How to read a tide chart: high tide, low tide and heights

What’s on a tide chart

In the US, people use “tide chart” for both a tide table (a list of times and heights) and a tide graph (a curve that rises and falls through the day). Either way, the information is the same:

• The time of each high tide and low tide.
• The height of the water at that moment, usually in feet.
• Whether it’s a high or a low, often marked H and L.

Along the East Coast, most days have four rows: two highs and two lows, a little over six hours apart. Some days have only three, because the tide arrives about 50 minutes later each day and one of the four slips past midnight into the next day. That isn’t a mistake. It’s the moon’s rhythm, which we explain in what causes tides.

A tide graph shows the same numbers as a curve. It’s handy for seeing at a glance whether the water is rising fast or slow at the hour you care about.

Times: local time, and watch daylight saving time

The tide times on this site are in local time, with daylight saving time applied when it’s in effect. That’s worth checking on any other source, especially printed tables. NOAA’s prediction pages, for example, let you choose local time with daylight saving (shown as LST/LDT), standard time only, or GMT. Pick the wrong one in summer and every tide is off by an hour.

Also keep in mind that high tide is the moment the water stops rising, but the level barely changes for a while around it. Half an hour before and after high tide, the water is within a few inches of its peak. The same goes for low tide.

Heights: feet above the chart datum

The height on a tide chart is not the depth of the water. It’s how far the water stands above a fixed reference level, the chart datum. In the US, NOAA uses mean lower low water (MLLW): the average of the lowest low tide of each day over a 19-year period. A height of 5.8 ft means the water is 5.8 feet above that average low.

Because MLLW is an average, roughly half of the lowest daily lows fall below it. Those show up with a minus sign and are called minus tides. They cluster around the new and full moon, when tides are biggest. A low of −0.2 ft in Charleston is routine; in Puget Sound, the best summer lows can drop below −3 ft, which is why clam diggers and tide poolers plan around them.

To know how much water you’ll have at a specific spot, add two numbers:

• the depth on the nautical chart at that spot (measured from the chart datum);
• the tide height at that moment.

If the chart shows 4 feet in a channel and the tide is at 3 feet, you’ll have about 7 feet of water. We cover this in more detail in chart datum and MLLW.

The difference between a high tide and the next low (or the previous one) is the range. It tells you more about the day than any other number: a big range means fast-moving water, stronger currents and more exposed flats and bars.

A worked example: a spring tide day in Charleston

Here’s what a tide chart might look like for a day of spring tides in Charleston Harbor (it’s an illustration, not the forecast for a real date):

Tide	Time	Height
Low	12:31 a.m.	0.4 ft
High	6:42 a.m.	5.8 ft
Low	12:55 p.m.	−0.2 ft
High	7:10 p.m.	6.1 ft

What it tells you:

• From the 12:55 p.m. low to the 7:10 p.m. high, the water rises 6.3 feet (6.1 minus −0.2) in a little over six hours. That’s the afternoon range, and it’s on the big side for Charleston, whose average range is a bit over 5 feet.
• The midday low is a minus tide. Expect more exposed oyster rakes and mud flats than usual.
• The two highs aren’t equal (5.8 and 6.1 ft). Small differences are normal. On the West Coast and in the Gulf, the gap can be large, as explained in types of tides.

US tide charts don’t include a tide coefficient, the number from 20 to 120 that French and Spanish tables use to rate each tide. To judge how big a day is, look at the range or check the moon phase: big ranges come with the new and full moon.

How fast the water rises: the rule of twelfths

The tide doesn’t rise at a steady rate. It starts slowly, speeds up in the middle hours and slows again near high tide. The rule of twelfths captures that: divide the range by 12, and in each hour of the rise the water climbs:

1. first hour: 1 twelfth;
2. second hour: 2 twelfths;
3. third hour: 3 twelfths;
4. fourth hour: 3 twelfths;
5. fifth hour: 2 twelfths;
6. sixth hour: 1 twelfth.

With our afternoon range of 6.3 feet, one twelfth is about half a foot. Starting from the 12:55 p.m. low at −0.2 ft:

About	Rises	Height
1:55 p.m.	0.5 ft	0.3 ft
2:55 p.m.	1.1 ft	1.4 ft
3:55 p.m.	1.6 ft	3.0 ft
4:55 p.m.	1.6 ft	4.5 ft
5:55 p.m.	1.1 ft	5.6 ft
7:10 p.m.	0.5 ft	6.1 ft

In the third and fourth hours, the water climbs more than a foot and a half per hour, three times faster than at the start. If your skiff needs 3 feet of tide to get over a bar at the mouth of a creek, the chart says you can expect it around 4 p.m., not right after lunch.

The rule is an approximation. It works best on coasts with two similar tides a day, like most of the East Coast, and poorly in long tidal rivers or places with one tide a day. For the exact height at a given time, check the tide curve for your spot.

Other things you may see

• Reference and subordinate stations. NOAA publishes full predictions for reference stations and, for many nearby spots, time and height adjustments. A creek a few miles up the marsh can have its high tide well after the inlet.
• Moon phase. It helps explain the range: spring tides follow the new and full moon, neap tides the quarter moons.
• Sunrise and sunset. Useful for knowing whether a good low falls in daylight.

What the chart doesn’t know: the weather

A tide chart predicts the astronomical tide, the part caused by the moon and sun. Weather changes it:

• Air pressure. Low pressure lets the sea rise above the prediction; high pressure holds it down. A rough rule is about 1 centimeter per millibar, or a bit over an inch for every 0.1 inch the barometer drops.
• Wind. Hours of onshore wind pile water against the coast and raise high tide. A strong offshore wind leaves the low lower than predicted.
• Rivers. After heavy rain, high river flow keeps lows higher and later in estuaries.

Usually it’s a few inches. During a nor’easter on a spring high tide, it can be much more.

Tips for using a tide chart on the water

• Use the station closest to where you’ll actually be, not the nearest big city.
• Check the range and the moon phase before you go: big tides move faster.
• Use the rule of twelfths to estimate when the water will reach the level that matters to you, and leave a margin.
• If a storm or strong onshore wind is forecast, expect the water to run higher than the chart says.

If you fish, the next step is choosing the right stage of the tide. We cover that in fishing the tides.