diff --git a/lib/fetch/body.js b/lib/fetch/body.js
index 920d873f2cb..3212631fb9b 100644
--- a/lib/fetch/body.js
+++ b/lib/fetch/body.js
@@ -99,7 +99,7 @@ function extractBody (object, keepalive = false) {
     // Set source to a copy of the bytes held by object.
     source = new Uint8Array(object.buffer.slice(object.byteOffset, object.byteOffset + object.byteLength))
   } else if (util.isFormDataLike(object)) {
-    const boundary = '----formdata-undici-' + Math.random()
+    const boundary = `----formdata-undici-${Math.random()}`.replace('.', '').slice(0, 32)
     const prefix = `--${boundary}\r\nContent-Disposition: form-data`
 
     /*! formdata-polyfill. MIT License. Jimmy Wärting <https://jimmy.warting.se/opensource> */
@@ -109,68 +109,49 @@ function extractBody (object, keepalive = false) {
 
     // Set action to this step: run the multipart/form-data
     // encoding algorithm, with object’s entry list and UTF-8.
-    action = async function * (object) {
-      const enc = new TextEncoder()
-
-      for (const [name, value] of object) {
-        if (typeof value === 'string') {
-          yield enc.encode(
-            prefix +
-              `; name="${escape(normalizeLinefeeds(name))}"` +
-              `\r\n\r\n${normalizeLinefeeds(value)}\r\n`
-          )
-        } else {
-          yield enc.encode(
-            prefix +
-              `; name="${escape(normalizeLinefeeds(name))}"` +
-              (value.name ? `; filename="${escape(value.name)}"` : '') +
-              '\r\n' +
-              `Content-Type: ${
-                value.type || 'application/octet-stream'
-              }\r\n\r\n`
-          )
-
-          yield * value.stream()
-
-          // '\r\n' encoded
-          yield new Uint8Array([13, 10])
-        }
+    // - This ensures that the body is immutable and can't be changed afterwords
+    // - That the content-length is calculated in advance.
+    // - And that all parts are pre-encoded and ready to be sent.
+
+    const enc = new TextEncoder()
+    const blobParts = []
+    const rn = new Uint8Array([13, 10]) // '\r\n'
+    length = 0
+
+    for (const [name, value] of object) {
+      if (typeof value === 'string') {
+        const chunk = enc.encode(prefix +
+          `; name="${escape(normalizeLinefeeds(name))}"` +
+          `\r\n\r\n${normalizeLinefeeds(value)}\r\n`)
+        blobParts.push(chunk)
+        length += chunk.byteLength
+      } else {
+        const chunk = enc.encode(`${prefix}; name="${escape(normalizeLinefeeds(name))}"` +
+          (value.name ? `; filename="${escape(value.name)}"` : '') + '\r\n' +
+          `Content-Type: ${
+            value.type || 'application/octet-stream'
+          }\r\n\r\n`)
+        blobParts.push(chunk, value, rn)
+        length += chunk.byteLength + value.size + rn.byteLength
       }
-
-      yield enc.encode(`--${boundary}--`)
     }
 
+    const chunk = enc.encode(`--${boundary}--`)
+    blobParts.push(chunk)
+    length += chunk.byteLength
+
     // Set source to object.
     source = object
 
-    // Set length to unclear, see html/6424 for improving this.
-    length = (() => {
-      const prefixLength = prefix.length
-      const boundaryLength = boundary.length
-      let bodyLength = 0
-
-      for (const [name, value] of object) {
-        if (typeof value === 'string') {
-          bodyLength +=
-            prefixLength +
-            Buffer.byteLength(`; name="${escape(normalizeLinefeeds(name))}"\r\n\r\n${normalizeLinefeeds(value)}\r\n`)
+    action = async function * () {
+      for (const part of blobParts) {
+        if (part.stream) {
+          yield * part.stream()
         } else {
-          bodyLength +=
-            prefixLength +
-            Buffer.byteLength(`; name="${escape(normalizeLinefeeds(name))}"` + (value.name ? `; filename="${escape(value.name)}"` : '')) +
-            2 + // \r\n
-            `Content-Type: ${
-              value.type || 'application/octet-stream'
-            }\r\n\r\n`.length
-
-          // value is a Blob or File, and \r\n
-          bodyLength += value.size + 2
+          yield part
         }
       }
-
-      bodyLength += boundaryLength + 4 // --boundary--
-      return bodyLength
-    })()
+    }
 
     // Set type to `multipart/form-data; boundary=`,
     // followed by the multipart/form-data boundary string generated