diff --git a/backend/src/graph/mod.rs b/backend/src/graph/mod.rs
index cef7e82..126b8a1 100644
--- a/backend/src/graph/mod.rs
+++ b/backend/src/graph/mod.rs
@@ -8,7 +8,7 @@ mod transit_route;
 use anyhow::Result;
 use enum_map::{Enum, EnumMap};
 use geo::{Coord, LineLocatePoint, LineString, MultiPolygon, Point, Polygon};
-use geojson::{Feature, GeoJson, Geometry};
+use geojson::{Feature, FeatureCollection, GeoJson, Geometry};
 use rstar::{primitives::GeomWithData, RTree};
 use serde::{Deserialize, Serialize};
 use utils::Mercator;
@@ -190,12 +190,27 @@ impl Graph {
     /// Returns a GeoJSON string
     pub fn render_zones(&self) -> Result<String> {
         let mut features = Vec::new();
+        let mut max_density: f64 = 0.0;
         for zone in &self.zones {
             let mut f = Feature::from(Geometry::from(&self.mercator.to_wgs84(&zone.geom)));
             f.set_property("population", zone.population);
+            f.set_property("density", zone.density);
             features.push(f);
+
+            max_density = max_density.max(zone.density);
         }
-        Ok(serde_json::to_string(&GeoJson::from(features))?)
+        Ok(serde_json::to_string(&FeatureCollection {
+            features,
+            bbox: None,
+            foreign_members: Some(
+                serde_json::json!({
+                    "max_density": max_density,
+                })
+                .as_object()
+                .unwrap()
+                .clone(),
+            ),
+        })?)
     }
 }
 
@@ -257,4 +272,6 @@ pub struct Zone {
     pub geom: MultiPolygon,
     // TODO Later on, this could be generic or user-supplied
     pub population: u32,
+    // People per square km
+    pub density: f64,
 }
diff --git a/backend/src/graph/scrape.rs b/backend/src/graph/scrape.rs
index aededb7..4dec7e4 100644
--- a/backend/src/graph/scrape.rs
+++ b/backend/src/graph/scrape.rs
@@ -3,7 +3,7 @@ use std::collections::HashMap;
 use anyhow::Result;
 use enum_map::EnumMap;
 use flatgeobuf::{FeatureProperties, FgbFeature, GeozeroGeometry, HttpFgbReader};
-use geo::{Coord, EuclideanLength, MultiPolygon};
+use geo::{Area, Coord, EuclideanLength, MultiPolygon};
 use muv_osm::{AccessLevel, TMode};
 use osm_reader::OsmID;
 use rstar::RTree;
@@ -303,10 +303,14 @@ async fn load_zones(url: String, mercator: &Mercator) -> Result<Vec<Zone>> {
         // TODO Could intersect with boundary_polygon, but some extras nearby won't hurt anything
         let mut geom = get_multipolygon(feature)?;
         mercator.to_mercator_in_place(&mut geom);
+        let area_km2 = 1e-6 * geom.unsigned_area();
+        // TODO Re-encode as UInt
+        let population = feature.property::<i64>("population")?.try_into()?;
+
         zones.push(Zone {
             geom,
-            // TODO Re-encode as UInt
-            population: feature.property::<i64>("population")?.try_into()?,
+            population,
+            density: (population as f64) / area_km2,
         });
     }
     Ok(zones)
diff --git a/web/src/App.svelte b/web/src/App.svelte
index 06925d7..51eb4e7 100644
--- a/web/src/App.svelte
+++ b/web/src/App.svelte
@@ -170,7 +170,9 @@
         {/if}
 
         {#if $showPopulation}
-          <PopulationLayer />
+          {#await notNull($backend).renderZones() then gj}
+            <PopulationLayer {gj} />
+          {/await}
         {/if}
       {/if}
     </MapLibre>
diff --git a/web/src/colors.ts b/web/src/colors.ts
index 0eb1f10..1489d7c 100644
--- a/web/src/colors.ts
+++ b/web/src/colors.ts
@@ -1,3 +1,4 @@
+// From colorbrewer
 export const colorScale = [
   "#CDE594",
   "#80C6A3",
@@ -5,3 +6,12 @@ export const colorScale = [
   "#186290",
   "#080C54",
 ];
+
+// Just something different,
+export const populationColorScale = [
+  "#ffffb2",
+  "#fecc5c",
+  "#fd8d3c",
+  "#f03b20",
+  "#bd0026",
+];
diff --git a/web/src/common/PopulationLayer.svelte b/web/src/common/PopulationLayer.svelte
index 83bcdfb..4171384 100644
--- a/web/src/common/PopulationLayer.svelte
+++ b/web/src/common/PopulationLayer.svelte
@@ -5,23 +5,35 @@
     FillLayer,
     hoverStateFilter,
   } from "svelte-maplibre";
-  import { notNull } from "svelte-utils";
-  import { Popup } from "svelte-utils/map";
-  import { backend } from "../stores";
+  import type { FeatureCollection } from "geojson";
+  import { makeColorRamp, Popup } from "svelte-utils/map";
+  import { populationColorScale } from "../colors";
+
+  export let gj: FeatureCollection;
+
+  // TODO Should the limits be fixed? But this varies so much regionally
+  let limits = Array.from(Array(6).keys()).map(
+    (i) => (gj.max_density / (6 - 1)) * i,
+  );
 </script>
 
-{#await notNull($backend).renderZones() then data}
-  <GeoJSON {data} generateId>
-    <FillLayer
-      manageHoverState
-      paint={{
-        "fill-color": "red",
-        "fill-opacity": hoverStateFilter(0.2, 0.8),
-      }}
+<GeoJSON data={gj} generateId>
+  <FillLayer
+    manageHoverState
+    paint={{
+      "fill-color": makeColorRamp(
+        ["get", "density"],
+        limits,
+        populationColorScale,
+      ),
+      "fill-opacity": hoverStateFilter(0.2, 0.8),
+    }}
+  >
+    <Popup openOn="hover" let:props
+      >{props.population.toLocaleString()} people live here ({Math.round(
+        props.density,
+      ).toLocaleString()} people / square kilometer)</Popup
     >
-      <Popup openOn="hover" let:props>{props.population.toLocaleString()}</Popup
-      >
-    </FillLayer>
-    <LineLayer paint={{ "line-color": "black", "line-width": 1 }} />
-  </GeoJSON>
-{/await}
+  </FillLayer>
+  <LineLayer paint={{ "line-color": "black", "line-width": 1 }} />
+</GeoJSON>