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Tide Time Estimate Calculator

Work out tide time estimate instantly with clear inputs, formula shown and shareable results.

About Tide Time Estimate Calculator

Semi-diurnal tides repeat every 12 hours 25 minutes, half the 24 hour 50 minute lunar day, so successive high waters drift about 50 minutes later daily. Between them the height follows roughly a cosine, which is the basis of the rule of twelfths. Three hours after a high tide of 4.2 m range the water has fallen to about 3.15 m above low water.

Formulas

Tidal period

semi-diurnal cycle = 12.42 hours

Height between tides

height = (range/2)(1 + cos(2 pi t / 12.42))

Worked example

With the default values below, Tide Time Estimate Calculator returns Next High Tide of 18.92. Change any input above and every figure here is recalculated in your browser.

Inputs used

Time of the last high tideaccepts 0–23.99 h
6.5 h
Tidal rangeminimum 0 m
4.2 m
Hours since that high tideminimum 0 h
3 h

Results

Next High Tide
18.92 h
Next Low Tide
12.71 h
Water Height Above Low Water Now
2.21 m
Semi-Diurnal Tidal Period
12.421 h

Step by step

  1. 1Values used
  2. 2Tidal period
  3. 3Height between tides
  4. 4Next High Tide= 18.92 h
  5. 5Next Low Tide= 12.71 h
  6. 6Water Height Above Low Water Now= 2.21 m
  7. 7Semi-Diurnal Tidal Period= 12.421 h

Assumptions and limits

  • The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

What is the rule of twelfths?

The tide moves 1, 2, 3, 3, 2 and 1 twelfths of the range in successive hours - a mental approximation to the cosine curve.

Are all tides semi-diurnal?

No. Parts of the Gulf of Mexico and South China Sea have one tide a day, and many coasts are mixed with unequal successive highs.

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