diff --git a/cmd/moby/build.go b/cmd/moby/build.go
index 110944ce5..dd67b8049 100644
--- a/cmd/moby/build.go
+++ b/cmd/moby/build.go
@@ -9,7 +9,6 @@ import (
 	"net/http"
 	"os"
 	"path/filepath"
-	"strconv"
 	"strings"
 
 	"github.com/moby/tool/src/moby"
@@ -32,27 +31,6 @@ func (f *formatList) Set(value string) error {
 	return nil
 }
 
-// Parse a string which is either a number in MB, or a number with
-// either M (for Megabytes) or G (for GigaBytes) as a suffix and
-// returns the number in MB. Return 0 if string is empty.
-func getDiskSizeMB(s string) (int, error) {
-	if s == "" {
-		return 0, nil
-	}
-	sz := len(s)
-	if strings.HasSuffix(s, "G") {
-		i, err := strconv.Atoi(s[:sz-1])
-		if err != nil {
-			return 0, err
-		}
-		return i * 1024, nil
-	}
-	if strings.HasSuffix(s, "M") {
-		s = s[:sz-1]
-	}
-	return strconv.Atoi(s)
-}
-
 // Process the build arguments and execute build
 func build(args []string) {
 	var buildFormats formatList
diff --git a/cmd/moby/util.go b/cmd/moby/util.go
new file mode 100644
index 000000000..56fd3a231
--- /dev/null
+++ b/cmd/moby/util.go
@@ -0,0 +1,29 @@
+package main
+
+import (
+	"strconv"
+	"strings"
+)
+
+// This function parses the "size" parameter of a disk specification
+// and returns the size in MB. The "size" parameter defaults to GB, but
+// the unit can be explicitly set with either a G (for GB) or M (for
+// MB). It returns the disk size in MB.
+func getDiskSizeMB(s string) (int, error) {
+	if s == "" {
+		return 0, nil
+	}
+	sz := len(s)
+	if strings.HasSuffix(s, "M") {
+		return strconv.Atoi(s[:sz-1])
+	}
+	if strings.HasSuffix(s, "G") {
+		s = s[:sz-1]
+	}
+
+	i, err := strconv.Atoi(s)
+	if err != nil {
+		return 0, err
+	}
+	return 1024 * i, nil
+}
diff --git a/src/moby/build.go b/src/moby/build.go
index 56892ae18..415c08c5e 100644
--- a/src/moby/build.go
+++ b/src/moby/build.go
@@ -142,7 +142,7 @@ func outputImage(image *Image, section string, prefix string, m Moby, idMap map[
 // Build performs the actual build process
 func Build(m Moby, w io.Writer, pull bool, tp string) error {
 	if MobyDir == "" {
-		return fmt.Errorf("MobyDir for temporary storage not set")
+		MobyDir = defaultMobyConfigDir()
 	}
 
 	iw := tar.NewWriter(w)
diff --git a/src/moby/util.go b/src/moby/util.go
index b262e0066..56afefb88 100644
--- a/src/moby/util.go
+++ b/src/moby/util.go
@@ -1,6 +1,16 @@
 package moby
 
+import (
+	"path/filepath"
+)
+
 var (
-	// MobyDir is the location of the cache directory which should be set by the caller
+	// MobyDir is the location of the cache directory, defaults to ~/.moby
 	MobyDir string
 )
+
+func defaultMobyConfigDir() string {
+	mobyDefaultDir := ".moby"
+	home := homeDir()
+	return filepath.Join(home, mobyDefaultDir)
+}