How to read a tide chart

A tide chart shows how high the water will be at a spot through the day. Here is what each part means and how to use it.

The curve

The wavy line is the predicted water height. Peaks are high tides and dips are low tides. The time between a high and the next low is usually a little over six hours, so on most of the US coast you get two of each a day. The steepest part of the curve, halfway between a high and a low, is when the water rises or falls fastest.

The numbers

Heights are measured in feet from a zero line called mean lower low water (MLLW): the average of the lowest tide of each day over many years. It's the same zero used on most US nautical charts, so a charted depth plus the tide height is roughly the water you'll have. A minus tide (for example -1.2 ft) means the water drops below that average low, uncovering more beach, rocks and flats than usual.

Why the two highs differ

On the Pacific coast, in Alaska and in Hawaii, one high tide each day is usually much higher than the other. That's a mixed tide. Most of the Atlantic coast has two highs of similar height (semidiurnal), and parts of the Gulf of Mexico get just one high and one low a day (diurnal). Each station page here says which kind it has.

Spring and neap tides

Around the new and full moon the sun and moon pull in line, so highs are higher and lows are lower. Those are spring tides (nothing to do with the season). Around the quarter moons the range shrinks: neap tides. Station pages list the days with the biggest swings in the month ahead.

Predictions vs. what you'll see

These are NOAA's astronomical predictions. Strong onshore wind, low air pressure or heavy river flow can raise the water above the prediction, and offshore wind can drop it. For boating, check current conditions and official notices too. This site is not for navigation.

Subordinate and reference stations

NOAA measures water levels for years at reference stations and predicts their tides directly. Thousands of other spots are subordinate stations: their times and heights are worked out by adjusting the nearest reference station. They're good for planning but can be off by a few minutes or a few tenths of a foot.

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