In college, I once push-pinned a sheet over my apartment's bedroom window and held it open when needed with a binder clip from my economics notebook. At the time, I thought this was quite inventive. Ugly, but inventive. I'm sure my landlord adored all those tiny tack holes in the molding. Now, I prefer beautiful curtains and shades, and I make sure to measure precisely to get just the look I want.
For curtains, the measuring tool you use is crucial. You'll get the most accurate measurement with a retractable metal tape measure. The kind the carpenters use. The retractable metal tape measurer is preferable to the sewer's cloth tape because it will remain flat and tight to the surface being measured. A cloth tape measure can slip or bend and result in a bad measurement.
Once you have your retractable metal tape measure in hand, you're ready to measure the window. Start by measuring the width of the window. Place the end of the tape measure along the outside edge of the molding of the window (not along the glass), and measure to the opposite outside molding edge of the window.
To measure the height of your window, place the end of the tape measure along the top edge of the molding of the window, and measure to the window sill.
This gives you the raw measurements for the actual window. You now need to decide how you want to mount your curtains. Depending on the overall look you're hoping to achieve, as well as the type of curtains you have in mind, you can choose between a Wall Mount, a Trim Mount, or an Inside Mount.
When using a Wall Mount, you'll place the curtain hardware on the wall around the window. This is a great option if you're trying to make your windows look bigger. You'll mount the hardware about 3" beyond the window trim on each side, and 4"- 6" above the top window trim. You'll need to add these measurements in to the width and height of your raw window dimensions when calculating the size necessary for your curtains.
The Trim Mount is a good option if you're hoping to draw attention to the actual size of your windows, and when you want a bit of the trim to show. When using this mount, you'll place the hardware directly on the molding on either side of the window. To calculate yardage for this mount, you will just use the measurements you took for the actual window, because these original raw measurements included the trim.
You will need to use the Inside Mount if you are making Roman Shades or if you have beautiful decorative molding around your windows that you want to be completely visible. The hardware will mount just inside the edge of the window, along the inside edge of the trim. To calculate the fabric necessary for this mount, you will need to subtract the width of the trim from your original measurements. You may find it easier to measure specifically for the Inside Mount. Measure from the inside edge of the molding to the opposite inside edge of the molding.
Depending on the thickness of the fabric you're using and how full you want your curtains, you can determine the width of fabric necessary for your curtains. For most curtains, two times the raw width of the window will be the perfect calculation. However, if you are using a very sheer fabric, you may want to multiply the window width by three. And, heavy fabrics may only require 1½ times the width of the window.
To determine the length of the fabric necessary, you will need to decide the height at which you'd like your curtains to fall. For regular curtains, you want them to hang about 4" past the bottom window sill. For floor length curtains, measure from the window sill to about 1" above the floor. If you want your curtains to "puddle" on the floor, measure to the floor and add another 6" - 10".
Remember, for each type of curtain, you'll also need to add about 4" for hems and casings at the top and 2"- 4" for hems at the bottom.