About 8 m at 15:00
Rising tide, range 12 m, each tidal hour 1 h 0 min.
| Tidal hour | Time | Change | Height (rule) | Height (sine curve) |
|---|---|---|---|---|
| 0 | 12:00 | - | 2 m | 2 m |
| 1 | 13:00 | +1 m | 3 m | 2.8 m |
| 2 | 14:00 | +2 m | 5 m | 5 m |
| 3 | 15:00 | +3 m | 8 m | 8 m |
| 4 | 16:00 | +3 m | 11 m | 11 m |
| 5 | 17:00 | +2 m | 13 m | 13.2 m |
| 6 | 18:00 | +1 m | 14 m | 14 m |
This is a rough guide, not a tide table. Take the times and heights from an official tide table for your port, and use the table's own figures wherever you can.
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How to use it
Put in the first water's time and height, then the next one's. Use the 24-hour clock and metres. It works for a rising or a falling tide, and across midnight. Add a time to get the height then. Add a height, such as the depth you need over a sill, to see roughly when the tide gets there.
How it is worked out
The rule says that between low and high water, the tide rises by 1, 2, 3, 3, 2 and then 1 twelfths of its range, one step for each sixth of the time. It falls the same way. So halfway through, it has risen half its range. The middle two steps, at 3 twelfths each, move more water than the others.
The source's own example: low water at noon, 2 m; high water at 6 pm, 14 m. The range is 12 m. By 3 pm the tide has risen 1 + 2 + 3 twelfths, which is 6 m, so the height is 8 m. Put those numbers in and you get 8 m.
We split the time between the two waters into six equal "tidal hours". The source notes that six hours is an underestimate, so a tidal hour is often a little longer than a clock hour. Between two steps we draw a straight line. The last column shows the smooth sine curve the rule is based on, so you can see how far apart they are.
When not to use it
It is a rough approximation, and an official tide table is always better. It doesn't suit places where the tide is not a smooth curve. The source names Poole Harbour and the Solent, which have double high waters, and Weymouth Bay, which has a double low water.
Sources
- Wikipedia, Rule of twelfths, read 30 October 2026
Craig Fearn runs Still & Wake. He writes the guides and journal.