What 20mm of rain actually means

20mm of rain means 20 litres of water landing on every square metre of ground. That is the whole trick to reading rainfall figures: one millimetre of rain equals one litre per square metre. Once you hold onto that, forecast numbers stop being abstract and start being volumes of water you can picture.

Why rain is measured in millimetres

Rainfall is measured as depth: how deep the water would sit if none of it drained, soaked in or evaporated. A rain gauge is essentially a straight-sided container — 20mm of rain fills it to a depth of 20mm.

Depth is useful because it scales with area. The same 20mm falls on your patio, your street and the whole river catchment upstream of your town. The depth stays the same; the volume grows with the area — and that is where the numbers get big.

20mm in real volumes

Take that 20 litres per square metre and multiply it up:

• A typical house roof (say 50m²): around 1,000 litres — a tonne of water — funnelled into the gutters and downpipes.
• A short residential street (roughly 2,000m² of road and pavement): about 40,000 litres, most of it heading for the drains because tarmac absorbs almost nothing.
• A football pitch (about 7,000m²): roughly 140,000 litres — enough to fill a decent-sized swimming pool.
• A modest river catchment of 100km²: around 2 billion litres. Even if only a fraction of that reaches the river, it explains why river levels can rise sharply hours after the rain has stopped falling upstream.

None of this water vanishes. It soaks into soil, sits on the surface, runs into drains, or makes its way to a watercourse. Flooding is what happens when more arrives than those routes can take.

Intensity vs total: 20mm in an hour is not 20mm in a day

The single most important distinction in any rainfall forecast is how fast the rain falls, not just how much.

20mm spread over 24 hours is a thoroughly wet day — steady rain for hours — but the ground and the drains usually have time to keep pace. Soil absorbs at a certain rate; drains carry water away at a certain rate. Give them a full day and 20mm is generally manageable, unless the ground is already saturated.

20mm in a single hour is an intense downpour, the kind that arrives with summer thunderstorms. Water lands faster than soil can absorb it and faster than urban drainage was designed to carry it, so it ponds on roads, backs up out of gullies and flows across the surface. This is the classic recipe for surface water flooding — which can happen well away from any river.

The same total, wildly different outcomes. It is why the Met Office issues warnings for short, intense bursts that would look unremarkable as daily totals, and why a forecast of "20mm possible in an hour" deserves more attention than "20mm through the day".

How much rain is normal in the UK?

For context: much of lowland England averages somewhere in the region of 600–900mm of rain a year, while parts of the western uplands — Snowdonia, the Lake District, the western Highlands — receive several times that. Spread across a year, that works out at a couple of millimetres a day on average, though of course rain doesn't fall on a schedule.

Against that backdrop:

• 1–5mm in a day — light rain, barely worth a mention.
• 10mm in a day — a properly wet day.
• 20–30mm in a day — a very wet day; often the sort of total that accompanies a named storm or a slow-moving front.
• 50mm+ in a day — exceptional for most of the UK, and frequently associated with flooding somewhere.

These are rough guides rather than thresholds — what counts as remarkable varies a lot by region. A total that would be unusual in Cambridgeshire is routine in Cumbria. You can browse what real gauges have recorded near you on FloodRadar's rainfall explorer.

When do totals start mattering for flooding?

There is no magic number at which flooding begins, because the rain is only half the story. The same 20mm may increase flood risk substantially in one situation and pass without incident in another. What tips the balance:

What the ground can take

Dry, well-drained soil can absorb a great deal. Saturated ground after a wet fortnight absorbs almost nothing — at that point even modest rain runs straight off, as if the whole landscape had been paved. This is why the same storm can be harmless in September and cause flooding in January.

Where it lands

Cities shed water fast: roofs, roads and car parks send nearly everything into drains built for typical storms, not extreme ones. Steep, rocky upland catchments also respond quickly, which is why small rivers in hilly areas can rise within an hour or two of heavy rain. Large lowland rivers respond more slowly but keep rising for days as water works its way downstream.

What has already fallen

Flooding is often caused less by one dramatic day than by an accumulation: a wet month that saturates the ground, then a final band of rain that has nowhere to go. Rivers already running high have little headroom left.

Reading a forecast like a hydrologist

Next time you see a rainfall figure, run through three quick questions:

1. How fast? A total crammed into an hour or two is far more likely to cause surface water problems than the same total over a day.
2. Onto what? Saturated ground, urban areas and steep catchments turn rain into runoff quickly; dry lowland soil buys time.
3. On top of what? Check whether it has been a wet week and whether rivers are already elevated — recent rain matters as much as forecast rain.

20mm is not, by itself, a flood. It is 20 litres on every square metre — and whether that water drains quietly away or ends up somewhere it shouldn't depends on speed, surface and what came before. The number tells you how much water is coming; the ground decides what happens next.