Frequently asked questions

How are tide times calculated?

The UK Hydrographic Office (UKHO) designates all UK ports as either a “standard port” or a “secondary port”. For example, Plymouth is considered a standard port and Mevagissey is considered a secondary port.

Tide time predictions for standard ports are based on detailed tide tables published by the UKHO. These tables show the exact time of each tide for each day of the year.

Tide time predictions for secondary ports are calculated by applying an adjustment to the tide times for the relevant standard port. For example, the high tide at a secondary port might occur +10 minutes after the high tide at the relevant standard port.

The UKHO provides two possible high tide adjustments for each secondary port. For example, a high tide that occurs at 13.00 might need an adjustment of +35 minutes while a high tide that occurs at 19.00 might need an adjustment of +45 minutes.

Advanced navigators in complex situations use interpolation to calculate the exact adjustment for any given hour of the day. For example, a high tide that occurs at 15.38 might need an adjustment of +39 minutes.

However, most people don't need to use interpolation to calculate an exact adjustment. Instead, they can use a simple average of the two possible adjustments provided by the UKHO. For example, an average adjustment of +40 minutes could be calculated from the two possible adjustments of +35 minutes and +45 minutes.

This website uses the average adjustment method to calculate tide times for secondary ports. For example, tide times for Mevagissey are based on Plymouth with a -18 minute average adjustment for high water.

This average adjustment method will result in small variations between the estimated and actual tide times, mainly due to the cycle of spring and neap tides. However, it's also important to remember that all tide predictions will be affected by local weather conditions. For example, high temperatures and/or strong winds can significantly change the times and heights of tides.

If you need extremely precise calculations for tide times, we recommend that you use the 7-day forecast available from the UKHO.

How are tide heights calculated?

The UKHO uses “Chart Datum” as a baseline when providing predictions for tide heights. Chart Datum is the plane to which all tidal heights in the UK are referred. It is also the plane below which all depths are published on a navigational chart, so adding the tidal height to the charted depth gives the true depth of water.

In the United Kingdom, Chart Datum is normally approximately the level of Lowest Astronomical Tide (LAT). This is the lowest level which can be expected under average weather conditions. However, it is not an absolute minimum and storm surges can cause significantly lower levels to occur.

Tide height predictions for standard ports are provided in the detailed tide tables published by the UKHO. These tables show the height above Chart Datum of each tide for each day of the year.

Tide height predictions for secondary ports can be calculated by applying an average adjustment to the tide height for the relevant standard port. For example, the high tide at a secondary port might be +0.2 metres higher on average than the high tide at the relevant standard port.

It's important to remember that tide predictions are always given in metres above Chart Datum. The actual depth of water will depend on the charted depth shown on navigational charts.

It's also important to remember that all tide predictions will be affected by local weather conditions. For example, high temperatures and/or strong winds can significantly change tide heights and times.

Why do your tide times and heights differ from other tide table books or websites?

Most tide table books use the same average adjustment method as above.

Most tide table websites are strangely unclear about exactly how they calculate tide times…

If you have any doubts about the accuracy of tide table books or websites, we recommend that you compare them with the 7-day forecast available from the UKHO.

Where can I learn more about tides?

We recommend Tides: The Science and Spirit of the Ocean by Jonathan White.