diff --git a/docs/CURRENT-STATE.md b/docs/CURRENT-STATE.md
index 87b918a4e..71aa36964 100644
--- a/docs/CURRENT-STATE.md
+++ b/docs/CURRENT-STATE.md
@@ -2221,6 +2221,7 @@ accessible name remains "Don't show this again". Control behavior is unchanged.
 ## FIRMS source status
 
 The FIRMS CSV parser rejects impossible acquisition calendar dates before the proxy filters records to the trailing 24 hours, while retaining valid UTC acquisition times.
+FIRMS ages round to whole hours before choosing a unit, so a 47.6-hour age reads "2d", not "48h".
 The `get_active_fires` tool reports acquisition times as UTC ISO timestamps,
 including zero-padded hours and minutes; missing or invalid dates and times
 remain `null`, including impossible calendar dates from a custom source.
diff --git a/src/layers/firms/model.js b/src/layers/firms/model.js
index 9e3cf7f95..7dcf255c8 100644
--- a/src/layers/firms/model.js
+++ b/src/layers/firms/model.js
@@ -526,7 +526,8 @@ export function createModel({
     if (!Number.isFinite(deltaMs) || deltaMs < 0) return '';
     const hours = deltaMs / 3600000;
     if (hours < 1) return '<1h';
-    if (hours < 48) return `${Math.round(hours)}h`;
+    const wholeHours = Math.round(hours);
+    if (wholeHours < 48) return `${wholeHours}h`;
     return `${Math.round(hours / 24)}d`;
   }
 
@@ -561,8 +562,9 @@ export function createModel({
     if (!Number.isFinite(deltaMs) || deltaMs < 0) return '';
     const hours = deltaMs / 3600000;
     if (hours < 1) return t('<1h', { context: 'age' });
-    if (hours < 48)
-      return t('{count}h', { context: 'age', count: Math.round(hours) });
+    const wholeHours = Math.round(hours);
+    if (wholeHours < 48)
+      return t('{count}h', { context: 'age', count: wholeHours });
     return t('{count}d', { context: 'age', count: Math.round(hours / 24) });
   }
 
diff --git a/src/layers/firms/model.test.mjs b/src/layers/firms/model.test.mjs
new file mode 100644
index 000000000..7ff12cf3f
--- /dev/null
+++ b/src/layers/firms/model.test.mjs
@@ -0,0 +1,24 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import { getLocale, setLocale } from '../../i18n/index.js';
+import { createModel } from './model.js';
+
+const HOUR_MS = 3600000;
+
+test('a fire age that rounds to 48 hours reads in days, as 48 hours does', () => {
+  const previousLocale = getLocale();
+  const { formatAge, formatAgeLabel } = createModel({
+    services: { anchors: {} },
+  });
+  try {
+    setLocale('en');
+    assert.equal(formatAge(47.4 * HOUR_MS), '47h');
+    assert.equal(formatAge(47.6 * HOUR_MS), '2d');
+    assert.equal(formatAge(48 * HOUR_MS), '2d');
+    assert.equal(formatAgeLabel(47.4 * HOUR_MS), '47h');
+    assert.equal(formatAgeLabel(47.6 * HOUR_MS), formatAgeLabel(48 * HOUR_MS));
+    assert.equal(formatAgeLabel(48 * HOUR_MS), '2d');
+  } finally {
+    setLocale(previousLocale);
+  }
+});