Tides / Guides / How tides work
How tides work
Updated October 15, 2026.
A tide is the ocean responding to gravity. The moon pulls on the water, and the water on the near side of the earth is drawn toward the moon. A second bulge forms on the far side, where the earth itself is pulled toward the moon a little more than the water left behind. As the earth turns, a place on the coast passes through both bulges. That is why so many harbors see two high tides and two low tides in a day.
The sun does the same thing, with less force, because it is much farther away. When the sun and the moon line up, the two pulls add together and the range from low to high gets larger. When they are at a right angle, the pulls partly cancel and the range gets smaller. Those are the spring tides and the neap tides. The names have nothing to do with the season.
Why the clock is not exactly 12 hours
The moon takes about 24 hours and 50 minutes to return to the same place in the sky. The next high tide is therefore about 12 hours and 25 minutes after the one before it, not 12 hours on the nose. Over a week the highs walk later through the day. A dawn high this Sunday can be a mid-morning high next Sunday.
The shape of the basin changes the schedule again. A wide continental shelf, a bay, or a narrow inlet can delay the wave and change how high it climbs. That is why two towns on the same coast, a few miles apart, do not share a tide table. Each chart on this site is one station's prediction, not a statewide clock.
One high a day, or two highs of different size
Most of the Atlantic and Pacific coasts of the United States are semidiurnal: two highs and two lows. Along parts of the Gulf of Mexico the pattern is diurnal, closer to one high and one low in a day. The Pacific is often mixed. The two highs on the same day are not the same height. The difference is the diurnal inequality. The page for a station says which pattern NOAA assigns it, and the week's table shows the two highs side by side when both exist.
What the number on the chart means
A predicted height is not the depth under the boat. In the United States the zero is mean lower low water, the average of the lower of the two lows over a tidal epoch. A prediction of 0.2 feet is a low tide near that average zero. It can still be plenty of water, or not enough, depending on the bar. In Australia and New Zealand the zero is chart datum, and the heights are in metres.
The prediction is the astronomical tide: moon, sun, and the station's published response to them. Wind, air pressure, river flow, and a storm surge are not inside that number. A northeast wind can hold water in a bay and make the low tide miss the prediction by a foot. The flood gauge, when a page has one, is a different instrument. It measures the water that is there, against flood stage. The tide table does not.
Slack water is the turn
Current does not stop for long just because the height stops rising. Slack is the short pause while the flow reverses. In a river or an inlet it can lag the high or low on the table by an hour or more, because the height at the station turns before the water in the channel has finished moving. Fishermen care about that lag. The table tells you when the height turns. It does not tell you when the water in a creek stops.