diff --git a/src/HTTP/Request.js b/src/HTTP/Request.js
index 1ce9e68..590de1f 100644
--- a/src/HTTP/Request.js
+++ b/src/HTTP/Request.js
@@ -1,2 +1,16 @@
 /** @type {(_: Blob) => () => Promise<ArrayBuffer>} */
 export const blobArrayBufferImpl = b => () => b.arrayBuffer()
+
+/** @type {(_: FormDataEntryValue) => number} */
+const formDataValueSize = v => (typeof v === 'string' ? v.length : v.size)
+
+/** @type {(_: ArrayBuffer | FormData) => () => number} */
+export const rawRequestBodySize = body => () =>
+  body instanceof ArrayBuffer
+    ? body.byteLength
+    : body instanceof FormData
+    ? Array.from(body.entries()).reduce(
+        (size, [k, v]) => size + k.length + formDataValueSize(v),
+        0,
+      )
+    : 0
diff --git a/src/HTTP/Request.purs b/src/HTTP/Request.purs
index 4907fff..8fa90c2 100644
--- a/src/HTTP/Request.purs
+++ b/src/HTTP/Request.purs
@@ -12,21 +12,21 @@ module HTTP.Request
   , requestHeaders
   , requestUrl
   , requestMethod
+  , rawRequestBodySize
   ) where
 
 import Prelude
 
 import Control.Promise (Promise)
 import Control.Promise as Promise
-import Data.ArrayBuffer.ArrayBuffer as ArrayBuffer
 import Data.ArrayBuffer.Types (ArrayBuffer)
 import Data.Map (Map)
 import Data.Map as Map
 import Data.Maybe (Maybe(..))
 import Data.Newtype (unwrap)
 import Data.Nullable as Nullable
-import Data.Tuple.Containing (class TupleContaining, extract)
-import Data.Tuple.Nested (type (/\), (/\))
+import Data.Tuple.Containing (extract)
+import Data.Tuple.Nested (type (/\))
 import Data.URL (URL)
 import Effect (Effect)
 import Effect.Aff.Class (class MonadAff, liftAff)
@@ -45,9 +45,11 @@ import Unsafe.Coerce (unsafeCoerce)
 import Web.File.Blob (Blob)
 import Web.File.Blob as Blob
 
-foreign import blobArrayBufferImpl :: Blob -> Effect (Promise ArrayBuffer)
 foreign import data RawRequestBody :: Type
 
+foreign import blobArrayBufferImpl :: Blob -> Effect (Promise ArrayBuffer)
+foreign import rawRequestBodySize :: RawRequestBody -> Effect Int
+
 unsafeEmptyRawRequestBody :: RawRequestBody
 unsafeEmptyRawRequestBody = unsafeCoerce Nullable.null