Quick Facts
| Spring Tidal Range | Largest range — occurs at new and full moon, roughly every 14 days |
| Neap Tidal Range | Smallest range — occurs at first and third quarter moon |
| UK Tidal Pattern | Semi-diurnal: two high waters and two low waters per day |
| Gulf of Mexico Pattern | Diurnal: one high water and one low water per day |
| Bay of Fundy Range | Up to 16 m — largest tidal range on Earth |
| Mediterranean Range | Less than 0.5 m in most areas — effectively tideless for passage planning |
Tides are the rhythmic rise and fall of sea level driven by the gravitational pull of the Moon and, to a lesser extent, the Sun. For coastal sailors and anyone navigating in shoal water, understanding how tides work is not optional. The difference between high water and low water determines whether you can enter a harbour, anchor safely, or transit a shallow bar — and timing that difference to the hour requires more than a glance at the tide table.
This guide covers the physical mechanics of tidal generation, the two broad tide patterns found around the world, the datum references used on charts, practical methods for estimating water depth at any hour, global tidal range variation, and how to extract useful information from standard tide tables including corrections for secondary ports.
Spring and Neap Tides: What Drives the Cycle
The primary driver of tides is the Moon. As it orbits the Earth every 29.5 days, its gravitational pull creates a bulge of water on the side of the Earth facing the Moon — and, through inertia, a corresponding bulge on the far side. The Earth rotates through these bulges, producing the daily rise and fall of sea level.
When the Sun and Moon are aligned — at new moon and full moon — their gravitational forces combine to produce the largest tidal range. These are spring tides, occurring roughly every 14 days. The name has nothing to do with the season; it comes from the Anglo-Saxon word meaning to "spring up." Spring tides produce the highest high waters and the lowest low waters of the lunar cycle. In practice, this means maximum exposure of intertidal zones, maximum tidal stream velocities, and the greatest depth difference between high and low water.
At first and third quarter moon, the Sun and Moon are at right angles to each other as seen from Earth, and their gravitational pulls partially cancel out. The result is neap tides — the smallest tidal range of the cycle. Neap tides mean more moderate tidal streams, less exposure of intertidal ground, and a smaller window of opportunity at tidal gates that require specific water heights.
The spring-neap cycle runs approximately every 14 days, with the full 29.5-day lunar month containing two spring and two neap sequences. Long-range passage planning on tidal coasts should account for where in this cycle your transit falls.
Diurnal vs Semi-Diurnal Tide Patterns
Not all coasts experience tides the same way. The shape of ocean basins, continental shelf geometry, and the interaction of the tidal wave with local topography produces three broad patterns.
Semi-diurnal tides — two high waters and two low waters per day of roughly equal height — are the most common pattern in the North Atlantic and around the British Isles. The period between successive high waters is approximately 12 hours 25 minutes, which means the sequence advances by about 50 minutes each day. Sailors on the UK coast, the US East Coast, and much of Europe work with this pattern as their baseline.
Diurnal tides produce just one high water and one low water per day. The Gulf of Mexico is the best-known example: on the Louisiana and Texas coast, the tidal range may be no more than 30–60 cm and the cycle takes 24 hours. Parts of the Philippines, the Gulf of Tonkin, and some Pacific islands also experience diurnal tides. The navigational implication is a single slack-water period per day rather than two.
Mixed semi-diurnal tides are common on the US West Coast, parts of the Pacific, and Southeast Asia. Two high waters and two low waters occur each day, but they are of markedly unequal height — the "higher high water" may be a metre or more above the "lower high water" on the same day. Chart tables for these areas provide four separate predicted heights, and ignoring the inequality can result in significantly less depth than expected at a given time.
Chart Datum: LAT, HAT, and Mean Sea Level
The depths shown on a nautical chart are not measured from the sea surface — they are measured from chart datum, a reference level set low enough that the tide will rarely fall below it. In UK and many international charts, chart datum is approximately Lowest Astronomical Tide (LAT): the lowest water level predicted to occur under average meteorological conditions and any combination of astronomical conditions. Soundings on the chart represent the minimum depth you should expect to find in normal conditions.
Highest Astronomical Tide (HAT) is the highest water level predicted under the same average meteorological conditions. Clearance heights for bridges and overhead cables are measured downward from HAT, so a bridge with 8 m clearance on the chart will have exactly 8 m of air at HAT and more at any other state of tide.
Mean High Water Springs (MHWS), Mean Low Water Springs (MLWS), Mean High Water Neaps (MHWN), and Mean Low Water Neaps (MLWN) are the averages used to characterise the tidal range at a given port and appear in tidal almanacs. Understanding the relationship between chart datum, these means, and the actual predicted tide for a specific day is the foundation of safe tidal pilotage.
The Rule of Twelfths
Tide tables give predicted heights at high water and low water, but sailors often need to estimate the water depth at an intermediate time — for example, to know whether there will be enough depth to enter a harbour two hours before high water. The rule of twelfths is a quick mental calculation that works well for semi-diurnal tides with a relatively symmetrical curve.
Divide the tidal range (the difference between high water and low water heights) into twelve equal parts. The tide rises or falls by the following amounts in each successive hour of the six-hour cycle:
- 1st hour: 1/12 of range
- 2nd hour: 2/12 of range
- 3rd hour: 3/12 of range
- 4th hour: 3/12 of range
- 5th hour: 2/12 of range
- 6th hour: 1/12 of range
As a practical example: if low water is 0.5 m and high water is 4.5 m (range = 4.0 m), the tide will have risen by 3/12 of 4.0 m = 1.0 m in the first two hours, giving 1.5 m total. The rule of twelfths is an approximation — it is most accurate for ports with a near-sinusoidal tidal curve and less accurate at ports with a double high water (such as Southampton) or highly asymmetric curves. For critical passages, use the full tidal curve from the almanac.
Tidal Range Around the World
The Bay of Fundy in eastern Canada produces the world's largest tidal ranges — up to 16 metres between high and low water at the head of Minas Basin. The resonance of the funnel-shaped bay amplifies the Atlantic tidal wave to extraordinary heights. Passages in this area require careful planning: intertidal zones extend for kilometres, tidal streams run at 4–6 knots in narrows, and anchoring must account for a boat that will take the bottom and need to float off again.
The UK's Bristol Channel and Severn Estuary see ranges of 12–14 metres at springs near Avonmouth — the second-largest in the world. Spring tidal streams in the Severn can exceed 5 knots. At the other extreme, the Mediterranean experiences astronomical tides of less than 0.5 m in most locations; weather-driven storm surges are the more significant water level variation. The Baltic Sea is similarly micro-tidal.
Tidal Diamonds and the Tidal Stream Atlas
Admiralty charts mark positions of known tidal stream data with a lettered diamond symbol — each letter refers to a table printed on the chart giving the tidal stream direction and speed for that position at each hour relative to high water at the standard port. Two columns are given: springs rates and neaps rates. Interpolating between these for an intermediate tidal state requires estimating where in the spring-neap cycle you are operating, typically expressed as a percentage of mean spring range.
Tidal stream atlases — published separately by the UKHO and equivalent hydrographic offices — show tidal stream vectors across a wider area, with a page for each hour of the tidal cycle. These are particularly useful for offshore passage planning where the set and drift of the current over the full passage duration must be accumulated. The standard reference for UK waters is the Admiralty Tidal Stream Atlas series.
Reading Tide Tables: HW/LW Times and Secondary Port Corrections
Published tide tables list predicted times and heights of High Water (HW) and Low Water (LW) for a set of standard ports — the primary reference stations for a given region. The Admiralty Tide Tables (ATT) and equivalent publications such as NOAA tide predictions cover hundreds of standard ports with hourly or four-times-daily predictions.
For ports not listed as standard ports, tide tables provide secondary port corrections: a time difference (positive or negative minutes to add to the standard port HW and LW times) and a height difference or ratio (to apply to the standard port heights). Secondary port corrections account for the time it takes the tidal wave to travel from the standard port to the secondary port, as well as local modifications to the tidal range caused by coastal geometry. Applying these corrections correctly is essential at secondary ports — failing to do so can result in errors of 30–90 minutes in timing and 0.5 m or more in predicted height.
Continue: Tides in Practice
Using Tides to Plan Passages → | Reading Tidal Stream Atlases → | Tidal Hazards →
← Tides & Tidal Currents | Tidal Passage Planning →
Frequently Asked Questions
What causes spring tides and neap tides and how often do they occur? Spring tides occur when the Sun and Moon are aligned — at new moon and full moon — and their gravitational forces combine to produce the largest tidal range, roughly every 14 days. The name has nothing to do with the season; it comes from the Anglo-Saxon word meaning to spring up. Spring tides produce the highest high waters, the lowest low waters, and maximum tidal stream velocities. Neap tides occur at first and third quarter moon, when the Sun and Moon are at right angles and their gravitational pulls partially cancel out, producing the smallest tidal range. The full spring-neap cycle runs approximately every 14 days within the 29.5-day lunar month.
What is the difference between semi-diurnal, diurnal, and mixed tidal patterns? Semi-diurnal tides — two high waters and two low waters per day of roughly equal height — are the most common pattern in the North Atlantic and around the British Isles, with successive high waters approximately 12 hours 25 minutes apart. Diurnal tides produce just one high water and one low water per day; the Gulf of Mexico is the best-known example, with a tidal range of only 30 to 60 cm on the Louisiana and Texas coast. Mixed semi-diurnal tides, common on the US West Coast and Southeast Asia, produce two high waters and two low waters per day of markedly unequal height — the higher high water may be a metre or more above the lower high water on the same day.
What is chart datum and how does it affect the depths shown on a nautical chart? Chart datum is the reference level from which depths on a nautical chart are measured — set low enough that the tide will rarely fall below it. In UK and many international charts, chart datum is approximately Lowest Astronomical Tide (LAT): the lowest water level predicted to occur under average meteorological conditions and any combination of astronomical conditions. Soundings on the chart represent the minimum depth expected in normal conditions. Highest Astronomical Tide (HAT) is the highest water level predicted under the same conditions; clearance heights for bridges and overhead cables are measured downward from HAT, so a bridge with 8 metres clearance will have exactly that clearance at HAT and more at any other tidal state.
How does the rule of twelfths work for estimating water depth at intermediate tidal states? The rule of twelfths divides the tidal range (HW height minus LW height) into twelve equal parts and estimates how much the tide rises or falls each hour of the six-hour cycle: 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. Practical example: if low water is 0.5 m and high water is 4.5 m (range 4.0 m), the tide rises by 3/12 of 4.0 m = 1.0 m in the first two hours, giving 1.5 m total at that point. The rule is most accurate for ports with a near-sinusoidal tidal curve; for critical passages, use the full tidal curve from the almanac.
What are the world's most extreme tidal ranges and where are they? The Bay of Fundy in eastern Canada produces the world's largest tidal ranges — up to 16 metres between high and low water at the head of Minas Basin, where the funnel-shaped bay resonates with and amplifies the Atlantic tidal wave. Passages in this area require careful planning: intertidal zones extend for kilometres, tidal streams run at 4 to 6 knots in narrows, and anchoring must account for a boat that will dry out and need to float off. The UK's Bristol Channel and Severn Estuary see ranges of 12 to 14 metres at springs near Avonmouth — the second-largest in the world. At the other extreme, the Mediterranean has astronomical tides of less than 0.5 m in most locations.
How do secondary port corrections work and why do they matter for tide table accuracy? Tide tables list predicted times and heights of high water and low water for standard ports — primary reference stations for a given region. For ports not listed as standard ports, tide tables provide secondary port corrections: a time difference in positive or negative minutes to add to the standard port HW and LW times, and a height difference or ratio to apply to the standard port heights. Secondary port corrections account for the time it takes the tidal wave to travel from the standard port to the secondary port and local modifications to the tidal range caused by coastal geometry. Failing to apply these corrections correctly can result in errors of 30 to 90 minutes in timing and 0.5 m or more in predicted height.