Rule of Twelfths

The Rule of Twelfths is a guideline for determining where the tide is in the rise and fall cycle and how swiftly the current is running.

Boaters generally use the Rule of Twelfths to determine if shallow creeks and rivers have enough depth for boats to pass. Kayakers use it more often to gauge the strength of the tidal flow, so as not to put themselves in situations where they would have to paddle long distances against a strong tidal flow.

The Rule of Twelfths is based on tidal changes that require roughly 6 to 6.5 hours for a tide change in either direction. The formula in the Rule of Twelfths is that 1/12 of the water moves in the first hour, 2/12 of the water moves in the second hour, 3/12 of the water moves in each of the third and fourth hours, 2/12 of the water moves in the fifth and 1/12 of the water moves in the sixth.

While this is not exact, it is surprisingly accurate when the weather is stable, without strong winds and an extreme full- or new-moon tide. The extra flow during those situations usually causes the tide to move more water later in the tide cycle.

Wind blowing strongly with the tide will accelerate the tidal flow and often push more water than was forecast, so the high tides are higher or the low tides are lower. Wind blowing strongly against the tide will retard the tidal flow and often allow less water to move than was originally forecast, which makes the low tides higher or the high tides lower than usual.