What Does a Tide Table Actually Show?
A traditional printed tide table shows four columns for each day: date, time of tide event, height of the tide in metres (or feet), and whether it is a high tide (HW or HT) or low tide (LW or LT).
A modern digital tide forecast — like those on TideTimes Global — typically shows the same information plus an hourly water level graph, making it far easier to visualise how the tide changes throughout the day.
The two essential pieces of information are:
Time: When will the water be at its highest or lowest? This tells you when to plan your activity.
Height: How high will the high tide be, and how low will the low tide be? This tells you whether there will be enough water depth for your purpose, or enough beach exposed for yours.
Understanding Chart Datum: The Baseline for All Heights
The height figures in a tide table are not measured from 'sea level' — they're measured from chart datum, which is defined as the Lowest Astronomical Tide (LAT): the lowest water level that can be predicted to occur under normal astronomical conditions.
This means tide table heights are always positive (or zero) — a height of 0.0 m doesn't mean the water has disappeared, it means the water is at the theoretical minimum. The actual depth of water at any location is the tide table height plus the depth printed on a nautical chart at that point.
Practical implication: if a tide table shows a low tide of 0.1 m and a high tide of 2.8 m, the tidal range for that day is 2.7 m. The absolute depth at any particular point depends on the chart datum depth at that location — tide tables tell you the change in water level, not the total depth.
The Rule of Twelfths: Knowing When the Tide is Rising Fastest
The tide doesn't rise and fall at a constant rate. It moves slowly near high and low tide, and fastest in the middle of the tidal cycle. The Rule of Twelfths gives a simple approximation of how much the water rises or falls in each hour of a 6-hour tidal cycle:
- Hour 1: 1/12 of the total range - Hour 2: 2/12 of the total range - Hours 3 and 4: 3/12 each (the fastest) - Hour 5: 2/12 - Hour 6: 1/12
Example: If the tidal range is 3.0 m, the water rises by approximately 0.25 m in hour 1, 0.50 m in hour 2, 0.75 m in each of hours 3 and 4, then slows again.
This is critical knowledge for safety: if you are on a beach or rocky platform that floods, the middle two hours of the incoming tide can raise water levels by 1.5 m — far faster than most people expect.
How to Use a Tide Table for Different Activities
Different activities need different information from the same tide table:
Fishing: Look for the time of tide change (when high tide transitions to low or vice versa). The 1–2 hours either side of a tide change produce the most active feeding. Also note whether it's a spring tide week (new/full moon) for maximum tidal currents.
Surfing: Find the predicted water height at the times you want to surf, and compare to your break's known preferred tidal range. Most local surfers can tell you whether their break works best at 'mid tide', '1.2–1.8 m', 'high tide only' etc.
Tide pooling and beachcombing: Identify days with the lowest predicted low tides (smallest numbers in the low tide column) during daylight hours. A low tide of 0.1 m exposes far more than one of 0.8 m.
Boating and sailing: Check that the predicted water height at your intended departure and arrival times exceeds your vessel's draft plus a safety margin. Pay attention to tidal currents in channels and harbour entrances.
Swimming: As a general guide, mid-incoming tide (water rising) tends to be cleaner and has lower rip current risk than outgoing tide. See our dedicated swimming guide for more detail.