How to Read a Tide Chart

A tide chart looks like a simple wave, but every part of it carries a specific meaning: the time scale, the height reference, and the shape of the curve. Here is how to read one without guessing.

Tide chart or tide table?

A tide table lists the predicted high and low waters for each day: a time and a height for every turn of the tide. A tide chart (or tide curve) draws the same information as a line, with time along the bottom and water height up the side. The two always agree, because the curve is built from the same predictions. TideRef shows both: the 7-day table, and a 48-hour curve with high and low markers and a line for the current moment.

The two axes

Time runs left to right. On NOAA predictions the clock is the local time at the station, switching between standard and daylight time as the calendar requires, so a chart for a Pacific station and one for an Atlantic station each use their own local clock. Height runs up the side, usually in feet in the United States or in meters elsewhere. Peaks in the curve are high tides and troughs are low tides. The steep parts between them are the rising (flood) and falling (ebb) tide.

What the zero line means: the datum

Heights on a tide chart are not depths of water. They are measured from a reference level called a datum. NOAA's tide predictions for U.S. waters use MLLW, Mean Lower Low Water: the average height of the lower of the two daily low tides, averaged over a 19-year National Tidal Datum Epoch. Zero on the chart is therefore a fairly low tide, and most low tides sit a little above it.

Two consequences follow. First, a low tide can be negative, meaning lower than that average low. Negative lows are what tidepoolers wait for, because they expose rocks and flats that are normally underwater. Second, because soundings on U.S. nautical charts are also referenced to MLLW, you can estimate the water over a charted spot by adding the predicted tide height to the charted depth, remembering that the prediction is not a substitute for a current local observation.

Reading a table row

The example below uses invented numbers purely to show the layout. Real values depend entirely on the station and the date.

Time	Event	Height (ft, MLLW)
2:10 AM	Low	0.4
8:25 AM	High	5.1
2:40 PM	Low	-0.3
8:55 PM	High	6.0

In this example the tide rises about 4.7 feet from the first low to the first high over roughly six hours, falls 5.4 feet to the second low, then rises 6.3 feet to the evening high. The difference between a high and the next low is the range, and it changes through the month with the Moon (see our guide to spring and neap tides).

Three tide patterns you will see

Semidiurnal: two highs and two lows of similar size each day. This is typical of much of the U.S. East Coast.
Mixed semidiurnal: two highs and two lows, but the two highs (or the two lows) differ noticeably in height. This is common on the U.S. West Coast, where you will see a "higher high" and a "lower high".
Diurnal: a single high and a single low per day, found in parts of the Gulf of Mexico.

On a curve, the pattern is obvious at a glance: count the peaks in a 24-hour span and compare their heights.

The shape of the curve and how fast water moves

Water does not rise at a constant speed. The change is slow near a high or a low, fastest around the midpoint, and slow again as it approaches the next extreme. Mariners use the rule of twelfths as a rough guide for a roughly six-hour rise or fall: about one twelfth of the range in the first hour, two twelfths in the second, three in the third, three in the fourth, two in the fifth and one in the last. It is an approximation, but it explains why the water seems to move so little around the turn and so quickly in the middle. TideRef draws its curve and estimates the current height by smoothly interpolating between the predicted highs and lows with a cosine curve, which has the same slow-fast-slow shape.

What a tide chart does not tell you

Weather. Predictions are astronomical. Strong onshore wind, storm surge or unusually high or low barometric pressure can push the real water level well away from the chart. As a rough rule, a drop of one millibar in pressure raises the sea surface by about a centimeter.
Currents. Tide height and tidal current are different things. In inlets, bays and channels the water can keep flowing in one direction after the height has already turned, so use a dedicated current table for anything involving boats.
Local conditions. River flow, ice, dredging and shifting sandbars all alter what you actually see at a specific beach or dock.

A quick routine for reading any chart

Confirm the station and its location matches the water you care about.
Confirm the date and that the times are local to the station.
Check the datum (MLLW for U.S. predictions) and the unit.
Find the highs and lows nearest your plan and note the range.
Allow a margin for weather, and verify against an official source if safety is involved.

Put it into practice: open the TideRef tide clock, pick a station, and compare what you just read with live NOAA predictions.