diff --git a/lib/fetch/body.js b/lib/fetch/body.js
index 130a0bc669b..ea245e646e0 100644
--- a/lib/fetch/body.js
+++ b/lib/fetch/body.js
@@ -142,7 +142,37 @@ function extractBody (object, keepalive = false) {
     source = object
 
     // Set length to unclear, see html/6424 for improving this.
-    // TODO
+    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="${escape(normalizeLinefeeds(name))}"`) +
+            Buffer.byteLength(`\r\n\r\n${normalizeLinefeeds(value)}\r\n`)
+        } else {
+          bodyLength +=
+            prefixLength +
+            Buffer.byteLength(`; name="${escape(normalizeLinefeeds(name))}"`) +
+            (value.name ? Buffer.byteLength(`; filename="${escape(value.name)}"`) : 0) +
+            2 + // \r\n
+            `Content-Type: ${
+              value.type || 'application/octet-stream'
+            }\r\n\r\n`.length
+
+          // value is a Blob or File
+          bodyLength += value.size
+
+          bodyLength += 2 // \r\n
+        }
+      }
+
+      bodyLength += boundaryLength + 4 // --boundary--
+      return bodyLength
+    })()
 
     // Set type to `multipart/form-data; boundary=`,
     // followed by the multipart/form-data boundary string generated
diff --git a/test/fetch/content-length.js b/test/fetch/content-length.js
new file mode 100644
index 00000000000..9264091069b
--- /dev/null
+++ b/test/fetch/content-length.js
@@ -0,0 +1,29 @@
+'use strict'
+
+const { test } = require('tap')
+const { createServer } = require('http')
+const { once } = require('events')
+const { Blob } = require('buffer')
+const { fetch, FormData } = require('../..')
+
+// https://github.com/nodejs/undici/issues/1783
+test('Content-Length is set when using a FormData body with fetch', async (t) => {
+  const server = createServer((req, res) => {
+    // TODO: check the length's value once the boundary has a fixed length
+    t.ok('content-length' in req.headers) // request has content-length header
+    t.ok(!Number.isNaN(Number(req.headers['content-length'])))
+    res.end()
+  }).listen(0)
+
+  await once(server, 'listening')
+  t.teardown(server.close.bind(server))
+
+  const fd = new FormData()
+  fd.set('file', new Blob(['hello world 👋'], { type: 'text/plain' }), 'readme.md')
+  fd.set('string', 'some string value')
+
+  await fetch(`http://localhost:${server.address().port}`, {
+    method: 'POST',
+    body: fd
+  })
+})