Methodology: How We Calculate And Present Flood Risk Data
Flood Risk Checker takes official flood risk data for every postcode, groups postcodes into towns using official settlement boundaries, and calculates each town's share of homes in each flood risk band. Every rule is fixed, published here and applied identically to every place.
This page explains how each figure on a town, county, river or gauge page is produced, how often it is refreshed, and what it cannot tell you. The data itself is listed in data sources. What the bands mean is explained in flood risk levels.
What Data Does A Place Page Use?
A place page uses four kinds of official data: flood risk bands for each source of flooding, Flood Zones for planning, river gauge records and live readings, and official postcode and settlement boundaries. None of the figures is produced by our own flood modelling.
| Data | Publisher | What we use it for |
|---|---|---|
| Flood risk bands (rivers and the sea; surface water) | Environment Agency; Natural Resources Wales; SEPA | Headline band and band distribution |
| Flood Zones 2 and 3 | Environment Agency; Natural Resources Wales | Flood Zone mix |
| Monitoring stations and readings | Environment Agency flood-monitoring API and the devolved agencies | Nearest gauge, river level, trend |
| Flood warnings and alerts | Environment Agency and the devolved agencies | Live warning modules |
| Postcodes, settlements, household counts | Ordnance Survey, Office for National Statistics | Grouping postcodes into places and weighting them |
We do not build flood models or adjust an agency's classification. Where we calculate something, such as a share or a rank, the formula is given below.
How Is A Place Defined?
A place is an official built-up area published by the Office for National Statistics. Each live postcode belongs to the built-up area that contains its centre point, and postcodes outside any built-up area are covered by county pages and the postcode checker instead.
The steps are:
- Take every live postcode and its centre point.
- Assign the postcode to the built-up area containing that point.
- Assign the place to a county and nation using the postcodes' official administrative codes, where the most common code wins.
A postcode district, such as the first half of a postcode, is listed on the page of the place that holds most of its postcodes. Where no place holds at least 40% of a district's postcodes, the district is labelled as split between places.
How Is The Headline Flood Risk Band Chosen?
A place's headline band for each source is the highest band that covers at least 5% of its homes. This avoids two errors: one riverside street making a whole town High, and a town where 30% of homes are High being labelled by the band most of its homes fall in.
The rule in full:
- Headline band = the highest band B where the share of homes in band B is 5% or more.
- If no band reaches 5%, the headline is the band that covers the most homes, and the page says so.
- If more than 10% of a place's postcodes have no band for a source, that source shows "Data not available" rather than an estimate.
The full distribution is always shown next to the headline, so the headline can be checked against it.
How Is The Share Of Homes In Each Band Calculated?
The share of homes in a band is the number of homes in postcodes with that band divided by the number of homes in postcodes with any band for the same source. Homes are estimated from Census household counts, apportioned from Census output areas to postcodes.
share(place, source, band) = homes in the place's postcodes with that band
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homes in the place's postcodes with any band for that source
- Shares are calculated per source and add up to 100% for each source. They are never added across sources, because one home has a separate band for each source.
- Shares are rounded to one decimal place for display.
How Is The Flood Zone Mix Calculated?
The Flood Zone mix is the share of a place's homes in postcodes whose centre point falls inside Flood Zone 3, inside Flood Zone 2, or in neither. Flood Zone 1 is not a mapped area; it is everything outside Zones 2 and 3, so its share is calculated by subtraction.
Three limits apply:
- No split between Flood Zone 3a and 3b is shown for a place. The national map does not separate them; the split is made in each local planning authority's own assessment, and Flood Zones explained covers the difference.
- Welsh places use Welsh Flood Zones, which follow different definitions, and are never given English zone numbers.
- Scottish and Northern Irish places have no Flood Zone section, because neither nation uses Flood Zones. Northern Irish flood map data is licensed commercially rather than openly, so Northern Irish places link to Flood Maps NI instead of showing calculated figures.
The Environment Agency's Flood Zones show the extent of land at present day risk of flooding from rivers and the sea, ignoring the benefits of defences, so a place's Flood Zone mix and its headline band can point in different directions. Where at least 10% of homes are in Flood Zone 3 but the rivers and sea band is Low or Very Low, the page explains the difference and links to why a Flood Zone 3 house can be Low risk.
How Is A Place Ranked Within Its County?
A place is ranked within its county by the share of its homes in a Medium or High band for flooding from rivers and the sea, highest first. The measure is named next to every rank, for example "ranks 3 of 14 in Cumbria for rivers and sea flood risk".
- Only places with a page are counted in the denominator, and the page says how many.
- Equal shares get equal ranks (1, 2, 2, 4), rather than being separated arbitrarily.
- Where surface water is a place's main source of risk, a second rank by surface water share is shown.
- Ranks for the whole county are recalculated whenever any place in it changes band.
How Is The Nearest River Gauge Chosen?
The gauge shown on a place page is the closest level-measuring station on the place's main river, or if there is none, the closest station on any river, provided it is within 15 kilometres of the place's centre. Distance is measured in a straight line, not along the river.
The place's main river is the named watercourse with the greatest length inside the place, or the nearest named watercourse within 2 kilometres. Stations are shown only if they have a named river, a named settlement, a typical range and a highest recorded level. If no station qualifies, the gauge section is left out.
How Are Live River Levels, Trends And Warnings Shown?
Live river levels and warnings are read from the Environment Agency's flood-monitoring API and its devolved equivalents every 15 minutes and stored, so pages stay available if an agency's service slows or fails. A page never calls a government service directly.
What each live figure means:
| Figure | Rule |
|---|---|
| Rising, falling or steady | Latest reading compared with the reading about 45 minutes earlier; changes under 2 cm count as steady |
| Above, within or below typical range | Latest reading compared with the station's published typical high and low levels |
| Height above typical range | Latest reading minus the typical high level, in metres |
| Warning duration | Time since the warning was raised |
The agency's own documentation explains a timing limit. Levels are measured usually every 15 minutes, but data transfer back to the Environment Agency is typically once or twice per day, but usually increases during times of heightened flood risk. A reading's time is therefore always shown next to it. If our last check fails, the page shows the previous reading with its time rather than an empty or all-clear state.
How Often Is Each Dataset Refreshed?
Flood risk bands and Flood Zones change only when an agency publishes a new version, which happens months or years apart. Live readings and warnings are checked every 15 minutes. Published statistics and rules are rechecked weekly against their source.
| Data | Changes when | We check |
|---|---|---|
| Flood risk bands, Flood Zones | The agency releases a new version | Dataset "last updated" dates, quarterly; pipeline re-run on change |
| Welsh Flood Map for Planning | late May and late November each year | After each update |
| Gauge metadata | Stations open, close or change range | With each live check |
| Live readings and warnings | Continuously | Every 15 minutes |
| Statistics, rules, scheme figures | The publisher revises its page | Weekly, by comparing the exact quoted wording |
A page's "last updated" date changes only when a headline band, rank, main gauge or at least 20% of a Flood Zone share changes, not when live readings change. Live figures carry their own "data last checked" time.
What Are The Limitations Of These Figures?
These figures describe the risk to an area, not to a single building. The Environment Agency states that its long term flood risk service does not tell you how likely it is that an individual property will flood, and the same limit applies to every figure built from that data.
The known limitations are:
- Area, not property. Floor levels, door thresholds and property flood resilience are not in any national dataset.
- Centre points. A postcode is counted wholly in or out of a zone by its centre point, so postcodes on a boundary can be misassigned.
- Estimated homes. Household counts are estimated from the Census, not counted address by address.
- Sources not mapped. Flooding from sewers, blocked drains and burst pipes is not modelled in the national datasets.
- Different systems in each nation. Welsh, Scottish and Northern Irish figures use their agency's own classification and are not directly comparable with English bands.
- History is curated, not complete. Past flood events appear only where we have verified a source; an empty history section does not mean a place has never flooded.
When Has This Methodology Changed?
Every change to a rule, threshold or data source on this page is recorded below with its date and reason.
| Date | Change | Reason |
|---|---|---|
| the site’s launch | Methodology first published | — |
Contains public sector information licensed under the Open Government Licence v3.0. Flood risk and river level data supplied by the Environment Agency, Natural Resources Wales and the Scottish Environment Protection Agency. floodriskchecker.co.uk is an independent website. It is not run by, or affiliated with, the Environment Agency, Defra, SEPA, Natural Resources Wales or any government body.