Rule of Twelfths Tide Calculator

Rule of Twelfths Tide Calculator

The Rule of Twelfths is a traditional mental-math shortcut mariners use to estimate tidal height at any point between a known high and low tide - no tide table lookup or tide curve needed for intermediate hours.

The 1-2-3-3-2-1 Pattern

Hour	Fraction of Total Range
1st hour	1/12
2nd hour	2/12
3rd hour	3/12
4th hour	3/12
5th hour	2/12
6th hour	1/12

Example

A 9-foot tidal range, 2 hours after low tide:

Twelfths elapsed = 3/12 → Height Change = 9 x (3/12) = 2.25 ft above low tide

Why This Pattern Works

This 1-2-3-3-2-1 pattern (which sums to 12, hence the name) approximates the smooth sinusoidal curve of a standard semi-diurnal tide - the tide rises slowly at first, accelerates through the middle hours, then slows again approaching the next high or low, mirroring how a sine wave's rate of change behaves.

A Useful Approximation, Not an Exact Calculation

This gives reasonably accurate estimates for standard semi-diurnal tides found in most locations, though it's a simplification - not exact for locations with unusual, mixed, or highly irregular tidal patterns, where consulting actual tide tables or tide curve software remains more reliable.