diff --git a/scripts/contrib/ddimage b/scripts/contrib/ddimage
index ab1d4c7b10..93ebeafc31 100755
--- a/scripts/contrib/ddimage
+++ b/scripts/contrib/ddimage
@@ -39,7 +39,7 @@ device_details() {
 		echo "   model: UNKNOWN"
 	fi
 	if [ -f "/sys/class/block/$DEV/size" ]; then
-		echo "    size: $[$(cat /sys/class/block/$DEV/size)*BLOCK_SIZE] bytes"
+		echo "    size: $(($(cat /sys/class/block/$DEV/size) * $BLOCK_SIZE)) bytes"
 	else
 		echo "    size: UNKNOWN"
 	fi
diff --git a/scripts/contrib/mkefidisk.sh b/scripts/contrib/mkefidisk.sh
index 0c15104bb3..bcc2411ca2 100755
--- a/scripts/contrib/mkefidisk.sh
+++ b/scripts/contrib/mkefidisk.sh
@@ -64,7 +64,7 @@ function device_details() {
 		echo "   model: UNKNOWN"
 	fi
 	if [ -f "/sys/class/block/$DEV/size" ]; then
-		echo "    size: $[$(cat /sys/class/block/$DEV/size)*BLOCK_SIZE] bytes"
+		echo "    size: $(($(cat /sys/class/block/$DEV/size) * $BLOCK_SIZE)) bytes"
 	else
 		echo "    size: UNKNOWN"
 	fi