Start with a collared shirt that fits you well. Lay the fully buttoned collar flat. Measure with tape inside of collar going around to the far end of opposite buttonhole until the tape overlaps.
Measure around the fullest part of your chest, just under your arms and across your shoulder blades, holding the tape firm and parallel to the floor.
With your arm relaxed at your side and slightly bent, start at the center back of your neck and run the tape over the shoulder and down the outside of your arm, ending at the wrist. Sleeve length is always in whole numbers, so round up if necessary.
Measure around your natural waistline. For belts, it varies where your pants fall. To ensure a perfect fit, order 2" larger than your normal pant waist size (e.g., if you wear a size 34 pant, order a size 36 belt). If between sizes, order the next size up.