diff --git a/runtime/indent/prolog.vim b/runtime/indent/prolog.vim
index afe448e..26a3bc3 100644
--- a/runtime/indent/prolog.vim
+++ b/runtime/indent/prolog.vim
@@ -41,18 +41,18 @@
     endif
     " Check for clause head on previous line
     if pline =~ ':-\s*\(%.*\)\?$'
-	let ind = ind + &sw
+	let ind = ind + shiftwidth()
     " Check for end of clause on previous line
     elseif pline =~ '\.\s*\(%.*\)\?$'
-	let ind = ind - &sw
+	let ind = ind - shiftwidth()
     endif
     " Check for opening conditional on previous line
     if pline =~ '^\s*\([(;]\|->\)'
-	let ind = ind + &sw
+	let ind = ind + shiftwidth()
     endif
     " Check for closing an unclosed paren, or middle ; or ->
     if line =~ '^\s*\([);]\|->\)'
-	let ind = ind - &sw
+	let ind = ind - shiftwidth()
     endif
     return ind
 endfunction