Rule of Twelfths Tide Calculator
Estimate intermediate water depth and hourly vertical rise or fall between high and low tide.
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How the Marine Rule of Twelfths Works
Ocean tides approximate a sinusoidal harmonic wave over an average 6-hour period between low and high water. The tidal range is divided into 12 equal fractions:
- 1st Hour: 1/12th of total range (slack water begins moving)
- 2nd Hour: 2/12ths of total range
- 3rd Hour: 3/12ths of total range (maximum current flow)
- 4th Hour: 3/12ths of total range (maximum current flow)
- 5th Hour: 2/12ths of total range
- 6th Hour: 1/12th of total range (easing into next slack)
Notice that in the middle 2 hours (hours 3 and 4), half of the entire tidal range moves (6/12ths = 50%), which creates the strongest tidal rips and rapids.
Frequently Asked Questions
Where does the Rule of Twelfths apply?
It applies to semi-diurnal tidal regimes (two high and two low tides daily of similar height), which prevail along the Atlantic coast of North America and Western Europe.
How accurate is the Rule of Twelfths?
It is typically accurate to within a few inches when compared to official sinusoidal tide curves, making it the standard mental math method taught in RYA and ASA coastal navigation courses.
What is chart datum?
Chart datum on US charts is Mean Lower Low Water (MLLW). Tide heights on tables are added to the charted soundings printed on nautical charts to obtain true water depth.