diff --git a/app/public/dashboard-ui.js b/app/public/dashboard-ui.js
index 900ee32..0b0d7f9 100644
--- a/app/public/dashboard-ui.js
+++ b/app/public/dashboard-ui.js
@@ -301,7 +301,13 @@ function medianIntervalMin() {
   d.sort((a, b) => a - b);
   return d[d.length >> 1] / 60000;
 }
-function coverageFor(row, perDay) { return row && perDay ? Math.min(100, (row.n / perDay) * 100) : null; }
+// Share of the day's 288 five-minute slots that have at least one reading, so it doesn't matter whether the
+// station reports every 60, 100 or 120 s. Days saved before slots were tracked fall back to reading count.
+function coverageFor(row, perDay) {
+  if (!row) return null;
+  if (row.slots != null) return Math.min(100, (row.slots / 288) * 100);
+  return perDay ? Math.min(100, (row.n / perDay) * 100) : null;
+}
 
 function renderHealthCard() {
   const host = $('healthCard');
@@ -311,7 +317,8 @@ function renderHealthCard() {
   const interval = medianIntervalMin();
   const perDay = interval ? 1440 / interval : null;
   const past = DAILY.filter((d) => d.d !== todayStr()).slice(-30);
-  const cov = perDay && past.length ? mean(past.map((d) => coverageFor(d, perDay))) : null;
+  const covVals = past.map((d) => coverageFor(d, perDay)).filter((v) => v != null);
+  const cov = covVals.length ? mean(covVals) : null;
   const t = todayRow();
   host.innerHTML = `<div class="ui-card"><h3>Station Health</h3><div class="kv">
     <div><b><span class="pill ${ok ? 'ok' : 'warn'}">${ok ? 'Online' : 'Delayed'}</span></b><span>Last report</span><small>${ageMin <= 0 ? 'just now' : ageMin + ' min ago'}</small></div>
@@ -530,8 +537,8 @@ function renderInsights() {
   const interval = medianIntervalMin();
   const perDay = interval ? 1440 / interval : null;
   const cov = past.slice(-45);
-  if (perDay) {
-    const vals = cov.map((d) => +coverageFor(d, perDay).toFixed(0));
+  if (cov.length) {
+    const vals = cov.map((d) => +(coverageFor(d, perDay) ?? 0).toFixed(0));
     const ds = barDataset('Coverage %', vals, c.cool);
     ds.backgroundColor = vals.map((v) => (v < 90 ? c.accent + 'bb' : c.cool + 'bb'));
     makeChart('inCoverage', 'bar', [ds], cov.map((d) => prettyDay(d.d)), '%', { scales: { y: { min: 0, max: 100 } } });
diff --git a/app/server.js b/app/server.js
index a12e6f4..1ef0abe 100644
--- a/app/server.js
+++ b/app/server.js
@@ -220,6 +220,27 @@ function stationLocalDay(r) {
   return f.format(new Date(r.dateutc));
 }
 
+// Local 5-minute slot (0-287) of a reading, for interval-independent coverage.
+const slotFmtCache = new Map();
+function stationLocalSlot(r) {
+  const tz = r.tz || stationTzFallback;
+  let f = slotFmtCache.get(tz);
+  if (!f) {
+    try { f = new Intl.DateTimeFormat('en-GB', { timeZone: tz, hourCycle: 'h23', hour: '2-digit', minute: '2-digit' }); }
+    catch { f = new Intl.DateTimeFormat('en-GB', { timeZone: stationTzFallback, hourCycle: 'h23', hour: '2-digit', minute: '2-digit' }); }
+    slotFmtCache.set(tz, f);
+  }
+  const p = Object.fromEntries(f.formatToParts(new Date(r.dateutc)).map(x => [x.type, x.value]));
+  return Math.min(287, Math.floor((Number(p.hour) * 60 + Number(p.minute)) / 5));
+}
+// day.sl is a 288-char string of 0/1: which 5-minute slots of the local day have at least one reading.
+function markSlot(day, slot) {
+  const sl = day.sl && day.sl.length === 288 ? day.sl : '0'.repeat(288);
+  if (sl[slot] === '1') { day.sl = sl; return false; }
+  day.sl = sl.slice(0, slot) + '1' + sl.slice(slot + 1);
+  return true;
+}
+
 // Per-day set of dateutc values already on disk. Shared by the live recorder and
 // the backfill so neither can write a reading the other (or a prior run) saved.
 const stationSeen = new Map();
@@ -365,6 +386,7 @@ function updateRecordsFromReading(obs) {
   applyReadingToRecords(obs);
   const day = recordsStore.days[stationLocalDay(obs)];
   day.n = (day.n || 0) + 1;
+  markSlot(day, stationLocalSlot(obs));
   addAggregates(day, obs);
   recordsDirty = true;
 }
@@ -375,6 +397,7 @@ function rebuildRecordsFromFiles() {
   try { files = fs.readdirSync(STATION_DIR).filter(f => f.endsWith('.jsonl')).sort(); } catch { return 0; }
   const counts = {};
   const fresh = {};
+  const slotsFresh = {};
   let scanned = 0;
   for (const f of files) {
     try {
@@ -387,6 +410,7 @@ function rebuildRecordsFromFiles() {
         applyReadingToRecords(r);
         const k = stationLocalDay(r);
         counts[k] = (counts[k] || 0) + 1;
+        markSlot(slotsFresh[k] || (slotsFresh[k] = {}), stationLocalSlot(r));
         addAggregates(fresh[k] || (fresh[k] = {}), r);
         scanned++;
       }
@@ -396,6 +420,9 @@ function rebuildRecordsFromFiles() {
   for (const [k, n] of Object.entries(counts)) {
     const day = recordsStore.days[k];
     day.n = Math.max(day.n || 0, n);
+    // union with what live recording already marked (pruned raw files must not lose coverage)
+    const fs_ = slotsFresh[k]?.sl;
+    if (fs_) { const cur = day.sl && day.sl.length === 288 ? day.sl : '0'.repeat(288); day.sl = [...fs_].map((c, i) => (c === '1' || cur[i] === '1') ? '1' : '0').join(''); }
     // Replace aggregates only when the files hold at least as many samples as the store did
     const f = fresh[k];
     if (f.tn != null && f.tn >= (day.tn || 0)) { day.tsum = f.tsum; day.tn = f.tn; }
@@ -1992,6 +2019,7 @@ app.get('/api/station/daily', requireDashboardAuth, (req, res) => {
   if (limit) entries = entries.slice(-limit);
   res.json(entries.map(([d, v]) => ({
     d, n: v.n || 0,
+    slots: v.sl ? (v.sl.match(/1/g) || []).length : null, // 5-min slots with data (of 288)
     hi: v.hi?.[0] ?? null, hiT: v.hi?.[1] ?? null, lo: v.lo?.[0] ?? null, loT: v.lo?.[1] ?? null,
     avg: v.tn ? +(v.tsum / v.tn).toFixed(1) : null,
     gust: v.gust?.[0] ?? null, gustT: v.gust?.[1] ?? null,