diff --git a/src/controllers/metrics.ts b/src/controllers/metrics.ts
index 567a3170..e85294c0 100644
--- a/src/controllers/metrics.ts
+++ b/src/controllers/metrics.ts
@@ -2,11 +2,12 @@ import { register } from 'prom-client';
 
 import { AppController } from '@/app.ts';
 import { DittoDB } from '@/db/DittoDB.ts';
-import { dbAvailableConnectionsGauge } from '@/metrics.ts';
+import { dbAvailableConnectionsGauge, dbPoolSizeGauge } from '@/metrics.ts';
 
 /** Prometheus/OpenMetrics controller. */
 export const metricsController: AppController = async (c) => {
   // Update some metrics at request time.
+  dbPoolSizeGauge.set(DittoDB.poolSize);
   dbAvailableConnectionsGauge.set(DittoDB.availableConnections);
 
   const metrics = await register.metrics();
diff --git a/src/metrics.ts b/src/metrics.ts
index ee73d083..ac1db2ee 100644
--- a/src/metrics.ts
+++ b/src/metrics.ts
@@ -70,6 +70,11 @@ export const dbEventsCounter = new Counter({
   labelNames: ['kind'],
 });
 
+export const dbPoolSizeGauge = new Gauge({
+  name: 'ditto_db_pool_size',
+  help: 'Number of connections in the database pool',
+});
+
 export const dbAvailableConnectionsGauge = new Gauge({
   name: 'ditto_db_available_connections',
   help: 'Number of available connections in the database pool',