Rule of Twelfths Tide Calculator
Tide height at any hour between tables: the 1-2-3-3-2-1 rule computed exactly, with the sine-curve truth it approximates.
Hours three and four carry HALF the entire range — the mid-tide sprint that strands the unwary picnic and floats the grounded keel. The rule assumes a regular semidiurnal tide; estuaries with double highs or strong shallow-water distortion need real curves.
Formula
⚠️ For planning and education only — verify with official charts, tide tables and your vessel's documentation. Not for navigation.
Tide height at any hour between tables: the 1-2-3-3-2-1 rule computed exactly, with the sine-curve truth it approximates.
About Rule of Twelfths Tide Calculator
Tide tables give the endpoints; the water in between follows a sine curve that the rule of twelfths approximates beautifully in your head: the range rises by 1, 2, 3, 3, 2, 1 twelfths through the six hours. This calculator computes the rule exactly, shows the true sine value beside it (they rarely differ by more than centimeters), and reports the current rise RATE — the mid-tide sprint where half the water moves in a third of the time.
How to use Rule of Twelfths Tide Calculator
- 1Enter — sensible defaults are pre-filled so you see a worked result immediately.
- 2Read the live results: .
- 3Check the "With your numbers" line to see the formula rise per hour: 1/12, 2/12, 3/12, 3/12, 2/12, 1/12 of the range — a stepwise sine curve substituted step by step.
- 4Adjust inputs (or flip the unit toggle) until the scenario matches yours, then copy or share the result.
Why use Rule of Twelfths Tide Calculator?
- ✓Instant, free and private — every calculation runs in your browser, nothing is uploaded
- ✓Built on the published formula rise per hour: 1/12, 2/12, 3/12, 3/12, 2/12, 1/12 of the range — a stepwise sine curve with sources cited on the page
- ✓Hours three and four carry HALF the entire range — the mid-tide sprint that strands the unwary picnic and floats the grounded keel. The rule assumes a regular semidiurnal tide; estuaries with double highs or strong shallow-water distortion need real curves.
- ✓Switch units, tweak any input and watch every result update live
Frequently asked questions
How do I use the rule for a real clearance decision?+
Anchor the endpoints from the table, count hours from the nearest LOW (or high, running the rule backwards), sum the twelfths: 2.5 hours after low with a 12-ft range = 1+2+(half of 3) = 4.5 twelfths = 4.5 ft above low water. Add the charted depth at your spot and compare against draft plus safety margin — the full chain our keel-clearance tool automates.
Why do hours three and four matter so much?+
The sine curve is steepest at its middle: 6/12 of the range — half of ALL the water movement — passes in those two hours, with rise rates triple the first hour's. Operationally this is when currents run hardest, when the drying bank floods fastest under the picnic, and when a grounded boat refloats (going up) or pins hard (going down). The rule's rhythm makes the danger window memorable.
Where does the rule break down?+
Wherever the tide isn't a clean 6.2-hour semidiurnal sine: the Solent's double highs, shallow estuaries that flood fast and ebb long, diurnal regimes (Gulf of Mexico's once-a-day tides stretch the clock), and wind/pressure surges the table never knew. The rule is the melody; real harbors add harmonics — official tidal curves for the port handle the exceptions.
Is there a matching rule for tidal current speed?+
The rule of thirds: current builds 50%, 90%, 100% of maximum by the end of hours one, two, three, mirroring back down — same sine logic applied to flow. Pair them: maximum current rides the mid-tide hours where the twelfths stack fastest. Passage timing through tidal gates is essentially scheduling around both rules at once.
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