Tide Rule of Twelfths Calculator
The Rule of Twelfths is a practical approximation used in coastal navigation to estimate the height of the tide at any point during the 6-hour tidal cycle between high and low water. The tide does not rise or fall at a uniform rate; it accelerates through the middle hours and slows at the extremes. Enter the heights of high water and low water and the number of complete hours elapsed since high (or low) water. The calculator applies the 1:2:3:3:2:1 rule to interpolate the approximate tidal height. Always verify against official NOAA tide predictions before entering shoal water.
Rule of Twelfths formula
Range = High water - Low water
Twelfth = Range / 12
Rise after hour 1: 1/12 of range
Rise after hour 2: 3/12 of range (cumulative)
Rise after hour 3: 6/12 of range (cumulative)
Rise after hour 4: 9/12 of range (cumulative)
Rise after hour 5: 11/12 of range (cumulative)
Rise after hour 6: 12/12 of range (full high water)
Height = Low water + cumulative rise at elapsed hours
The cumulative fractions are 1, 3, 6, 9, 11, 12 twelfths for hours 1 through 6.
Tide height in practice
- The rule assumes a 6-hour tidal period. Actual tidal periods vary; check NOAA tide tables for your port.
- Always use the rule as an approximation only. Safety margins are essential when navigating shoal water.
- Height of tide is the vertical distance from chart datum. Add it to the charted depth to get total water depth.
- Official NOAA tide predictions are available at tidesandcurrents.noaa.gov for hundreds of US locations.
Tide Rule of Twelfths: frequently asked questions
What is the Rule of Twelfths?
The Rule of Twelfths is a simple approximation for estimating the height of the tide at any hour during a 6-hour tidal period. The tide rises or falls by 1/12 of the range in the first hour, 2/12 in the second, 3/12 in the third, 3/12 in the fourth, 2/12 in the fifth, and 1/12 in the sixth hour.
How accurate is the Rule of Twelfths?
The Rule of Twelfths is a simplified approximation that works well for semi-diurnal tidal regimes with nearly equal highs and lows. For precise navigation, always consult official NOAA tide prediction tables. The rule can be off by up to 20% in some tidal conditions.
What is meant by tidal range?
Tidal range is the difference in height between consecutive high water and low water. A range of 6 feet means the tide rises or falls 6 feet between high and low water. The Rule of Twelfths divides this range into twelfths.
Can I use this for depth-clearance calculations?
The Rule of Twelfths gives an approximate height for depth calculations. Always add the low water depth to the estimated rise to get approximate depth over a shoal or bar. Add a safety margin and cross-check with official NOAA tide tables before entering shoal water.
Does the Rule of Twelfths work for diurnal tides?
The Rule of Twelfths was designed for semi-diurnal (two highs, two lows per day) tidal patterns. In areas with diurnal or mixed tidal patterns, such as the Gulf of Mexico or US West Coast, the rule is less reliable and official tide tables should be used.
Official sources
- NOAA Tides and Currents: tidesandcurrents.noaa.gov.
- National Geospatial-Intelligence Agency: Bowditch American Practical Navigator (Pub. 9).
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.