Habanero Cycles Stem Calculator

Note: The above method of measuring stems is only one of a plethora of methods employed - but is probably the most popular one currently.  Some manufacturers measure stems in "degrees above horizontal" (common on road bikes, and assuming ~72-3 degree head tube), "degrees from 90 degrees", etc.  If in doubt, print out the chart, and use it to guesstimate your stem's "degrees above horizontal" value by aligning the bottom of the chart with a horizontal surface and sighting the stem extension against the extension lines on the chart. You'll usually be pretty close in estimating your head tube angle at 73 degrees (road) or 71 degrees (MTB).  So, to arrive at "stem angle", add the head tube angle to the "extension angle" from the chart and you'll have a number that most bike shops can use.  

To calculate the effect of raising a stem, add 0.96cm of rise for every 1.0cm of quill or steer tube you expose, and subtract 0.30cm of reach.