Tidal Height Calculator

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, which is what the rising or falling selector above switches between. The twelfths are symmetric, so a falling tide is simply the same curve read backwards.

Where the rule stops working

It assumes two things, and plenty of places break one or both. The first is that the tide takes six hours to go from low to high. In practice a semi-diurnal cycle runs a little over six and a quarter hours between them, and the flood and the ebb are often different lengths, so an hour counted off a six-hour assumption drifts. The second is that the curve is a clean sinusoid. Somewhere like the Solent, with its double high water, or an estuary with a strong river flow, the real curve is nothing like it, and the tidal curve diagram printed in the almanac for that port is the only thing worth trusting.

Use the twelfths for planning and for a sanity check on the almanac. Do not use it to shave a margin on a falling tide over a hard bottom.

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.