diff --git a/nagios/univention-nagios/usr/lib/nagios/plugins/check_univention_slapd_mdb_maxsize b/nagios/univention-nagios/usr/lib/nagios/plugins/check_univention_slapd_mdb_maxsize
index eab78f7f32..138f43fed8 100755
--- a/nagios/univention-nagios/usr/lib/nagios/plugins/check_univention_slapd_mdb_maxsize
+++ b/nagios/univention-nagios/usr/lib/nagios/plugins/check_univention_slapd_mdb_maxsize
@@ -82,13 +82,13 @@ warnings = []
 success = []
 for mdb_dir in files:
 	try:
-		output = subprocess.check_output(['/usr/bin/mdb_stat', '-e', mdb_dir], close_fds=True, env={'LC_ALL': 'C'})
+		output = subprocess.check_output(['/usr/bin/mdb_stat', '-ef', mdb_dir], close_fds=True, env={'LC_ALL': 'C'})
 	except subprocess.CalledProcessError:
-		nagios_exit(STATE_CRITICAL, 'mdb_stat -e %s failed' % (mdb_dir,))
+		nagios_exit(STATE_CRITICAL, 'mdb_stat -ef %s failed' % (mdb_dir,))
 
 	stat = dict(line.strip().lower().split(': ', 1) for line in output.splitlines() if ': ' in line)
 	try:
-		in_use = int((int(stat['number of pages used']) * 100) / int(stat['max pages']))
+		in_use = 100 * (int(stat['number of pages used']) - int(stat['free pages'])) / int(stat['max pages'])
 	except KeyError:  # API change in the future
 		nagios_exit(STATE_CRITICAL, 'output of "mdb_stat -e %s" could not be parsed: %s' % (mdb_dir, output,))