The Rule of Twelfths

Since the average interval between high and low tide is just over six hours, we can divide the cycle into six segments of one hour each. On average the tide rises or falls approximately according to the following:

1^st hour - ¹⁄₁₂
2^nd hour - ²⁄₁₂
3^rd hour - ³⁄₁₂
4^th hour - ³⁄₁₂
5^th hour - ²⁄₁₂
6^th hour - ¹⁄₁₂

Vineyard Haven Harbor, for example, has an average tide of 19 inches (1.6 feet.) 1 hour after low water, the tide will have risen approximately 1.6 inches (¹⁄₁₂ × 19 inches;) 2 hours after low tide, it will have increased an additional 3.2 inches (²⁄₁₂ × 19 inches,) for a total increase of 4.8 inches from low tide; and so on.