How to Read a Tide Chart: Highs, Lows and Minus Tides
To read a tide chart, find the highs and lows for the day, note the time of each, and read the height as feet above or below zero, where zero is mean lower low water (MLLW). The curve between those points shows how fast the water rises and falls in between.
That is most of it. The rest is knowing why the Marin coast gets two highs and two lows that never match, and which of them matters for what you plan to do.
What the Heights on a Tide Chart Mean
Every height on a NOAA tide chart is measured from a zero line called a datum. In the United States that line is mean lower low water, the average height of the lower of the two daily low tides over a 19-year period.
So a high of +6.0 feet means the water is predicted to stand six feet above that average low. A low of +2.5 feet means the water will still be fairly high at its lowest point. A low of -1.0 foot means it will drop a foot below the usual low line, which uncovers reef and sand that are normally under water.
Heights are predictions from the pull of the moon and sun. They do not include wind, storm surge or big river flows, so the real water level can run higher or lower, sometimes by a foot or more in a storm.
Highs and Lows on the Marin Coast
The Marin coast has what NOAA calls mixed semidiurnal tides. That means two high tides and two low tides in about 24 hours and 50 minutes, and the two highs and two lows are different sizes. Most tide charts name all four.
| Name | What it means |
|---|---|
| Higher high water | The bigger of the two daily highs |
| Lower high water | The smaller high |
| Higher low water | The shallower of the two lows, sometimes only a little below the lower high |
| Lower low water | The deeper low, the one that matters for tidepools |
The gap can be large. On one mid-October day in 2026, NOAA predicted a morning high of about 4.1 feet at Point Reyes, a low of 3.4 feet, an afternoon high of 5.4 feet and a night low of 0.4 feet. The morning swing was under a foot. The evening swing was five feet.
That is why a chart line on the coast often looks lopsided, with a small bump and a big wave each day. When you plan around the tide, look for the low you actually need, not just the next low.
Timing: Your Spot Versus the Reference Station
Tides arrive at different times along the coast and inside the bays. NOAA predicts many small stations from a nearby reference station by adding a time and height correction. Use the station nearest where you will be.
The differences are real in Marin. On average, NOAA predicts the tide at Sausalito about 50 minutes later than at Point Reyes, and at Inverness on Tomales Bay about an hour and a half later. For a tidepool trip or a kayak launch, that much can decide whether you make it.
Check the station for your beach: Point Reyes, Inverness, Sausalito or Stinson Beach.
The Rule of Twelfths (and Its Limits)
The rule of twelfths is a quick way to guess the water level between a high and a low. It assumes the tide takes about six hours to change and moves this share of its total range each hour:
- First hour: 1/12
- Second hour: 2/12
- Third hour: 3/12
- Fourth hour: 3/12
- Fifth hour: 2/12
- Sixth hour: 1/12
The takeaway is that the water moves slowly near high and low, and fastest in the middle of the change. Half of the change happens in the third and fourth hours.
Treat it as rough. The rule assumes a smooth, even six-hour tide, and Marin’s mixed tides are rarely even. When the higher low and lower high are close together, the “six hours” can be much shorter and the curve flatter. A chart with the full predicted curve beats the rule every time.
Using a Tide Chart to Plan a Tidepool Trip
Start with the lower low water. For tidepools you want a minus tide, a low below 0.0 feet, and the lower it goes the more reef it uncovers. The National Park Service suggests arriving at least an hour before the low and leaving no later than an hour after the tide starts to rise.
Then check the time against daylight. In Marin, daytime minus tides come in the early morning in late spring and summer and in the afternoon in late fall and winter. A deep low at 4 a.m. is no help on a reef.
Last, look at the rising tide. If your spot can cut you off at high water, like Sculptured Beach, watch how fast the water climbs after the low. The tide pools guide covers the best spots, and the minus tides guide explains why some lows drop so far.
Read a Tide Chart Questions
What do the numbers on a tide chart mean?
Each number is a predicted water height in feet above or below a zero line called mean lower low water, or MLLW. Zero is the average height of the lowest low tide each day. A 6-foot high is six feet above that line, and a -1.0 low is a foot below it.
Why are there two high tides and two low tides a day?
The moon's pull raises a bulge of water on the side of Earth facing it, and a second bulge forms on the far side. As Earth turns, most coasts pass through both bulges and both low spots between them every lunar day of about 24 hours and 50 minutes, which gives two highs and two lows.
Why are the two high tides different heights in California?
California has mixed semidiurnal tides, meaning the two daily highs and two daily lows are different sizes. That happens because the moon sits north or south of the equator for much of its orbit, so the two bulges hit the coast unevenly. Charts label them higher high, lower high, higher low and lower low.
What is the rule of twelfths?
The rule of twelfths is a rough way to estimate the tide between a high and a low. It splits the roughly six-hour change into six parts and says the water moves 1, 2, 3, 3, 2 and 1 twelfths of its total range in each hour. It is only an approximation, and it fits Marin's uneven tides poorly.