Tide Time Calculator
Estimating the Tide Between Official Tables
Most coastlines experience a semi-diurnal tide — roughly two high tides and two low tides every 24 hours and 50 minutes — which makes the next tide reasonably predictable from the last one. This calculator applies that regular pattern two ways: projecting the next tide's timing, or estimating how high the water sits right now using the centuries-old navigator's shortcut known as the Rule of Twelfths.
The Formulas
Rule of Twelfths (height change per 1/6 of the tidal period, cumulative):
1/12, 3/12, 6/12, 9/12, 11/12, 12/12 of the total tidal range
The default tidal period, 12.4167 hours, is the standard approximation for the semi-diurnal (M2) lunar tide cycle.
Worked Examples
| Mode | Inputs | Result |
|---|---|---|
| Next Tide Time | Last high tide at 6:30, period 12.4167 hr | Next low tide at 18:55; following high tide at 07:20 |
| Current Tide Height | Last high 6:30, now 9:30, range 2.0–8.0 (units of your choice) | ≈7.05, using 1.90 of 12 twelfths elapsed |
The Rule of Twelfths, Interval by Interval
| Interval | Change this interval | Cumulative change |
|---|---|---|
| 1st sixth | 1/12 | 1/12 |
| 2nd sixth | 2/12 | 3/12 |
| 3rd sixth | 3/12 | 6/12 |
| 4th sixth | 3/12 | 9/12 |
| 5th sixth | 2/12 | 11/12 |
| 6th sixth | 1/12 | 12/12 |
The pattern (1, 2, 3, 3, 2, 1 twelfths) reflects how tidal flow accelerates through the middle of the cycle and eases near each high and low — the water rises or falls fastest at mid-tide, not at the turn.
How to Use This Calculator
- Choose the mode: Next Tide Time or Current Tide Height.
- For Next Tide Time: enter the Last Tide Time (24-hour decimal, e.g. 6.5 for 6:30), whether it was high or low, and the Tidal Period (defaults to 12.4167 hours).
- For Current Tide Height: also enter the Current Time, Low Tide Height, and High Tide Height.
- Select Calculate to see the projected tide time or estimated current height.
Related Calculations
Plan a passage around the tide with the Marine Distance Calculator, or check anchoring depth needs with the Anchor Chain Length Calculator.