diff --git a/libisofs/system_area.c b/libisofs/system_area.c
index 43e82ff..c67021f 100644
--- a/libisofs/system_area.c
+++ b/libisofs/system_area.c
@@ -747,7 +747,7 @@ static int iso_write_apm_entry(Ecma119Image *t, int apm_block_size,
     uint8_t *wpt;
     int block_fac;
 
-    block_fac = apm_block_size / 512;
+    block_fac = 2048 / apm_block_size;
 
     memset(buf, apm_block_size, 0);
     wpt = buf;