Tides are caused mainly by the gravitational pull of the Moon, and to a lesser extent the Sun, on the oceans. The Moon raises a bulge of water on the side of the Earth facing it and another on the opposite side; as the Earth rotates, most coastlines pass through two high waters and two low waters roughly every 24 hours and 50 minutes (a semi-diurnal pattern). The Sun reinforces or opposes the Moon depending on their alignment, producing spring and neap tides.

Chart datum is the reference level from which charted depths and drying heights are measured. On modern charts it is usually Lowest Astronomical Tide (LAT) — the lowest level the tide is expected to reach under average meteorological conditions. Charted depths are the depth below chart datum, so the actual depth of water equals the charted depth plus the height of tide at that moment. Using LAT means the real depth is almost always at least the charted depth.

What is the difference between tidal height and tidal stream?

Tidal height is the vertical rise and fall of the water — it determines depth and clearance. Tidal stream is the horizontal flow of water (the current) caused by the tide — it sets your boat off course and speeds you up or slows you down. They are related but not the same: high or low water does not always coincide with slack water, and the strongest streams often run mid-tide.

Understanding Tides: A Practical Guide for Sailors

Q: What is the difference between spring and neap tides?

Spring tides occur around the full and new Moon, when the Sun and Moon are aligned and their gravitational pulls combine — giving the largest tidal range (higher highs, lower lows) and the strongest tidal streams. Neap tides occur around the first and last quarter Moon, when the Sun and Moon are at right angles and partly cancel — giving the smallest range and weakest streams. The cycle from springs to neaps and back takes about two weeks.

Q: What is the Rule of Twelfths?

The Rule of Twelfths estimates how much the tide rises or falls each hour over a roughly six-hour tide. Of the total range, the tide moves about 1/12 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. It is a quick mental approximation for the height of tide between high and low water; for precise figures use tidal curves in an almanac.

Tides decide where you can go, when, and how deep the water really is. This guide explains what causes them, why some are bigger than others, how to read depth from a chart, and the quick mental tools every skipper uses to stay off the bottom.

Last updated: 19 June 2026 · By Askolds Hermanis, Founder & Sailing Instructor (SkipperCheck / Nautica, since 2008)

Quick answer: Tides are the rise and fall of the sea caused by the Moon and Sun. Around full and new Moon you get big spring tides; around the quarter Moons, small neap tides. Charts show depths below chart datum (usually LAT), so actual depth = charted depth + height of tide. Use the Rule of Twelfths to estimate the height between high and low water, and don't confuse tidal height (depth) with tidal stream (current).

Watch: chart datum and tide heights explained. More clips in our video lessons.

On this page

What causes tides
Spring and neap tides
Chart datum and height of tide
The Rule of Twelfths
Tidal streams vs tidal height
Why it matters in practice
Frequently asked questions

What causes tides

Tides are driven by the gravitational pull of the Moon, and to a lesser degree the Sun, on the world's oceans. The Moon pulls a bulge of water toward it on the near side of the Earth, with a matching bulge on the far side. As the Earth rotates beneath these bulges, most coastlines experience two high waters and two low waters in roughly 24 hours and 50 minutes — a semi-diurnal tide, which is the pattern across most of Europe.

The extra 50 minutes a day is why high water is a little later each day, and why the tidal pattern slowly walks around the clock over a fortnight.

Spring and neap tides

The Sun's pull either reinforces or opposes the Moon's, depending on alignment:

Spring tides — around the full and new Moon, the Sun and Moon line up and pull together. The result is the largest tidal range (higher highs, lower lows) and the strongest tidal streams.
Neap tides — around the first and last quarter Moon, the Sun and Moon pull at right angles and partly cancel. The result is the smallest range and the weakest streams.

The cycle from springs to neaps and back takes about two weeks. "Spring" has nothing to do with the season — it comes from the water "springing up".

Chart datum and height of tide

Charted depths have to be measured from a fixed reference, and that reference is chart datum. On modern charts it is usually Lowest Astronomical Tide (LAT) — the lowest level the tide is expected to fall to under average conditions.

The key formula: Actual depth = charted depth + height of tide. Because chart datum is set at the lowest astronomical tide, the real depth is almost always at least the charted figure — a built-in safety margin. Drying heights (areas that uncover at low water) are shown with an underline and measured above datum.

To find the height of tide at a given time, use the tide tables and tidal curves in a nautical almanac: full predictions for standard ports, and time/height differences for nearby secondary ports.

The Rule of Twelfths

Between high and low water you often need a quick estimate of the height of tide without drawing a full tidal curve. The Rule of Twelfths assumes a roughly six-hour, sinusoidal tide and splits the total range like this:

Hour after HW (or LW)	Fraction of range that moves
1st hour	1/12
2nd hour	2/12
3rd hour	3/12
4th hour	3/12
5th hour	2/12
6th hour	1/12

So the water moves slowly near high and low water and fastest in the middle two hours. It is an approximation — for tight clearances use the almanac's tidal curve — but it is fast, reliable enough for planning, and worth committing to memory.

Tidal streams vs tidal height

This catches a lot of new skippers out. Tidal height is the vertical rise and fall — it sets your depth and clearance. Tidal stream is the horizontal flow of water (the current) — it pushes your boat sideways and speeds you up or slows you down.

They are related but not simultaneous: high water is not the same as slack water, and the strongest streams often run mid-tide.
Streams flood (rising) and ebb (falling), with slack water in between.
Like heights, streams are stronger at springs and weaker at neaps. Find them in tidal stream atlases and the tidal diamonds printed on charts.

Why it matters in practice

Clearance: enough water under the keel at low tide, and under bridges/cables at high tide.
Anchoring: plan scope on the high-water depth, and check you won't dry out at low water.
Passage planning: carry a fair tide where you can — a 2–3 knot stream is free speed one way and a wall the other.
Pilotage: entering drying harbours and crossing bars safely depends on getting the height of tide right.

Frequently asked questions

What causes tides?

The gravitational pull of the Moon, and to a lesser extent the Sun, on the oceans. Most coasts get two high and two low waters about every 24 hours 50 minutes. The Sun reinforces or opposes the Moon to produce spring and neap tides.

What is the difference between spring and neap tides?

Springs (full and new Moon) have the largest range and strongest streams because the Sun and Moon pull together. Neaps (quarter Moons) have the smallest range and weakest streams because they partly cancel. The cycle takes about two weeks.

What is chart datum?

The reference level for charted depths, usually Lowest Astronomical Tide (LAT). Actual depth = charted depth + height of tide, so real depth is almost always at least the charted figure.

What is the Rule of Twelfths?

A quick way to estimate the height of tide between high and low water: the tide moves 1, 2, 3, 3, 2, 1 twelfths of the range in successive hours of a roughly six-hour tide — slow near the turns, fast in the middle.

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