Tide Height Calculator
The middle two hours carry half the range.
The middle two hours carry half the range.
Height of tide now
3 m
4 m range · 3 twelfths risen · rising 1.05 m/h
The rule of twelfths splits the range 1-2-3-3-2-1 across the six hours from low to high. The middle two hours carry half the range between them, which is why the tide seems to sit still at the turns and race in the middle. It is an approximation to a sine curve, and it holds reasonably on an open coast. In estuaries, near tidal gates, and anywhere with a double high water or a long stand it fails — and those are precisely the places where being wrong puts you on the ground. Charted depth is measured from chart datum, which is roughly the lowest astronomical tide. The height of tide adds to it, so total water is charted depth plus tide, and clearance is that minus your draught.
How the Tide Height Calculator works
Height of tide by the rule of twelfths, with the rate of rise, clearance under the keel and when a chosen height is reached. The rule fails in estuaries, and the page says where.
Also known as: rule of twelfths calculator · height of tide at 3 hours · will i have enough water to cross · under keel clearance calculation
Where to go next
The Tide Height question rarely arrives on its own. These are the ones that usually come with it:
- Anchor Scope Calculator — Not the sounder reading. Add the tide and the bow.
- Dead Reckoning Calculator — Departure is not longitude until you divide by cos.
- Percentage Calculator — Every common percentage question in one place.
- Age Calculator — Exact age in years, months, days, and total units.
Frequently asked questions
What is the rule of twelfths?
The tide rises 1, 2, 3, 3, 2, 1 twelfths of its range in the six hours from low to high. The middle two hours carry half the range between them.
How accurate is the rule of twelfths?
Reasonable on an open coast with a roughly sinusoidal tide. Poor in estuaries, near tidal gates, and anywhere with a double high water or a long stand — precisely where getting it wrong grounds you.
How do I work out clearance under the keel?
Charted depth plus height of tide, minus your draught. Charted depth is measured from chart datum, roughly the lowest astronomical tide, so the tide almost always adds to it.
How fast does the tide rise?
Fastest at mid-tide and effectively still at the turns. On a 4 m range the middle hours run at over a metre an hour, which is why a boat that took the ground comes off quickly or not for hours.
What is chart datum?
The reference level charted depths are measured from, set at approximately the lowest astronomical tide. Negative tides below it are possible but rare.
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