diff --git a/handlers/max_request_size.go b/handlers/max_request_size.go
index 939c7dde..9700ca67 100644
--- a/handlers/max_request_size.go
+++ b/handlers/max_request_size.go
@@ -41,18 +41,24 @@ func NewMaxRequestSize(cfg *config.Config, logger *slog.Logger) *MaxRequestSize
 
 func (m *MaxRequestSize) ServeHTTP(rw http.ResponseWriter, r *http.Request, next http.HandlerFunc) {
 	logger := LoggerWithTraceInfo(m.logger, r)
-	reqSize := len(r.Method) + len(r.URL.RequestURI()) + len(r.Proto) + 5 // add 5 bytes for space-separation of method, URI, protocol, and /r/n
 
-	for k, v := range r.Header {
-		valueLen := 0
-		for _, value := range r.Header.Values(k) {
-			valueLen += len(value)
+	// Four additional bytes for the two spaces and \r\n:
+	//   GET / HTTP/1.1\r\n
+	reqSize := len(r.Method) + len(r.URL.RequestURI()) + len(r.Proto) + 4
+
+	// Host header which is not passed on to us, plus eight bytes for 'Host: ' and \r\n
+	reqSize += len(r.Host) + 8
+
+	// Go doesn't split header values on commas, instead it only splits the value when it's
+	// provided via repeated header keys. Therefore we have to account for each value of a repeated
+	// header as well as its key.
+	for k, vv := range r.Header {
+		for _, v := range vv {
+			// Four additional bytes for the colon and space after the header key and \r\n.
+			reqSize += len(k) + len(v) + 4
 		}
-		reqSize += len(k) + valueLen + 4 + len(v) - 1 // add padding for ': ' and newlines and comma delimiting of multiple values
 	}
 
-	reqSize += len(r.Host) + 8 // add padding for "Host: " and newlines
-
 	if reqSize >= m.MaxSize {
 		reqInfo, err := ContextRequestInfo(r)
 		if err != nil {
diff --git a/handlers/max_request_size_test.go b/handlers/max_request_size_test.go
index 79406a84..863cce50 100644
--- a/handlers/max_request_size_test.go
+++ b/handlers/max_request_size_test.go
@@ -166,6 +166,17 @@ var _ = Describe("MaxRequestSize", func() {
 			Expect(result.StatusCode).To(Equal(http.StatusRequestHeaderFieldsTooLarge))
 		})
 	})
+	Context("when a repeated header has a short value and long key taking it over the limit", func() {
+		BeforeEach(func() {
+			for i := 0; i < 10; i++ {
+				header.Add("foobar", "m")
+			}
+		})
+		It("throws an http 431", func() {
+			handleRequest()
+			Expect(result.StatusCode).To(Equal(http.StatusRequestHeaderFieldsTooLarge))
+		})
+	})
 	Context("when enough normally-sized headers put the request over the limit", func() {
 		BeforeEach(func() {
 			header.Add("header1", "smallRequest")