diff --git a/src/gfloat/round.py b/src/gfloat/round.py
index 4fe5ce7..0280aae 100644
--- a/src/gfloat/round.py
+++ b/src/gfloat/round.py
@@ -60,7 +60,7 @@ def round_float(
 
     else:
         # Extract exponent
-        expval = int(np.floor(np.log2(vpos)))
+        expval = math.frexp(vpos)[1] - 1
 
         # Effective precision, accounting for right shift for subnormal values
         if fi.has_subnormals: