How to read a tide chart

Two axes and a zero that is not the seabed

Every curve on this site plots two quantities. Along the bottom runs local clock time at the station, midnight to midnight, in the 12-hour format with daylight saving time applied wherever the state observes it. Up the left side runs water height in feet. The line marked 0 ft is neither sea level nor the bottom: it is Mean Lower Low Water, the long-term average of the lower of each day's two lows at that gauge, computed by NOAA over its 19-year National Tidal Datum Epoch. US nautical charts measure their soundings from the same zero, which is why a chart depth and a predicted height can simply be added; the tide table guide walks through that sum.

A reading of 4 ft therefore means the surface stands four feet above that average low, at that gauge, at that minute. It tells you nothing about how deep the water is where you happen to be standing; that depends on the ground underneath. At Charleston an ordinary high reaches about 5.4 ft and an ordinary low settles near 0.3 ft, so on most days the line wanders between those two marks.

The vertical scale fits the station being shown. A curve that fills the whole panel at Pensacola Beach, where the average day spans 1.4 ft, would be a ripple next to Anchorage, where it spans 28.1 ft. Glance at the numbers on the axis before comparing two pictures side by side.

Peaks, troughs and the labels that matter

The labeled points carry the real information. Each peak is a predicted high water and each trough a predicted low, both with NOAA's time to the minute and height in feet. Those values are published by the National Ocean Service and shown unchanged. The line joining them is drawn by this site: from NOAA's hourly predictions at reference stations, and from the shape of the reference curve at subordinate stations, as the method page sets out. When an exact figure matters, read the label instead of estimating from the line.

The rhythm of those peaks also reveals which family of tide a place belongs to, and each family is read a little differently.

Matching twins

Charleston shows the usual Atlantic pattern, two highs and two lows of similar size each day. The morning and afternoon tides look like copies of each other, so either low will uncover roughly the same strip of mud along the harbor, and planning comes down to which one suits your schedule.

Unequal pairs

Seattle draws something lopsided. Its two highs differ by 1.8 ft on an average day, and its two lows by 6.3 ft. One low barely sags while the other drains the shoreline, which is why West Coast tables name a higher high and a lower low. On a chart like this, pick the low by its label, never by its position in the day.

A single slow wave

Along much of the northern Gulf the picture flattens into one rise and one fall. Pensacola Beach has a single high on 86% of days. The water creeps for many hours in the same direction, and a stiff north or south wind can move it as much as the moon does.

The slope is the speed

Look at how steep the line is, not only where it peaks. Near each turn the curve rounds off into a plateau; for a short while on either side of a high or low the height hardly changes, a pause often called the stand of the tide. Halfway between a low and the next high the line is at its steepest, and that is when water climbs a beach or a piling fastest.

For a semidiurnal tide the middle hours of a rise move roughly a quarter of the range each, the peak of the rule of twelfths covered in the fishing tides guide. At Bar Harbor, with an average range of 11.1 ft, that comes to about 2.8 ft in a single hour; at Charleston, about 1.4 ft. A flat bar that is dry at 10 AM can be knee deep by 11. The mini tool above reads the height at any hour you choose so you can test that against your own plan.

Light, darkness and the moon on the same picture

The gray shading on the chart covers the hours from sunset to sunrise. It answers a question people often forget to ask: does that big low happen while you can actually see the rocks? Many of the deepest lows of a season fall in darkness at one coast and in daylight at another, a pattern explored in the negative low tides guide.

Below the time axis, green bars mark the solunar periods, two longer ones around the moon's passage overhead and underfoot and two shorter ones around moonrise and moonset. They are a fishing tradition, explained with its limits in the best fishing times guide, and you can ignore them entirely if you came for the beach.

Reading a week instead of a day

Stack seven days and two patterns appear. First, each tide arrives later than the one the day before, by roughly 50 minutes on a twice-daily coast, so a low you liked at 9 AM on Saturday drifts toward lunch by Monday. Second, the envelope of the curve breathes: highs climb and lows sink for a few days around each new and full moon, then the swing shrinks near the quarter moons. The coefficient printed for each day puts a number on that breathing, with 100 meaning an ordinary day at that station; the spring and neap tides guide shows where it comes from, and the king tides page covers the far end of it.

Four ways people misread a tide chart

The wrong gauge. A station inside a sound and one on the ocean beach a few miles away can differ by an hour or more and by a large share of the height. Open the page of the station closest to where you will be, on the same side of the inlet.

Zero taken for dry. A 0 ft low is simply an average lower low. Plenty of lows fall below it, and on a gently sloping flat the water at 0 ft may still be a long walk out.

Weather left out. The curve is the astronomical tide. Onshore wind, low pressure, swell and river flow push real water above or below it, sometimes by a foot or more, and nothing on the chart warns you.

The wrong clock. Times are those of the station, not of your phone's home zone. Someone in Denver planning a morning at a California tide pool reads Pacific Time on that page.

None of these charts is meant for navigation. They are a planning picture for the shore, the dock and the ramp; for anything afloat in tight water, use official charts and NOAA's observed water levels.