diff --git a/src/Standard.php b/src/Standard.php
index d9fda55..ccc3e36 100644
--- a/src/Standard.php
+++ b/src/Standard.php
@@ -547,6 +547,29 @@ public function toArrayPreservingKeys(): array
         return $this->toArray(true);
     }
 
+    /**
+     * Counts seen values online.
+     *
+     * @param ?int &$count the current count; initialized unless provided
+     *
+     * @param-out int $count
+     *
+     * @return $this
+     */
+    public function runningCount(
+        ?int &$count
+    ): self {
+        $count ??= 0;
+
+        $this->cast(static function ($input) use (&$count) {
+            ++$count;
+
+            return $input;
+        });
+
+        return $this;
+    }
+
     /**
      * {@inheritdoc}
      *
diff --git a/tests/CountTest.php b/tests/CountTest.php
index 62d2aeb..ed854ba 100644
--- a/tests/CountTest.php
+++ b/tests/CountTest.php
@@ -25,6 +25,7 @@
 use function Pipeline\fromArray;
 use function Pipeline\map;
 use function Pipeline\take;
+use function range;
 
 /**
  * @covers \Pipeline\Standard
@@ -61,4 +62,20 @@ public function testCountValues(): void
             yield 2 => 3;
         }));
     }
+
+    public function testRunningCount(): void
+    {
+        $pipeline = map(fn () => yield from range(0, 100))
+            ->runningCount($countAll)
+            ->filter(fn (int $n) => $n % 2)
+            ->runningCount($countEvent)
+            ->filter(fn (int $n) => $n % 3);
+
+        $this->assertSame(0, $countAll);
+        $this->assertSame(0, $countEvent);
+
+        $this->assertSame(33, $pipeline->count());
+        $this->assertSame(101, $countAll);
+        $this->assertSame(50, $countEvent);
+    }
 }