Tide Height Rule Of Twelfths
Calculator
Results
- Tide height (m)
- 2.15
- Tide risen since low water (m)
- 1.35
- Tide range (m)
- 3.6
Aviation and marine results
| Tide height (m) | 2.15 |
| Tide risen since low water (m) | 1.35 |
| Tide range (m) | 3.6 |
formula-map diagram
- Tide height (m)
- 2.15
- Tide risen since low water (m)
- 1.35
- Tide range (m)
- 3.6
Aviation and marine relationship
Formula
Rule of twelfths: 1/12, 2/12, 3/12, 3/12, 2/12, 1/12 of the range per hour= 2.15
Note
This is a simplified model: it applies a standard navigation, performance or seamanship formula to the numbers you entered. True airspeed and density altitude use the ISA troposphere model for dry air and ignore humidity, compressibility and instrument or position error, so they are not a substitute for your aircraft's flight manual or an air data computer. Wind, climb, descent and fuel figures assume a steady wind and a constant speed and fuel flow for the whole leg; they ignore manoeuvring, holding, taxi and start-up fuel, terrain, turbulence and air traffic control routing. Takeoff distance scales an entered sea level figure by a rule-of-thumb percentage per 1000 ft of density altitude and is not a certified performance chart. Weight and balance results depend entirely on the arms and weights you enter and must be checked against the approved loading envelope. Hull speed is the classic 1.34 × √LWL displacement estimate, boat fuel burn is a horsepower-based approximation, anchor scope is a rule of thumb, and the rule of twelfths is a linear approximation of a sinusoidal tide that does not hold in all locations. Always use official charts, tide tables, the aircraft or vessel manuals and current weather, and treat these numbers as planning estimates only.
More in Aviation and marine
See all →Frequently asked questions
What is the rule of twelfths used for?+
It's a quick mental (or calculated) method to estimate the height of the tide at any point between low and high tide, without needing a full tide curve. It's useful for judging water depth at a specific time, for example to know when a shallow channel becomes passable.
How does the rule of twelfths distribute the tidal range across six hours?+
It assumes the roughly 6-hour period between low and high tide is divided into six equal time intervals, with the tide rising (or falling) by 1/12, 2/12, 3/12, 3/12, 2/12, and 1/12 of the total range in each successive hour — reflecting the fact that tide moves fastest in the middle of the cycle and slowest near the turn.
Why does the tide rise more slowly near high and low water than in the middle of the cycle?+
Tidal height follows roughly a sine-wave pattern, and the rate of change of a sine curve is smallest at its peaks and troughs and greatest at the midpoint. This is why the first and last hours only account for 1/12 of the range each, while the middle two hours account for 3/12 each.
What's a common misconception about the rule of twelfths?+
That it applies precisely to every location and every tide cycle. It's an approximation based on an idealized semi-diurnal (twice-daily) tide with a roughly 6-hour rise/fall; actual tides can be affected by local geography, weather, and diurnal patterns, so official tide tables are more accurate for critical navigation decisions.
What inputs does this calculator need to estimate tide height at a given time?+
It typically needs the times and heights of the preceding low tide and following high tide (or vice versa), plus the specific time you want to check. It then applies the twelfths distribution to the elapsed time within that roughly 6-hour window.