Why Tide Tables Are Essential

Whether you're a recreational sailor, a kayaker, or a coastal walker, understanding how to read a tide table could be the difference between a safe outing and a dangerous situation. Tide tables tell you precisely when high and low tides will occur at a given location, and how high or low the water will be.

What's in a Tide Table?

A standard tide table lists the following information for each day:

  • Date — the calendar date
  • Time of high water (HW) — when the tide reaches its peak
  • Time of low water (LW) — when the tide reaches its lowest point
  • Height of high water — measured in metres above Chart Datum
  • Height of low water — measured in metres above Chart Datum

Chart Datum (CD) is the baseline used for tide measurements — it's roughly the level of the lowest astronomical tides at that location. All depths on nautical charts are measured from Chart Datum, which means at any state of the tide you can calculate the actual water depth.

Step-by-Step: Reading a Tide Table

  1. Find the correct port. Tide tables are published for specific reference ports. If you're not at a reference port, you'll need to apply a tidal difference from a secondary port table.
  2. Identify the date you're planning for. Locate the relevant row in the table.
  3. Note the times of high and low water. Check whether times are given in UTC or local time — many official tables use UTC, so you may need to add an hour for BST or local summer time.
  4. Note the tidal heights. The height at high water tells you how much water is above Chart Datum at the peak. The height at low water tells you the minimum.
  5. Calculate the tidal range. Subtract low water height from high water height. A large range means faster-moving tides and stronger currents.

Calculating Water Depth at Any Time

You don't always arrive at high or low water — so how do you know the depth at a specific time? Use the Rule of Twelfths, a simple approximation:

  • In the 1st hour after low water: tide rises by 1/12 of the tidal range
  • In the 2nd hour: rises by 2/12
  • In the 3rd hour: rises by 3/12
  • In the 4th hour: rises by 3/12
  • In the 5th hour: rises by 2/12
  • In the 6th hour: rises by 1/12

This means the tide rises and falls most rapidly in the middle two hours — an important consideration when crossing a sandbar or anchoring in a tidal area.

Tidal Curves for Greater Accuracy

For more precise calculations, nautical almanacs include tidal curves — graphical representations of the rise and fall of the tide at a port throughout the tidal cycle. These allow you to interpolate water depth at any point in time with much greater accuracy than the Rule of Twelfths.

Common Mistakes to Avoid

  • Confusing UTC with local time — always check the time zone used in your tide table
  • Using the wrong port — tides can vary significantly over short distances
  • Ignoring tidal range — a spring tide produces much stronger currents than a neap tide
  • Forgetting atmospheric pressure — low pressure systems can raise actual sea level above predicted heights

Where to Find Reliable Tide Tables

Tide tables are published annually by national hydrographic offices such as the UK Hydrographic Office (UKHO) and NOAA in the United States. Many free online tools and apps also provide accurate tide predictions, though always cross-reference with official sources for safety-critical navigation.