What you're looking at

Each shaded area is a postcode sector — the postcode up to the first digit after the space (e.g. SW1A 1). A sector covers roughly 3,000 addresses, and there are about 9,000 across Great Britain, so it's a good middle ground: finer than a town, broader than a street. Every active supporter is placed in their sector from their (decrypted) postcode.

The three views

• Volume — raw supporter count, shaded pale amber → deep red. A square-root scale keeps smaller areas visible. This mostly shows where people live — cities dominate.
• Penetration — supporters per 1,000 residents, shaded pale → deep teal. This divides out population, so it shows engagement depth: where we reach a high share of the local population, regardless of how big the area is.
• vs Population — an index on a blue–white–red scale. Blue = under-indexed (room to grow), white ≈ in line, red = over-indexed (we punch above our weight here).

How the “vs Population” index (weighting) works

It's a location quotient: a sector's share of our supporters divided by that sector's share of the GB population.

index = (sector supporters ÷ all supporters) ÷ (sector population ÷ all population)

So 1.0 means our presence there is exactly proportional to its population. 1.8 means we're 80% over-represented; 0.3 means we have less than a third of the supporters that area's size would predict. It strips out the “big places have more people” effect, surfacing genuine over- and under-performance.

Why some areas drop out of the per-capita views

Penetration and the index need a reliable resident population. Sectors with fewer than 500 residents (city-centre / business postcodes like a Westminster or a city-centre sector) are excluded from those two views — a handful of supporters over almost no residents would otherwise read as a wild over-index and drown out the real signal. They still appear under Volume.

Privacy & the charity toggle

Postcodes are decrypted on the server only and reduced to sector; any sector with fewer than 5 supporters is hidden, and individual postcodes never reach your browser. Include charity-located (off by default) optionally adds supporters who have no postcode, placed at their charity's registered location; the <5 suppression is re-applied whenever you change that.

Data & accuracy

Population is from the 2021 Census (ONS), apportioned to sectors via the ONS Postcode Directory. Boundaries are reconstructed postcode-sector polygons from Open Door Logistics. Both are close approximations rather than survey-exact — ideal for spotting patterns, not for legal precision. Scotland & Northern Ireland don't yet have population loaded, so they appear under Volume but are blank in the per-capita views.