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Open the collar and spread it out. Measure from where the button is stitched to the end of the paired buttonhole. They are on opposite ends of the collar.
While still keeping the shirt flat and facing up, measure from one side of the shirt right below the armpit to the same point on the opposite side. Keep the tape straight.
While still keeping the shirt flat and facing up, find the point on the shirt where your belly button would be and measure from one side of the shirt across to the same point on the opposite side.
While still keeping the shirt flat facing up, measure from the lowest point on one side of the shirt to the same point on the opposite side of the shirt.
Spread the shirt out and place it face down. Find the seam where the shoulder meets the back of the shirt, then measure from the top of this seam across the shirt to the seam on the opposite side.
Lay the shirt flat down and facing up, measure from the top of the shoulder seam down in a straight line along the outer edge of the shirt sleeve to the end of the cuff.
While still keeping the shirt flat facing up, measure from the outer side of the sleeve where the bicep would be to the armpit of the shirt.
Lay the cuff flat with the inside of the cuff facing up, then measure from the end of the buttonhole to where the button is stitched on the opposite end.
While still keeping the shirt flat facing up, measure down along the shirt in a straight line from the seam close to the collar to where the dress shirt ends.
Wrap the measuring tape vertically around your armpit. Leave room for two fingers under the measuring tape.
Please view picture for details. 
Normal is traditional fit. If you want roomy around your body circumference then choose loose fitting. Athletic fit tapers to fit your athletic body. Slim fit is just near your body to make you look slimmer. 
