Consider the Setting

• Before you measure the window, decide what kind of window you want to replace it. There is a wide range of styles you can buy, depending on the style of your house and on the location of the room. White vinyl windows are cheaper and easier to work with than wood windows, but some home styles require wood. Double-hung windows have top and bottom sashes that hinge inward so they can be cleaned from inside the house. They are much more expensive than single-hung windows, but worth it for upper floors. Mullions--the decorative square frames across the glass--look great on old houses, but out of place on some modern ones.
  
  

Measure the Opening

• Take measurements of the width and height of the window opening. Measure from the jambs, which are the flat wood that the window panes sit against, further out than the trim. Take three measurements from three different points vertically and horizontally, and record the smallest of the three measurements. Subtract 1/4 inch from the smallest width measurement, and 1/4 inch from the smallest height measurement. Those are your height and width numbers that you will use to order your window.
  
  

Remove and Replace

• Don't take out the old window until the new one arrives and you've checked the measurements to make sure it will fit. Carefully remove the interior border of 1" x 2" trim that's holding in the window sashes (you'll re-use the trim), and take out the thinner vertical center piece. Remove the sashes. Pop the new window into the opening, shim it level and plumb with wood shims, and attach it to the side jambs with long wood screws in the holes on the side of the window unit. Stuff insulation into the cracks on the side. Put the trim back on and caulk all around.