diff --git a/src/diagnostics.test.ts b/src/diagnostics.test.ts
index 56f92ec..fa49f49 100644
--- a/src/diagnostics.test.ts
+++ b/src/diagnostics.test.ts
@@ -1,6 +1,6 @@
 import { describe, it } from "node:test";
 import assert from "node:assert/strict";
-import { parseSfuMetrics } from "./diagnostics";
+import { parseSfuMetrics, sfuMetricsUrl } from "./diagnostics";
 
 describe("parseSfuMetrics", () => {
   it("keeps gryt_sfu series, sums labelled ones and drops the rest", () => {
@@ -19,3 +19,13 @@ describe("parseSfuMetrics", () => {
     });
   });
 });
+
+describe("sfuMetricsUrl", () => {
+  it("prefers SFU_METRICS_URL, and otherwise guesses port 9091 on the SFU's host", () => {
+    assert.equal(sfuMetricsUrl({ SFU_METRICS_URL: "http://x:1/m", SFU_WS_HOST: "ws://sfu:9092" }), "http://x:1/m");
+    assert.equal(sfuMetricsUrl({ SFU_WS_HOST: "ws://sfu:9092" }), "http://sfu:9091/metrics");
+    assert.equal(sfuMetricsUrl({ SFU_WS_HOST: "wss://sfu.example.com" }), "http://sfu.example.com:9091/metrics");
+    assert.equal(sfuMetricsUrl({ SFU_WS_HOST: "ws://[::1]:9092" }), "http://[::1]:9091/metrics");
+    assert.equal(sfuMetricsUrl({}), undefined);
+  });
+});
diff --git a/src/diagnostics.ts b/src/diagnostics.ts
index f373fc9..562c77e 100644
--- a/src/diagnostics.ts
+++ b/src/diagnostics.ts
@@ -6,7 +6,7 @@ export interface DiagnosticsSnapshot {
   uptimeSeconds: number;
   online: number;
   sfu: ReturnType<SFUClient["getConnectionStatus"]> | null;
-  /** Null when SFU_METRICS_URL is unset or the SFU did not answer. */
+  /** Null when the SFU's metrics did not answer. */
   sfuMetrics: Record<string, number> | null;
   voice: {
     channelId: string;
@@ -34,6 +34,17 @@ export function parseSfuMetrics(text: string): Record<string, number> {
   return out;
 }
 
+/** SFU_METRICS_URL, or the SFU's default metrics port on the host SFU_WS_HOST names. */
+export function sfuMetricsUrl(env: NodeJS.ProcessEnv = process.env): string | undefined {
+  if (env.SFU_METRICS_URL) return env.SFU_METRICS_URL;
+  try {
+    const host = new URL(env.SFU_WS_HOST ?? "").hostname;
+    return host ? `http://${host}:9091/metrics` : undefined;
+  } catch {
+    return undefined;
+  }
+}
+
 async function readSfuMetrics(url: string | undefined): Promise<Record<string, number> | null> {
   if (!url) return null;
   try {
@@ -72,7 +83,7 @@ export async function collectDiagnostics(
     uptimeSeconds: Math.round(process.uptime()),
     online,
     sfu: sfuClient?.getConnectionStatus() ?? null,
-    sfuMetrics: await readSfuMetrics(process.env.SFU_METRICS_URL),
+    sfuMetrics: await readSfuMetrics(sfuMetricsUrl()),
     voice: [...rooms].map(([channelId, members]) => ({ channelId, members })),
   };
 }