Place the tape measure on the outside of the window casing. Measure across the window to the outer edge of the casing. Write down the measurement, rounding up to the nearest quarter-inch.
Measure the window from the top, outside the casing to the bottom outer edge of the casing. Round up to the nearest quarter-inch.
Divide the length measurement by three, and multiply the result by two. Tier curtains generally cover the bottom two-thirds of the window, so this number represents the ideal length for the tiers. Purchase tiers that are as close to that measurement as possible, always rounding up to ensure that the curtains are long enough.