diff --git a/disk/disk_freebsd.go b/disk/disk_freebsd.go
index 24549b7..7380850 100644
--- a/disk/disk_freebsd.go
+++ b/disk/disk_freebsd.go
@@ -99,14 +99,12 @@ func DiskIOCounters() (map[string]DiskIOCountersStat, error) {
 	buf := []byte(r)
 	length := len(buf)
 
-	ds := Devstat{}
-	devstatLen := int(unsafe.Sizeof(ds))
-	count := int(uint64(length) / uint64(devstatLen))
+	count := int(uint64(length) / uint64(sizeOfDevstat))
 
 	buf = buf[8:] // devstat.all has version in the head.
 	// parse buf to Devstat
 	for i := 0; i < count; i++ {
-		b := buf[i*devstatLen : i*devstatLen+devstatLen]
+		b := buf[i*sizeOfDevstat : i*sizeOfDevstat+sizeOfDevstat]
 		d, err := parseDevstat(b)
 		if err != nil {
 			continue
diff --git a/disk/disk_freebsd_amd64.go b/disk/disk_freebsd_amd64.go
index bbae159..8579100 100644
--- a/disk/disk_freebsd_amd64.go
+++ b/disk/disk_freebsd_amd64.go
@@ -39,6 +39,10 @@ const (
 	MNT_SUSPEND = 4
 )
 
+const (
+	sizeOfDevstat = 0x120
+)
+
 type (
 	_C_short       int16
 	_C_int         int32
diff --git a/disk/types_freebsd.go b/disk/types_freebsd.go
index 4486904..dd6ddc4 100644
--- a/disk/types_freebsd.go
+++ b/disk/types_freebsd.go
@@ -65,7 +65,10 @@ const (
 	MNT_NOWAIT  = 2 /* start all I/O, but do not wait for it */
 	MNT_LAZY    = 3 /* push data not written by filesystem syncer */
 	MNT_SUSPEND = 4 /* Suspend file system after sync */
+)
 
+const (
+	sizeOfDevstat = C.sizeof_struct_devstat
 )
 
 // Basic types