How to read a tide chart

A tide chart shows water height over time as a smooth curve. The peaks are high tides, the troughs are low tides, and the height is measured against a fixed reference called a datum. Read the curve left to right across the day, and you can see when the water will be high, when it will be low, and how fast it is moving in between.

The curve

The wave shape of the curve is the tide. On most coasts you see two highs and two lows a day, a pattern called semidiurnal. Some places, like much of the Gulf of Mexico, see one high and one low a day, a diurnal pattern. The steepest parts of the curve are usually when the current runs hardest, and the flat parts at the top and bottom are usually the turn. How closely the current actually tracks the height depends on where you are, and at some locations the two are hours apart: see what slack water is and when it actually happens.

Height and the datum

Tide height is given against a datum, the zero line for measurements. In the United States the common datum is Mean Lower Low Water, or MLLW, the average of the lower low tides. A height of 3.2 ft means the water is 3.2 feet above that line. Heights can briefly go below zero on a strong low, which is normal and just means the water dropped below the average low.

High and low times

The numbers next to each peak and trough are the time and height of that high or low. Plan around them: a clammer wants the low, a boater clearing a shallow bar wants the high, and an angler often wants the moving water on either side of the turn. Times are always local to the station, so check the time zone if the spot is far from home.

Reading it fast

1. Find where you are in the day on the horizontal axis.
2. Look at the curve. Rising means the tide is coming in (flooding), falling means it is going out (ebbing).
3. Find the next peak or trough to see when the tide turns and how high or low it will be.

Slackwater draws this curve for your spot and marks the next turn, computed on your device from NOAA harmonic data. See tide times by location, or read how long slack water lasts at your spot for why the turn matters.