diff --git a/lib/form_data.js b/lib/form_data.js
index 5d4ee88..e6551be 100644
--- a/lib/form_data.js
+++ b/lib/form_data.js
@@ -35,18 +35,28 @@ FormData.prototype.append = function(field, value, options) {
   append(value);
   append(footer);
 
-  this._trackLength(header, value)
+  // pass along options.knownLength
+  this._trackLength(header, value, options);
 };
 
-FormData.prototype._trackLength = function(header, value) {
+FormData.prototype._trackLength = function(header, value, options) {
   var valueLength = 0;
-  if (Buffer.isBuffer(value)) {
+  
+  // used w/ trackLengthSync(), when length is known.
+  // e.g. for streaming directly from a remote server,
+  // w/ a known file a size, and not wanting to wait for
+  // incoming file to finish to get its size.
+  if (options.knownLength != null) {
+    valueLength += +options.knownLength;
+  } else if (Buffer.isBuffer(value)) {
     valueLength = value.length;
   } else if (typeof value === 'string') {
     valueLength = Buffer.byteLength(value);
   }
 
   this._valueLength += valueLength;
+
+  // @check why add CRLF? does this account for custom/multiple CRLFs?
   this._overheadLength +=
     Buffer.byteLength(header) +
     + FormData.LINE_BREAK.length;
@@ -58,8 +68,13 @@ FormData.prototype._trackLength = function(header, value) {
 
   this._lengthRetrievers.push(function(next) {
 
+    // do we already know the size?
+    // 0 additional leaves value from getSyncLength()
+    if (options.knownLength != null) {
+      next(null, 0);
+
     // check if it's local file
-    if (value.hasOwnProperty('fd')) {
+    } else if (value.hasOwnProperty('fd')) {
       fs.stat(value.path, function(err, stat) {
         if (err) {
           next(err);