What is the Rule of Twelfths?

The Rule of Twelfths is a quick approximation used by mariners to estimate tidal height between high and low water. It assumes the tide follows a roughly sinusoidal curve and divides the six-hour tidal cycle into six one-hour segments. The tidal range changes by approximately 1/12, 2/12, 3/12, 3/12, 2/12, and 1/12 of the total range in successive hours.

How accurate is the Rule of Twelfths?

It is a good first approximation for semi-diurnal tides (two highs and two lows per day) in areas without strong meteorological or atmospheric surge effects. For precise navigation, always refer to official tide tables from your national hydrographic office. Add an appropriate under-keel clearance safety margin.

Does this work for diurnal tides (one high/low per day)?

The rule is designed for semi-diurnal tidal cycles of roughly six hours. For diurnal tides (approximately twelve hours per half-cycle), the proportions are less reliable. Use official tables for diurnal tide regions.

Chart datum (CD) is the baseline reference level used on nautical charts, typically set at or near the Lowest Astronomical Tide (LAT). All tide heights are measured above this level. Depths on your chart are depths at chart datum; add the current tide height to get the actual water depth.

What does "tidal state" mean in the output?

"Flooding" means the tide is rising toward high water. "Ebbing" means the tide is falling toward low water. "Slack" (shown near high or low water) means the tidal current is at or near its turning point with minimal flow.

Tide Height Estimator

Enter high and low tide data to estimate water depth at any point in the tidal cycle using the Rule of Twelfths.

Tide Parameters

Reference Tide Type

Start from High Tide Start from Low Tide

High Tide Time

HH:MM

Height above CD

Low Tide Time

HH:MM

Height above CD

Estimated Time

Time to estimate

Result

Enter tide data and press Estimate

Summary

This tool applies the Rule of Twelfths, a standard mariner's approximation, to estimate water depth at any moment within a tidal cycle. The rule divides the tidal range into twelfths and models how the tide rises or falls over each hour of the six-hour cycle. It is accurate enough for passage planning and anchor clearance checks in areas with semi-diurnal tides.

How it works

Enter the time and height of the preceding high tide.
Enter the time and height of the following low tide (or vice versa).
Enter the time at which you want to know the estimated water depth.
The tool calculates elapsed time, determines which twelfth-hour segment applies, and sums the proportional rise or fall.
The result shows estimated height above chart datum and the current tidal state (flooding, ebbing, or slack).

Use cases

Check minimum depth under keel before navigating a shallow bar or channel.
Plan the best window to cross a tidal estuary at safe water depth.
Estimate anchor rode scope by knowing the maximum tide height overnight.
Teach trainees the Rule of Twelfths with an interactive worked example.
Verify whether a boat will remain afloat at low tide when left on a drying mooring.
Cross-check tide table predictions against a quick mental arithmetic method.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu