diff --git a/targets/arduino/wiring.c b/targets/arduino/wiring.c
index 98ed9a330..d5d18a63f 100755
--- a/targets/arduino/wiring.c
+++ b/targets/arduino/wiring.c
@@ -277,7 +277,7 @@ unsigned long millis()
 	// overflows when it reaches 256.  we calculate the total
 	// number of clock cycles, then divide by the number of clock
 	// cycles per millisecond.
-	return timer0GetOverflowCount() * timer0GetPrescaler() * 256L / F_CPU * 1000L;
+	return timer0GetOverflowCount() * timer0GetPrescaler() * 256L / (F_CPU / 1000L);
 }
 
 void delay(unsigned long ms)