Seamanship
Height of tide
The height of tide at any time between low and high water from the tide tables, and the time it reaches the height you need for your draught.
To find when there is enough water
Fill in the fields to see the result.
How it works
The rule of twelfths: in each sixth of the time from low to high water, the tide rises 1, 2, 3, 3, 2 and 1 twelfths of the range. Halfway through it has risen half the range; the fastest change is in the middle two sixths. The cosine curve, (1 − cos πt/T) / 2, is the smooth version.
Both assume a regular, semi-diurnal tide. Where the tide is irregular (shallow-water ports, double high waters, mixed tides), use the tidal curves of the Admiralty Tide Tables or the port's own predictions, and allow for weather and barometric pressure.
For training and cross-checking. Not a substitute for approved navigation equipment, nautical publications and the ship’s own procedures. Calculations run in your browser and nothing you type is stored; a PDF you download is made on our server and not kept.
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