The Rule of Twelfths provides an approximation of what?

• A. Exactly measure water level hourly
• B. Determine wind strength
• C. Approximate tide height change by hour
• D. Predict current speed

Answer: (C)

The Rule of Twelfths is a simple way to estimate how the water level will change from hour to hour during a rising or falling tide. It uses the total tidal range (the difference between high and low tide) and splits it across six hours in roughly 1/12, 2/12, 3/12, 3/12, 2/12, 1/12 portions. In the first hour after low tide the water rises about one twelfth of the total range, the second hour about two twelfths, the third and fourth about three twelfths each, then two twelfths, then one twelfth in the sixth hour. For example, with a 6-foot tidal range, the estimated changes would be about 0.5 ft, 1 ft, 1.5 ft, 1.5 ft, 1 ft, and 0.5 ft across the six hours. This helps gauge depths and plan passages, especially in shallow areas.

It’s not a precise measurement tool for exact water levels, and it doesn’t address wind, currents, or current speed.