Get a tailored jacket by picking your standard shirt size for a relaxed look.
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Button up the shirt and lay it down on a flat surface. Measure it from the bottom of one armhole to the other (the widest part of the shirt).
Double the result for your final chest measurement.
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Hold the tape 8 inches below each armhole of your shirt to determine your waist.
Measure across, from one end to the other and double the result for your final waist measurement.
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Start from where the shoulder seam and collar connect and measure down the shirt till you reach your preferred length.