Tidal Height - Rule of 12ths

The tide doesn't fall evenly between High Water and Low Water. The Rule of 12ths is the sailor's mental shortcut, the tide moves 1, 2, 3, 3, 2, 1 twelfths of the range in the six successive hours. This calculator shows that rule on a chart alongside the smooth cosine curve it approximates, and adds a depth check for your boat.

What it does

• Pick HW height, LW height, and which way the tide is going.

• Scrub the time slider between 0h (HW) and 6h (LW).

• Read the current tide height by both methods, they agree at the hour breakpoints and differ by up to a few percent of range in between.

Optional under-keel clearance

Switch on the depth-check block and enter:

• Your vessel's draft.

• The charted depth at the spot you're worried about.

• Your safety margin.

The chart draws your boat, the seabed, the current waterline and your keel. The keel line goes green when you have safe water and red when you don't. A read-out tells you how long until the tide drops to (or rises through) the critical depth.

When to use it

• Drying moorings & bar crossings, pick a safe arrival window.

• Approach planning: plan low-water transits, anchor scope, and timing for shallow harbours.

• Teaching: visualise why the Rule of 12ths gives accurate-enough numbers despite being so simple.

Caveats

The Rule of 12ths assumes a roughly sinusoidal, two-tides-a-day pattern. For diurnal or distorted curves, fall back on a real tide table for the day, use this tool for first-pass planning.