Tidal Height Calculator
Estimate the height of tide at any time between high and low water using the 1/12 rule.
Find out if there is enough depth to enter a harbour or anchorage.
Why tidal height matters
Every port and anchorage has a chart datum — the lowest level to which tides are predicted to fall. All depths on nautical charts are measured from chart datum. The actual depth at any time = charted depth + current tidal height above datum.
If the chart shows 1.5 m of water and the tide is currently 2.0 m above datum, you have 3.5 m of water available. If your vessel draws 1.8 m plus a safety margin of 0.5 m, you need at least 2.3 m — so this berth is safe.
The 1/12 rule
Between low water and high water, the tide rises in a predictable sinusoidal curve that can be approximated with the 1/12 rule:
- Hour 1 (from LW): tide rises 1/12 of the total range
- Hour 2: rises a further 2/12 (cumulative 3/12)
- Hour 3: rises a further 3/12 (cumulative 6/12 — half way)
- Hour 4: rises a further 3/12 (cumulative 9/12)
- Hour 5: rises a further 2/12 (cumulative 11/12)
- Hour 6: rises a further 1/12 (reaches HW)
The same rule applies in reverse from HW to the next LW.
Charted depth vs actual depth
This calculator gives tidal height above chart datum. To get actual depth: add the tidal height to the chart depth. To check clearance: subtract your vessel’s draft (plus safety margin) from actual depth. Always allow at least 0.5 m safety margin — local conditions, wave action, and inaccuracies in the 1/12 rule all warrant a buffer.