How to Measure a Shirt: Shirt Length
With the shirt spread out on its front, measure from the base of the back of the collar straight down to the center of the very bottom of the dress shirt. Do not stretch the shirt, but be sure it is lying flat and fully extended.
*Note: The number of buttons depends on the shirt length and the placement of the top button below the collar. With the top button placement set to its default position: a 26 to 27 inch shirt length has 6 buttons, 27.5 to 30 inch shirt length has 7 buttons, 30.5 to 33.5 inch shirt length has 8 buttons, 34 to 37 inch shirt length has 9 buttons, and 37.5 to 38 inch shirt length has 10 buttons. The number of buttons will be displayed in your Custom Size profile and update instantly in response to modifications of shirt length and top button placement.