How to Read a Tide Chart: High Tide, Low Tide & Tidal Range

Tides shove a surprising amount of water around, and if you boat anywhere tidal they shape almost everything: whether there's room under your keel, how hard the current runs, and when you can sneak in and out of a shallow harbor. A tide chart is how you see it coming, and it takes about two minutes to learn.

High and low

Most US coasts get two highs and two lows a day, roughly six hours apart. (If you want the term, that's a semidiurnal tide.) The chart gives the clock time of each and how high the water will be. Between a high and the next low the tide is falling, or ebbing. Between a low and the next high it's rising, or flooding.

What the height means

Heights are in feet, measured from a fixed line called the datum. In the US that datum is Mean Lower Low Water, or MLLW, the average of the lower low tide each day. The depths printed on a nautical chart use the same line, so you can add the tide height to the charted depth to get the real depth right now. A tide of 0.0 ft sits at the datum. A negative tide, say −1.2 ft, drops below it, and those are the ones to watch: the least water you'll ever see there.

Range, springs and neaps

The gap between a high and the next low is the range, and it isn't fixed. It grows and shrinks over a two-week cycle tied to the moon. Around the new and full moon you get spring tides, with the biggest range and the fastest currents. Around the quarter moons you get neaps, with the smallest. Bigger range also means faster water ripping through inlets, which catches a lot of new boaters off guard.

Reading the curve

A good chart draws a smooth curve between the highs and lows. Water doesn't rise at a steady clip; it moves fastest around mid-tide and barely at all near the turns. Sailors have used the Rule of Twelfths for this forever: over the six hours from low to high, the water comes up about a twelfth of the range in the first hour, two twelfths in the second, then three, three, two, and one.

Putting it to work

Before you leave, check when the next high or low lands, how high the water will be then, and whether it's rising or falling while you're out. If your draft plus a little margin is more than the charted depth plus the tide, you touch bottom. Plan around it.

Pathr shows a live tide curve, today's highs and lows, and a week ahead for thousands of US spots. Browse them on our US tide charts, or jump straight to California or Florida.

Frequently asked

What does a negative tide mean?+

It's a low tide that falls below the MLLW datum. A −1.0 ft tide means the water is a foot lower than the '0' reference. Those are the shallowest conditions of the cycle, so it's when you're most likely to find the bottom.

Why are there two high tides a day?+

The moon's pull raises a bulge of water on both the near and far sides of the Earth. As the planet turns, a spot passes through both bulges each day, which works out to roughly two highs and two lows every 24 hours and change.

How accurate are tide predictions?+

The astronomical part is very accurate. But strong wind, low pressure, and storms can push the real water level well above or below the prediction, so treat the numbers as a solid baseline, not a guarantee.