Tide Height Calculator

The Rule of Twelfths is a traditional mariner's approximation stating that tide height changes by 1/12, 2/12, 3/12, 3/12, 2/12, and 1/12 of the total range across six equal intervals between high and low tide. It approximates the real sinusoidal shape of tidal motion closely enough for practical navigation without requiring the harmonic constituent data a precise tide prediction would need, which is why it remains a standard quick-reference tool for boaters.

The Rule of Twelfths assigns the largest fractions (3/12 and 3/12) to the third and fourth intervals, which fall right around the midpoint between high and low tide, and the smallest fractions (1/12 each) to the intervals right after a high or low. This mirrors real tidal motion, which moves slowest near the turn of the tide (at high or low water) and fastest around the midpoint, similar to how a pendulum swings slowest at its extremes and fastest through the middle.

Tidal Period sets how long a full high-to-high cycle takes, and the calculator scales the six Rule-of-Twelfths intervals to fit within half of that period. The default of 12.42 hours reflects the real period of the M2 lunar tidal constituent, the dominant driver of most semidiurnal (twice-daily) tides -- adjusting this input lets the calculator match locations with different tidal patterns.

It normalizes Hours Since High Tide against the full Tidal Period and checks whether that position falls within the first half of the cycle (falling, moving away from high tide) or the second half (rising, moving toward the next high tide). Current Tide Height is then calculated by adjusting from the nearer reference point -- High Tide Height while falling, Low Tide Height while rising.

No -- it is a linear-ish approximation of a smooth curve and works best for tides that are close to a simple sinusoidal shape. Real tide predictions from harmonic analysis (as published in official tide tables) account for multiple overlapping tidal constituents, local coastal geometry, and weather effects that can shift actual tide height meaningfully from this rule's estimate, especially in areas with unusual or mixed tidal patterns.