diff --git a/common.scad b/common.scad
index c3e8273..719a8b3 100644
--- a/common.scad
+++ b/common.scad
@@ -1,3 +1,6 @@
 Guitar_Scale_Length_mm = 648;
+ln2 = ln(2);
+function log2(x) = ln(x)/ln2;
 function fret_scale_length(n) = Guitar_Scale_Length_mm * 2^(-n/12);
+function mm_to_fret_number(mm) = -log2(mm/Guitar_Scale_Length_mm)*12;
 function lerp(start, end, amount) = start + (end-start)*amount;