🌊 Rule of Twelfths Calculator
Estimate the tide height at any time between High Water and Low Water.
What is this all about?
Whether you are a sailor planning to cross a shallow bar, an angler looking for the best feeding times, or a beachcomber wanting to know when the tide pools are accessible, understanding the tides is crucial. While modern apps provide detailed tidal curves, the Rule of Twelfths remains the most reliable and fastest method to estimate the tide height at any given time between High Water and Low Water.
What is the Rule of Twelfths?
The Rule of Twelfths is a simple mathematical approximation used in navigation and coastal activities to estimate the height of the tide at any intermediate time between High Water (HW) and Low Water (LW).
It assumes that tides follow a smooth, sinusoidal curve and that the time between High and Low water is approximately six hours. Instead of complex trigonometry, it uses a simple sequence of fractions to estimate how much the water level rises or falls each hour.
The Magic Sequence: 1-2-3-3-2-1
The rule states that in the first hour after High (or Low) water, the tide moves by 1/12th of the total range. In the second hour, it moves by 2/12ths, and so on.
The sequence of twelfths over a 6-hour period is:
- Hour 1: 1/12
- Hour 2: 2/12
- Hour 3: 3/12
- Hour 4: 3/12
- Hour 5: 2/12
- Hour 6: 1/12
How to Calculate Tidal Height (Step-by-Step)
To use our Rule of Twelfths Calculator above, you only need three pieces of information from your local tide table:
- The time and height of the nearest High Water (HW).
- The time and height of the nearest Low Water (LW).
- The specific time for which you want to know the water level.
Example Calculation:
Imagine High Water is at 12:00 with a height of 4.5 meters, and Low Water is at 18:00 with a height of 1.5 meters. You want to know the tide height at 15:00.
- Find the Total Range: 4.5m (HW) – 1.5m (LW) = 3.0 meters.
- Find the Elapsed Time: From 12:00 to 15:00 is 3 hours.
- Apply the Rule:
- Hour 1 (12:00-13:00): 1/12 of 3.0m = 0.25m
- Hour 2 (13:00-14:00): 2/12 of 3.0m = 0.50m
- Hour 3 (14:00-15:00): 3/12 of 3.0m = 0.75m
- Add them up: 0.25 + 0.50 + 0.75 = 1.50 meters of fall.
- Final Height: 4.5m (Start) – 1.5m (Fall) = 3.0 meters at 15:00.
(You can verify this exact example using our calculator above!)
When Should You NOT Use the Rule of Twelfths?
While incredibly useful for quick mental math, the Rule of Twelfths is an approximation. Do not rely on it in the following situations:
- Shallow or Restricted Waters: In rivers, estuaries, or very shallow areas, the tide does not follow a smooth sine wave. The water might rise very quickly and fall slowly (or vice versa).
- Double High/Low Waters: Some locations (like Southampton in the UK) experience a “double high” or “double low” water where the tide stands still or dips briefly. The rule fails here.
- Extreme Precision Needed: If you are navigating a large vessel with a deep draft in a tight channel, always use the official Admiralty Tide Tables or a harmonic tidal prediction software.
Frequently Asked Questions (FAQ)
Is the Rule of Twelfths accurate?
It is an approximation, usually accurate within 10-15% for standard semi-diurnal tides (two highs and two lows per day). It is perfect for small boats, kayaking, and general planning.
Does the rule work for rising tides as well as falling tides?
Yes. The sequence (1-2-3-3-2-1) applies exactly the same way. If the tide is rising, you add the fractions to the Low Water height. If it is falling, you subtract them from the High Water height. Our calculator handles both directions automatically.