diff --git a/cmd/size/size.go b/cmd/size/size.go
index a396ddaa1..b30aa5dfe 100644
--- a/cmd/size/size.go
+++ b/cmd/size/size.go
@@ -52,9 +52,10 @@ of the size command.`,
 		cmd.Run(false, false, command, func() error {
 			var err error
 			var results struct {
-				Count    int64 `json:"count"`
-				Bytes    int64 `json:"bytes"`
-				Sizeless int64 `json:"sizeless"`
+				Count    int64  `json:"count"`
+				Bytes    int64  `json:"bytes"`
+				Sizeless int64  `json:"sizeless"`
+				Fs       string `json:"fs"`
 			}
 
 			results.Count, results.Bytes, results.Sizeless, err = operations.Count(context.Background(), fsrc)
@@ -65,6 +66,7 @@ of the size command.`,
 				fs.Logf(fsrc, "Size may be underestimated due to %d objects with unknown size", results.Sizeless)
 			}
 			if jsonOutput {
+				results.Fs = args[0]
 				return json.NewEncoder(os.Stdout).Encode(results)
 			}
 			count := strconv.FormatInt(results.Count, 10)