diff --git a/fsutil/filesize_unix.go b/fsutil/filesize_unix.go
new file mode 100644
index 000000000..e45ccca17
--- /dev/null
+++ b/fsutil/filesize_unix.go
@@ -0,0 +1,25 @@
+package fsutil
+
+import (
+	"syscall"
+
+	"golang.org/x/xerrors"
+)
+
+type SizeInfo struct {
+	OnDisk int64
+}
+
+// FileSize returns bytes used by a file on disk
+func FileSize(path string) (SizeInfo, error) {
+	var stat syscall.Stat_t
+	if err := syscall.Stat(path, &stat); err != nil {
+		return SizeInfo{}, xerrors.Errorf("stat: %w", err)
+	}
+
+	// NOTE: stat.Blocks is in 512B blocks, NOT in stat.Blksize
+	//  See https://www.gnu.org/software/libc/manual/html_node/Attribute-Meanings.html
+	return SizeInfo{
+		stat.Blocks * 512,
+	}, nil
+}
\ No newline at end of file