How accurate are these tide times?

Every tide time on this site is computed, not copied from an official table. To find out where that works, we compared our method with official predictions at 61 tide stations run by NOAA (United States, Puerto Rico, Pacific islands) and the Canadian Hydrographic Service, for every high and low tide over 30 days.

Open coasts and islands (semidiurnal or mixed tides, range 0.5 m or more): times within 8 minutes typically, and within 31 minutes nine times in ten. Heights within 3 cm typically, 10 cm nine times in ten. The same held six months ahead.

Where it is weaker: tides that come mainly once a day (Gulf of Mexico, parts of the Caribbean, southwest Australia, Gulf of Thailand) were often 1 to 2 hours off; inside enclosed bays, sounds, estuaries and rivers the model cannot follow the water and times can be 1 to 3 hours off. Places like that carry a warning on their page, and we do not publish places deep inside bays or rivers.

Method

For each place we take a year of hourly sea level from the Open-Meteo marine API (Meteo-France SMOC ocean model via the E.U. Copernicus Marine Service, about 8 km grid, tides included) at the nearest ocean cell, fit 26 tidal constituents by least squares, and predict the curve forward. Highs and lows are the turning points of that curve. One global correction of 33 minutes is applied to all times: the model's turning points come that much early on open coasts everywhere we tested (two independent halves of the stations gave 30 and 34 minutes). Heights are from the mean sea level at the point, not chart datum, so low tides are negative numbers. Official tables and nautical charts usually measure from the lowest tide level instead.

Not for navigation

Use these tables to plan a swim, a surf, a dive or a beach walk. For boating, harbour entrances and anything where a wrong time is dangerous, use the official tide tables of the national hydrographic office and local knowledge. Wind, swell and air pressure move the sea on top of the tide.