diff --git a/free-gigs b/free-gigs
index 7e1b566..8d8a2d9 100755
--- a/free-gigs
+++ b/free-gigs
@@ -1,20 +1,29 @@
-#!/bin/bash
+#!/bin/sh
 # script-friendly df
 
-factor=1024  # default
+factor=1024  # for the default (GiB); use -H for GB
 
-OPTSTRING="H"
-while getopts ${OPTSTRING} opt; do
-  case ${opt} in
-    H)
-      factor=1000
-      ;;
+optstring="H"
+while getopts "$optstring" opt; do
+  case $opt in
+    h) factor=1024 ;;  # default (GiB)
+    H) factor=1000 ;;  # decimal (GB)
   esac
 done
 shift $((OPTIND-1))
 
 target=${1:-.}
 
-free_bytes=$(findmnt --df -T "$target" -oAVAIL --bytes | sed '1d')
-free_gigs=$((free_bytes/(factor**3)))
-echo $free_gigs
+if free_bytes=$(stat -f --format="%a*%S" "$target" 2>/dev/null); then
+    # GNU stat (Linux)
+    :
+elif free_bytes=$(stat -f "%a*%S" "$target" 2>/dev/null); then
+    # BSD/macOS stat
+    :
+else
+    # BusyBox stat fallback
+    free_bytes=$(stat -f "$target" | awk '/Block size:/ {bs=$3} /Available:/ {av=$2} END {print av*bs}')
+fi
+
+free_gb=$((free_bytes / factor / factor / factor))
+echo "$free_gb"