The rule of twelfths divides a tidal cycle (approximately 6 hours 12 minutes) into six equal periods. The tide rises or falls by 1/12, 2/12, 3/12, 3/12, 2/12, and 1/12 of its total range each hour. Current speed follows the same proportional pattern.
- Hour 1: 1/12 of range — current at ~33% of maximum speed
- Hour 2: 2/12 of range — current at ~67% of maximum speed
- Hours 3–4: 3/12 of range each — current at 100% maximum speed
- Hour 5: 2/12 of range — current drops to ~67% of maximum
- Hour 6: 1/12 of range — current weakest at ~33% before slack
| Hour | Fraction of Range | Current Speed (% of Max) |
|---|---|---|
| Hour 1 | 1/12 | 33% |
| Hour 2 | 2/12 | 67% |
| Hour 3 | 3/12 | 100% |
| Hour 4 | 3/12 | 100% |
| Hour 5 | 2/12 | 67% |
| Hour 6 | 1/12 | 33% |