London below the tide: every postcode measured against the Thames at high water
Twice a month, around full and new moon, the tides run higher: spring tides. On an ordinary one the Thames at London Bridge rises to 3.9 m above Ordnance Datum, the zero of British maps. 10,528 of Greater London's 177,041 postcodes stand on ground lower than that. River walls hold the daily tide back. Since 1982 the Thames Barrier has been closed against storm tides and, when the river is in flood, against ordinary ones: 221 times by November 2025.
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Ground: the Environment Agency's laser survey. Tides: the Port of London Authority's 2026 tables and its hand-written gauge ledgers for 1911–1995, published as data in 2021. Nothing here is a forecast: the page sets surveyed ground beside river levels recorded at London Bridge.
The map needs a browser that can draw it. The figures are in the text and in the table of districts below.
At London Bridge a spring tide lifts the Thames from 2.7 m below Ordnance Datum at low water to 3.9 m above it at high water. A week later, at neaps, high water reaches 2.7 m. These are the Port of London Authority's standard levels, from its 2026 tables.
The staff on the map carries those levels. Drag its marker, or step through the tides with the button: the map picks out every piece of ground lower than the level you set. It shows height and nothing else. Whether water could reach a place depends on the walls between it and the river.
Greater London has 177,041 postcodes with ground of their own. The ground at 10,528 of them, about 6%, is lower than 3.9 m. The districts with most are SE1 (1,418), E16 (757), SE16 (730), SE17 (678) and SW1P (537).
In area that is 66 km² of the capital's 1,552 km² of land. Nearly all of it lies beside the tidal river and its creeks. 310 of those postcodes are outside the Environment Agency's flood zones for the sea altogether: low spots away from the river, which the survey records as ground like any other.
The lowest reading at any postcode is 5.3 m below Ordnance Datum, at E20 1YY in Stratford.
High water does not stand at one height along the Thames: by the same tables an average spring tide reaches about 3.0 m at Southend, on the estuary, and about 4.2 m at Richmond. Every comparison on this page uses London Bridge.
At 01:10 on 7 January 1928 the gauge by London Bridge read 5.21 m, 1.3 m above today's ordinary spring tide. A surge from the North Sea had met a river swollen by snowmelt.
The Thames poured over the embankments at Southwark, Lambeth, Temple Pier and the Houses of Parliament. Fourteen people drowned, ten of them in the slums on the Westminster side of Lambeth Bridge. “The people who died were poor people living in crowded basements,” Anna Carlsson-Hyslop of the University of Manchester told the BBC. It was the last time a storm surge flooded central London.
On the night of 31 January 1953 a storm surge ran down the North Sea and killed 307 people along the east coast of England. It reached London Bridge at 02:06: 5.41 m, the highest of the 57,909 high waters logged there between 1911 and 1995.
In central London the river “did not quite overtop the Thames embankment”, as a 2015 study of the surge puts it. Downriver it did more. In the East End 100 yards of sea wall collapsed, more than 1,130 houses were flooded and one person died at Tidal Basin.
18,724 of today's postcodes stand on ground lower than that tide.
The Thames Barrier crosses the river near Woolwich: 520 m wide, ten steel gates, the main ones 3,300 tonnes each. It has been operational since 1982, and the Environment Agency's log dates its first closure against a tide to 1 February 1983.
The Agency describes the division of work like this. Upstream of the Barrier, “the walls and embankments protect against daily high tides. The Thames Barrier protects these areas against storm tides.” Downstream the walls are higher (by about 2 m from one side of the gates to the other, the Agency wrote in 2015), and “the Thames Barrier does not protect these areas from flooding”.
The City of London gives the level the Agency requires of its river walls, upstream of London Bridge, as 5.41 m above Ordnance Datum. The ledgers give the 1953 tide the same figure.
From the first closure to the end of the ledgers in 1995, the highest tide at London Bridge was 4.81 m, on 1 March 1990. The Environment Agency's own gauge at Tower Pier, running since 1988, lists that same tide as its highest (read on 9 October 2026).
Before 1983 the ledgers hold 13 higher tides. The chart shows the highest tide of each year.
Highest tide of each year at London Bridge, 1912–1995, in metres above Ordnance Datum (the ledgers begin in May 1911). Hollow marks: from 1983, the year of the Barrier's first closure.
For planning, the Environment Agency maps the land that would flood “ignoring the benefits of defences”. Its Flood Zone 3 from the sea is the land with a chance of 1 in 200 or more each year in that case. 18,150 Greater London postcodes lie inside it, about one in ten; it covers 112 km².
The outline on the map is that zone together with Flood Zone 2, which runs out to 1 in 1,000. The Agency adds that the zones “are not suitable for showing whether an individual property is at risk of flooding”.
221 times for flood defence between 1982 and 17 November 2025, by the Environment Agency's count: 119 against the tide alone and 102 when a high tide met a river in flood. Pick a winter.
Dates, levels and flows to March 2007 are the Environment Agency's closure log. It gives the Southend levels without naming a datum; they agree with heights above Ordnance Datum in the tide ledgers. For later seasons the Agency has published counts, not dates.
One winter against eighteen. In the flood season of 2013–14 the Barrier closed 50 times. In its first eighteen seasons together, 1982–83 to 1999–2000, it closed 39 times. Seven complete seasons since 1982 have had no closure at all, the latest of them 2024–25.
Why 2013–14 stands so high. 41 of those 50 closures were of the combined kind: a tide coming in to meet a river in flood. The Barrier “may also be closed during periods of high flow over Teddington Weir”, the Agency writes. “This is to reduce the risk of river flooding in some areas of west London including Richmond and Twickenham.” Closed just after low tide, it keeps the incoming tide out of the reach above, which leaves room for the river coming down. The Agency closed it on 48 occasions between 2 January and 5 March 2014.
What the count cannot show. A closure is a decision. “The barrier has no individual trigger level for closure”, says the Agency: a matrix of tide, surge and river flow guides the Duty Controller, who decides. So the bars measure each winter's weather and the decisions taken on it, and they are not a record of sea level.
What comes next. The Agency expects that “as sea levels rise, the barrier will be closed more and more often”, and that closing it too often would leave too little time to maintain it. Its plan is to raise the walls upstream, so that higher tides can be let through, and to choose by 2040 among seven options that run from upgrading the Barrier to building a new one downriver.
271 districts with 30 postcodes or more in Greater London, ordered by the share of their postcodes that stand below an ordinary spring high tide at London Bridge (3.9 m). Each has its own page to send.
How many times has the Thames Barrier been closed?
221 times for flood defence since it became operational in 1982, according to the Environment Agency's Barrier page as at 17 November 2025: 119 closures against tidal flooding and 102 against combined tidal and river flooding. The season with most was 2013–14, with 50. The Agency's 2026 review of its estuary plan gives 211 closures “between 1982 and 2025”; the two figures are the Agency's and the difference is not explained in either place.
What is the highest tide recorded at London Bridge?
In the Port of London Authority's ledgers for 1911–1995, 5.41 m above Ordnance Datum at 02:06 on 1 February 1953. The ledgers were digitised by the University of Southampton and the Environment Agency. The next highest are 10 December 1965 (5.24 m), 7 January 1928 and 31 December 1978 (both 5.21 m). The ledgers begin in May 1911; earlier tides are not covered, and for the years after 1995 this page has only the maximum the Agency's gauge lists, 4.81 m.
How much of London is below high tide?
By the survey of the ground, 66 km² of Greater London's 1,552 km² of land is lower than an ordinary spring high tide at London Bridge (3.9 m above Ordnance Datum), and 10,528 of 177,041 postcodes stand on such ground. Against the 1953 tide (5.41 m) the figures are 109 km² and 18,724 postcodes. The Authority gives the spring level to a tenth of a metre: at 3.85 m the count would be 10,287, at 3.95 m 10,754.
Is my home at risk of flooding?
This page cannot say. It compares the height of the ground with the height of past tides at London Bridge, and takes no account of walls, gates, drains or the route water would have to travel. For the long-term flood risk of the area around an address there is the government's service “Check the long term flood risk for an area in England”; it too does not tell you how likely a single property is to flood.
Why Ordnance Datum and not Chart Datum?
Tide tables give heights above Chart Datum, a low-water level chosen for navigation; maps and surveys give heights above Ordnance Datum Newlyn. At London Bridge, Chart Datum lies 3.20 m below Ordnance Datum, so a tide-table height of 7.1 m is 3.9 m on the map's scale. The offset changes from one tide station to the next.
How exact are the ground levels?
A postcode's level is the middle value of the Environment Agency's terrain survey in the 50 m square around the postcode's point. Where that square slopes or holds a cutting, a dock or a raised deck, the page adds the band in which most of the square lies (its 10th to 90th percentile). The survey records the ground as it is now, made ground and excavations included, so the lowest readings are often sunken structures.
Ground levels come from the Environment Agency's LIDAR Composite Digital Terrain Model (1 m, averaged to 10 m cells). A postcode's level is the middle value of the 25 cells, a 50 m square, around the point Ordnance Survey's Code-Point Open gives for it; 3,406 PO-box and single-user codes are left out. Areas of ground are counted on the 10 m grid inside the borough outlines of the Office for National Statistics, water left out; the flood zone's area is measured on its own outline. River levels are the Port of London Authority's standard levels from its 2026 tables, moved from Chart Datum to Ordnance Datum, and the high waters in its ledgers for 1911–1995. Closures per season were transcribed from the Environment Agency's chart and checked against its totals; dated closures to March 2007 come from the Agency's log and agree with the chart season by season. Every postcode is compared with London Bridge, wherever it is, and the comparison is of heights alone.
Environment Agency, The Thames Barrier: closure totals, closures by flood season, how the Barrier works. gov.uk
Environment Agency, Flood Defence Closures, long version, issued 31 March 2007 (closures 1 to 103), as hosted by RealClimate. realclimate.org
Environment Agency, LIDAR Composite Digital Terrain Model, 1 m. Open Government Licence. data.gov.uk
Environment Agency, Flood Map for Planning: Flood Zones. Open Government Licence. data.gov.uk
Environment Agency blog, Keeping London safe from flooding (2015; the figure for the walls is in the Agency's reply to a comment) and The calm after the storms (2014). environmentagency.blog.gov.uk
Environment Agency, flood-monitoring service, station 0007 Tower Pier. environment.data.gov.uk
Port of London Authority, Handbook of Tide Tables and Port Information 2026: chart datums and standard levels. pla.co.uk
Haigh, Inayatillah and others (2021), Historic sea level records in the Thames Estuary, UK, 1911–1995. British Oceanographic Data Centre. Open Government Licence. doi.org
Inayatillah and others (2022), Digitising historical sea level records in the Thames Estuary, UK. Scientific Data 9. doi.org
Wadey and others (2015), A comparison of the 31 January–1 February 1953 and 5–6 December 2013 coastal flood events around the UK. Frontiers in Marine Science 2. doi.org
Jon Kelly, The great 1928 flood of London. BBC News Magazine, 16 February 2014. bbc.co.uk
City of London Corporation, riverside strategy papers: statutory flood defence levels. democracy.cityoflondon.gov.uk
