How to Read a Tide Chart

Whether you're planning a beach day, a fishing trip, or a coastal hike, understanding how to read a tide chart is one of the most useful skills you can pick up. A tide chart gives you a visual snapshot of how water levels rise and fall over time at a specific location, helping you decide the best time to head out.

What Does a Tide Chart Show?

A tide chart is a graph that plots predicted water height on the vertical axis (y-axis) against time on the horizontal axis (x-axis). The smooth curve traces the continuous rise and fall of the tide throughout the day. The peaks of the curve represent high tides, while the troughs represent low tides.

Most coastal locations experience two high tides and two low tides every 24 hours and 50 minutes — a cycle driven by the gravitational pull of the moon and sun. Some locations, particularly in Southeast Asia and the Gulf of Mexico, have a single high and low tide per day (diurnal tides), while others show a mix of the two patterns (mixed semidiurnal).

Understanding the Axes

The X-Axis: Time

The horizontal axis shows the time of day, typically displayed in the station's local timezone. On multi-day charts, vertical bands or alternating shading help you distinguish one day from the next. On TideCheck's interactive charts, the current day is highlighted in teal for easy reference, and a "Now" dot marks the present moment on the curve.

The Y-Axis: Height

The vertical axis shows water height, measured in metres (or feet, depending on your region). But height relative to what? This is where the datum comes in.

What Is the Datum Line?

The datum is the baseline reference level from which all tide heights are measured. Think of it as the "zero" on the ruler. There are several common datums:

• LAT (Lowest Astronomical Tide) — the international standard used in most countries. It represents the lowest tide level that can be predicted under normal conditions. Most tide heights will be positive relative to LAT.
• MLLW (Mean Lower Low Water) — the US standard, representing the average of the lower low water readings over a tidal epoch. Very similar to LAT for practical purposes.
• MSL (Mean Sea Level) — the long-term average water level. Tide heights swing above and below MSL, so you'll see both positive and negative values.

On TideCheck, the datum line is shown as a bold dashed line across the chart. You can switch between LAT, MLLW, and MSL using the datum selector tabs above the chart. The fill colour below the curve also changes: blue above the datum and amber below it, giving you an instant visual sense of the tidal state.

Reading the Curve Shape

The shape of the tide curve tells you a lot:

• Steep rise or fall — Water levels are changing quickly. Tidal currents will be strongest during these periods. This matters for boaters, kayakers, and anyone wading in tidal areas.
• Flat peaks and troughs — At high and low tide, the water level briefly pauses before reversing direction. This period is called slack water, and it's when currents are weakest — ideal for snorkelling, diving, or crossing tidal flats.
• Unequal highs or lows — Many locations show one high tide that's noticeably higher than the other (or one low tide lower than the other). This is the "mixed semidiurnal" pattern and is completely normal.

Spring and Neap Patterns

If you look at a tide chart spanning two weeks or more, you'll notice the tidal range (the difference between high and low water) isn't constant. It follows a roughly fortnightly cycle:

• Spring tides occur around new moon and full moon, when the sun and moon align. The highs are higher and the lows are lower, creating the largest tidal range. Despite the name, spring tides happen year-round — roughly every two weeks.
• Neap tides occur around first quarter and last quarter moons, when the sun and moon pull at right angles. The tidal range is smaller, with more moderate highs and lows.

TideCheck marks spring and neap indicators in the daily forecast, so you can quickly see where you are in the cycle. Try the 15-day forecast for Dover to see how the range shifts between spring and neap.

Day and Night Shading

TideCheck's charts include subtle amber shading for nighttime hours, based on actual sunrise and sunset times for the station's location. This helps you immediately identify which tides occur during daylight — essential for planning activities safely.

Using TideCheck's Interactive Features

Beyond the static curve, TideCheck's interactive chart lets you:

• Hover or tap anywhere on the curve to see the exact predicted height and time at that point.
• View extremes — High and low tide markers are labelled directly on the chart with their exact times and heights.
• Track "Now" — A glowing dot shows the current position on the curve, so you can see at a glance whether the tide is rising or falling and how far along it is.
• Switch datums — Toggle between LAT, MLLW, and MSL to see heights relative to the reference level that matters to you.

Understanding how to read a tide chart transforms it from a squiggly line into a powerful planning tool. Once you know what you're looking at, you'll never head to the coast without checking the tides first.