Tidal height calculator
Estimate tidal height at a given time using the Rule of Twelfths or a sine approximation. For passage planning only — always use official tide tables.
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Worked example
HW = 4.5 m, LW = 0.8 m. What is the tidal height 2 hours after low water?
- Tidal range: 4.5 − 0.8 = 3.7 m
- Rule of Twelfths: in 2 hours the tide rises 1/12 + 2/12 = 3/12 of range
- Rise = 3/12 × 3.7 m = 0.925 m
- Height = 0.8 + 0.925 = 1.73 m
Try 4.5 / 0.8 / 2h 0m / after LW above to verify.
The Rule of Twelfths
The Rule of Twelfths is a simple mental approximation for how quickly the tide rises or falls in each of the six hours of a half-tide cycle:
| Hour | Rise / fall | Cumulative |
|---|---|---|
| 1st hour | 1/12 of range | 1/12 |
| 2nd hour | 2/12 of range | 3/12 |
| 3rd hour | 3/12 of range | 6/12 |
| 4th hour | 3/12 of range | 9/12 |
| 5th hour | 2/12 of range | 11/12 |
| 6th hour | 1/12 of range | 12/12 |
The tide moves fastest in the 3rd and 4th hours — together accounting for half the total range.
Frequently asked questions
How accurate is the Rule of Twelfths?
It’s a useful approximation, not a precise calculation. It assumes a regular semi-diurnal tide with equal rise and fall times, which is reasonably accurate in UK coastal waters. For critical decisions — entering a harbour over a bar, anchoring overnight — always use official UKHO tide tables or a certified tide prediction app.
Where do I find official HW and LW times and heights?
The UK Hydrographic Office publishes tide tables for all standard ports. Free predictions are available via the UKHO EasyTide service, the Met Office, and apps such as Reeds Nautical Almanac. Your chart plotter may also include tide predictions.
What is the difference between Chart Datum and tidal height?
Chart Datum (CD) is the reference level used on Admiralty charts — approximately the Lowest Astronomical Tide (LAT). Depths shown on charts are below CD. Tidal heights are measured above CD. The actual depth of water = charted depth + tidal height.
What’s the difference between this calculator and a tide table?
Tide tables give measured or predicted heights at specific times from observation. This calculator interpolates between two known points (HW and LW) using a mathematical model. It’s accurate enough for passage planning but should not be used for precise navigation near hazards.