The Rule of Twelfths: How to Work Out the Tide at Any Time

You know the times and heights of low and high water from the tide tables. But you want to cross a shallow bar at 10:20, and the real question is: how much water will there be at that time? The rule of twelfths answers it in a minute, no calculator needed.

How it works

The tide does not rise at a steady rate: it starts slowly, speeds up at mid-tide and slows again near high water. The rule approximates that curve by splitting the range (high water height minus low water height) into 12 parts:

Hour after low water	Rise that hour	Total
1st	1/12	1/12
2nd	2/12	3/12 (a quarter)
3rd	3/12	6/12 (half)
4th	3/12	9/12 (three quarters)
5th	2/12	11/12
6th	1/12	12/12 (high water)

On a falling tide it works the same way, counting down from high water. If the tide does not last exactly 6 hours, divide the time from low to high water by 6 and use those "tidal hours".

Example: will there be enough water at 10:20?

Tide table: low water at 08:00, 1 ft; high water at 14:00, 13 ft. Range: 12 ft, so each twelfth is 1 ft. Handy.

Your boat draws 6 ft and you want 2 ft under the keel: you need 8 ft of water. The chart shows 3 ft at the bar, so you need the tide at 5 ft, which is 4 ft above low water.

• 09:00 (1st hour): +1 ft, total 1 ft.
• 10:00 (2nd hour): +2 ft, total 3 ft.
• 3rd hour: rises 3 ft. You need 1 more foot, a third of that hour: about 20 minutes.

Answer: from about 10:20 you have just enough water. It is an estimate, so build in a margin: go at 10:45, not 10:20.

The other reading: when the current runs hardest

If half the water comes in during the 3rd and 4th hours, that is also when tidal current runs hardest through inlets and channels. If you can, avoid crossing a narrow inlet at mid-tide with wind against current: that is when the steepest seas build.

Limits: when not to trust the rule

• It is an approximation. It works best with regular semidiurnal tides, like most of the US Atlantic coast. Always check NOAA tide predictions.
• Mixed and diurnal tides: on the US Pacific coast and in the Gulf of Mexico, tides are often mixed or once a day, and the rule does not fit well.
• Pressure and wind: high pressure lowers the water level and low pressure raises it (roughly 4 in per 10 hPa). A strong onshore or offshore wind can change it more.

If you anchor in tidal waters, combine this with how much rode to let out: the tide still to rise counts in your total height.