springs or neaps from the range
- Tidal stream rate from a diamond
- Secondary port: time of HW
- Secondary port: height of HW
Frequently asked questions
Is it better to work from high water or low water?
Work from whichever is nearer the time you want, so you count fewer hours. On a rising tide before half-tide, count from low water; after half-tide, count back from high water.