diff --git a/module/rnrs/arithmetic/fixnums.scm b/module/rnrs/arithmetic/fixnums.scm
index dbf9ee746..7a5a6215e 100644
--- a/module/rnrs/arithmetic/fixnums.scm
+++ b/module/rnrs/arithmetic/fixnums.scm
@@ -95,8 +95,11 @@
 	  (rnrs exceptions (6))
 	  (rnrs lists (6)))
 
-  (define fixnum-width 
-    (let ((w (inexact->exact (round (/ (log (+ most-positive-fixnum 1)) (log 2))))))
+  (define fixnum-width
+    (let ((w (do ((i 0 (+ 1 i))
+                  (n 1 (* 2 n)))
+                 ((> n most-positive-fixnum)
+                  (+ 1 i)))))
       (lambda () w)))
 
   (define (greatest-fixnum) most-positive-fixnum)
diff --git a/test-suite/tests/r6rs-arithmetic-fixnums.test b/test-suite/tests/r6rs-arithmetic-fixnums.test
index 01a7a89b2..60c3b87e9 100644
--- a/test-suite/tests/r6rs-arithmetic-fixnums.test
+++ b/test-suite/tests/r6rs-arithmetic-fixnums.test
@@ -23,6 +23,14 @@
   :use-module ((rnrs exceptions) :version (6))
   :use-module (test-suite lib))
 
+(with-test-prefix "fixnum-width"
+  (pass-if-equal "consistent with least-fixnum"
+      (- (expt 2 (- (fixnum-width) 1)))
+    (least-fixnum))
+  (pass-if-equal "consistent with greatest-fixnum"
+      (- (expt 2 (- (fixnum-width) 1)) 1)
+    (greatest-fixnum)))
+
 (with-test-prefix "fixnum?"
   (pass-if "fixnum? is #t for fixnums" (fixnum? 0))