How to Read a Tide Chart
Learn how to read a tide chart step by step: high and low tide times, heights in feet, tidal range, MLLW, negative tides and the rule of twelfths, with a real Key West example.
A tide chart shows the predicted water level at one place over time: a curve that rises to a high tide and falls to a low tide, with the times and heights printed beside each peak and trough. To read it, find today's date, look at the high and low times, check the height in feet at each one, and subtract to get the range. The bigger the range, the more the water will move.
A real example: the Key West tide chart
The clearest way to learn is to look at a real chart. Here are the values from the Key West tide page on Marinalo for Saturday, October 17, 2026 (Marinalo data, October 2026):
| Event | Time | Height |
|---|---|---|
| High tide | 1:08 AM | 2.2 ft |
| Low tide | 9:01 AM | 0.6 ft |
| High tide | 3:15 PM | 1.4 ft |
| Low tide | 7:13 PM | 1.1 ft |
The page also shows a "Right now" box: at 12:24 PM it showed the tide rising at 1.1 ft, with the next high in about 2 hours 51 minutes. Below that, a curve for the day plots the same four points, with a marker for the current time, and a table gives the next seven days. You can open the live Key West tide chart to follow along.
How to read a tide chart, step by step
- Find the right place and date. Tides differ from one place to the next, even a few miles apart. Use the station or town nearest your ramp, inlet or dock. Marinalo's tide pages list places by state, such as Florida and Texas.
- Check the time zone. Charts are normally in local time. Confirm that daylight saving time is handled properly before relying on a time.
- Find the highs and lows. Each peak is a high tide (H), each trough a low tide (L). In the Key West example, there are two highs and two lows in the day, which is typical for many US coasts.
- Read the height at each. Heights are in feet above or below a reference level (see datums below). The high at 1:08 AM is 2.2 ft; the low at 9:01 AM is 0.6 ft.
- Work out the tidal range. Range is high minus low. From 2.2 ft to 0.6 ft is 1.6 ft, which is the "Tidal range today" figure on the page. A range of 1.6 ft is modest; many places have far bigger ranges.
- Notice that highs and lows are not equal. At Key West that day, one high is 2.2 ft and the other only 1.4 ft; the lows are 0.6 ft and 1.1 ft. Don't assume the two highs match.
- Check the time between tides. Here, the fall from the 1:08 AM high to the 9:01 AM low takes just under 8 hours, and the rise to 3:15 PM takes just over 6 hours. Tides don't keep a fixed schedule; the times shift later each day.
- Look ahead. The seven-day table shows how the times and heights change. The page tags some days with moon-related terms such as neap tide. See our guides to spring and neap tides, king tides and how the moon affects tides for the reasons.
Datums: what "0 ft" means
Every tide height is measured from a reference level called a datum. For US tide predictions and nautical charts, NOAA uses Mean Lower Low Water (MLLW): the average of the lower of the two daily low tides, taken over the National Tidal Datum Epoch. NOAA uses MLLW as the reference for predictions and for nautical charting, which means the depth printed on a chart and the tide height on a tide table share the same zero point.
In practice: water depth at a spot is the charted depth plus the tide height. If your chart shows 3 ft at a shoal and the tide table says 1.4 ft, you have about 4.4 ft of water there (not counting waves and wind). If the tide is lower than zero, you subtract.
Keep in mind that tide predictions are not navigation instructions: the Marinalo page itself notes the data is not for navigation. Wind, storms, pressure and river flow change real water levels from the predicted values.
Negative tides
A negative tide is a low tide with a height below zero on the chart. Since MLLW is an average of lower low waters, some lows fall below it. On the Key West page, the lowest tide listed for the year is -0.7 ft on January 21, 2027 at 2:20 AM, and the highest is 2.7 ft on October 27, 2026 at 10:10 PM. Negative tides expose flats, bars and oyster beds, and shallow ramps can become unusable, so check the heights before you launch. The Key West boat ramp page and our Florida boat ramp directory help you plan where to launch.
Tide height vs tidal current
A tide chart tells you the water level, not how fast the water is moving. The strongest current often comes between high and low, and slack water can come at a different time than high or low tide. For the full story, read our guide to ebb, flood and slack tide. Use current predictions as well as a tide table when you cross an inlet or work a narrow channel.
The rule of twelfths (an estimate only)
If you need a rough idea of the water level between a high and a low, the rule of twelfths is a shortcut. It assumes the tide takes about six hours to go from low to high (or the reverse) and that the change follows a smooth sine-curve pattern: 1/12 of the range in the first hour, 2/12 in the second, 3/12 in the third, 3/12 in the fourth, 2/12 in the fifth and 1/12 in the sixth.
| Hour | Share of the range | Falling from 2.2 ft with a 1.6 ft range (Key West example) |
|---|---|---|
| 1 | 1/12 | about 0.13 ft drop (2.2 to about 2.07 ft) |
| 2 | 2/12 | about 0.27 ft drop (to about 1.8 ft) |
| 3 | 3/12 | about 0.4 ft drop (to about 1.4 ft) |
| 4 | 3/12 | about 0.4 ft drop (to about 1.0 ft) |
| 5 | 2/12 | about 0.27 ft drop (to about 0.73 ft) |
| 6 | 1/12 | about 0.13 ft drop (to 0.6 ft) |
This is a rough approximation. Real tides don't always take exactly six hours (the Key West fall that morning took just under 8), and the rule fails where the tide curve doesn't look like a sine wave. Use it only if you have no table, and use the printed times and heights whenever you can.
Tips for boaters
- Check the chart for your arrival and return times, not only departure.
- Allow extra depth for your boat's draft, wind and waves.
- Leave early when the return will be close to low tide in shallow water.
- Carry both a tide table and a chart, and compare them.
- For fishing, a tide change is often a popular time; the Marinalo tide pages also list solunar periods.
FAQ
How do you read a tide chart?
Find the date, read the time and height of each high and low, and take the difference to get the range. Add the tide height to the charted depth to get the water depth at a spot.
What does a negative tide mean?
It means the water is below the chart's zero level (MLLW), so shallow areas are more exposed than usual.
What is MLLW?
Mean Lower Low Water, the average of the lower of the two daily low tides over a NOAA tidal datum epoch. It is the zero point for NOAA tide predictions and nautical charts.
How often are there high tides?
Many US coasts have two highs and two lows a day, as the Key West example shows, but some places have different patterns. Check your own station's chart.
Are tide charts exact?
No. They are predictions based on astronomical forces. Wind, storms and pressure can change actual water levels, and the data is not for navigation.