How to Read a Tide Table — A Sailor's Step-by-Step Guide

A tide table lists the times and heights of high water (HW) and low water (LW) at a port for each day. Heights are measured above chart datum, the level that charted depths are measured from, so depth of water = charted depth + height of tide. Check the time zone (UK tables are usually in UT, so add an hour in summer) and whether the day is near springs or neaps.

A printed tide table listing dates with the times and heights of high and low water — A printed tide table: each day lists the times of high and low water and their heights above chart datum. Photo: Tewy, CC BY 2.5.

What is in a tide table

Date and day, often with the moon phase marked, so you can spot springs and neaps.
Time of each high and low water. There are usually two of each per day, about 12 hours 25 minutes apart, so the times get roughly 50 minutes later each day.
Height in metres above chart datum. A height of 0.4 m at low water means the sea is 0.4 m above the level the chart depths are measured from.
Time zone note. ADMIRALTY tables for UK ports are in UT (GMT). During British Summer Time you add one hour.

Step by step: from the table to the depth under your keel

Side-view diagram of tidal heights: chart datum, low water and high water levels, with height of tide, charted depth, depth of water, range and a drying height marked — Every height in a tide table is measured from chart datum. Add the height of tide to the charted depth to get the depth of water.

Find the right port. Tables are published for standard ports; smaller places are secondary ports, worked out from a nearby standard port with time and height differences.
Find the date and convert the times to local time.
Note the HW and LW heights either side of the time you care about. The difference is the range.
Work out the height of tide at your time using the port's tidal curve, or roughly with the rule of twelfths.
Add the height of tide to the charted depth to get the actual depth. For a drying height (an underlined figure on the chart), subtract it instead.
Subtract your draught and a safety margin to get the clearance under the keel.

Worked example (illustrative figures).
Step	Value
LW (local time)	08:10, 1.2 m
HW (local time)	14:25, 5.0 m
Range	3.8 m
Height of tide at 11:15 (about half tide)	about 3.1 m
Charted depth at the harbour entrance	1.5 m
Depth of water at 11:15	4.6 m
Clearance for a 1.8 m draught	2.8 m

Tenby harbour at low tide, with boats sitting on the sand below the harbour wall — Tenby harbour at low water. The table tells you when the boats will float again. Photo: David Dixon, CC BY-SA 2.0.

Springs and neaps

The range changes through the month. Around new and full moon you get spring tides: higher highs, lower lows and stronger streams. Around the quarter moons you get neap tides with a smaller range. A quick check is to compare the day's range with the port's mean spring and mean neap ranges, printed in the almanac. See spring and neap tides explained.

Practise with our tide calculator, then work through the tidal heights lesson.

Worked examples

Related guides

Frequently asked questions

Are tide table times in GMT or BST?

ADMIRALTY tide tables for UK ports are published in UT (GMT). Add one hour during British Summer Time. Many apps convert automatically, but always check the note at the top of the table.

What is chart datum?

Chart datum is the reference level for charted depths and tide heights, close to the lowest astronomical tide on modern charts. The water is very rarely below it, so charted depths are close to the least you should find.

Why are there two high tides a day?

Most places around the UK have semi-diurnal tides: two highs and two lows in about 24 hours 50 minutes, because of the way the moon's pull and the earth's rotation combine.

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