diff --git a/src/util/PeakBuffer.hxx b/src/util/PeakBuffer.hxx
index c3d871035..febf8bd73 100644
--- a/src/util/PeakBuffer.hxx
+++ b/src/util/PeakBuffer.hxx
@@ -55,6 +55,10 @@ public:
 	PeakBuffer(const PeakBuffer &) = delete;
 	PeakBuffer &operator=(const PeakBuffer &) = delete;
 
+	std::size_t max_size() const noexcept {
+		return normal_size + peak_size;
+	}
+
 	gcc_pure
 	bool empty() const noexcept;