Blinds Mounted on Head Rail

• Place one end of a tape measure against the outside edge of the leftmost window frame, extend it across the window and read the measurement on the outside edge of the rightmost window frame. This gives you the width of the blind. Measure from the top of the head rail to the top of the windowsill. This gives you the minimum length of the blind. This will cover the window and the framing.
  
  

Blinds Mounted Inside Window Frame

• Place one end of the tape measure against the inside edge of the leftmost window frame, extend it across the window and read the measurement on the inside edge of the rightmost window frame. This gives you the width of the blind. Measure from the bottom edge of the head rail to the top edge of the windowsill. This gives you the minimum length needed for the window blind. This allows you to secure the blind to the bottom of the head rail and countersink the blind into the window frame. With the blind down, you will still see the window frame.
  
  

Curtains Mounted on Head Rail

• Place one end of the tape measure against the outside edge of the leftmost window frame, extend it across the window and read the measurement on the outside edge of the rightmost window frame. This is the width of the curtain. Measure from the top of the head rail to the top of the floor molding. This gives you the minimum length of the blind. This will cover the window and the framing but will not touch the floor. This allows for easier vacuuming without fear of catching the curtains.
  
  

Curtains Mounted Inside Window Frame

• Place one end of the tape measure against the inside edge of the leftmost window frame, extend it across the window and find the measurement on the inside edge of the rightmost window frame. This gives the width of the blind. If you plan to use two curtains, cut the number by half. Measure from the bottom edge of the head rail to the top edge of the windowsill. This gives the minimum length needed for the curtain and creates a tight fit for the curtain inside the window frame. If you want to use longer curtains, measure from the bottom edge of the head rail to the top edge of the floor molding. This gives a longer curtain and still protects the curtain from a vacuum cleaner.