Curtain Maths: Calculating Curtain Sizes

Curtains do much more than enhance the design of the rooms inside of your home. Pleated and gathered curtains trap heat during winter and prevent sunlight from destroying the ambiance of a living room on sweltering summer days. Because homeowners need to gather or pleat curtains for several reasons, curtains typically range in size between 1.5 and 2.5 times a window size to ensure adequate coverage.

However, very few homeowners understand how to purchase the right size curtains for their homes.

You don’t need an engineering degree to calculate curtain sizes. What you need to do involves calculating the three primary components that comprise curtain hangings.

Pole and Curtain Style

Three types of curtain designs influence how the fabric attaches to a pole. Tab and eyelet curtains typically fit on a standard pole, which means you don’t have to add hooks or rings to ensure the stability of a curtain. Eyelet curtains begin above the curtain pole, while tab curtains attach below. Homeowners that measure a curtain size for the same pole have to add length for eyelet curtains.

Pleat style curtains include pinch pleat, goblet pleat, and double box pleat. However, measuring each of three pleat styles requires the same method. You can fit curtains to both curtain poles and rails. The only difference lies in the vertical positioning of the curtains. Homeowners that use curtain poles typically hang curtains lower than the height they use to hang curtains on rails.

The Position of a Pole or Rail

One of the first tasks for performing curtain maths requires you to select a pole or rail and then fit it. A curtain pole, rail, or even a track should sit around 15 centimetres above a window, with each side at least 15 centimetres wider than the ends. After you fit a curtain on a pole or rail, you measure the total width of the pole or rail (except the ends), and then add 20 more millimeteres of width to provide space for two curtains to overlap in the middle of a bedroom window or living room bay window.

If you fail to add the 20 millimetres, you can expect an air and light gap that diminishes room décor and comfort levels. Homeowners that decide to hang a single curtain above a window don’t have to add the 20 millimetres.

We now must multiply the width by 1.5 to 2.5 times for ensuring your curtains gather for aesthetic purposes.

Here are a few time-tested rules:

  • For tab top curtains on a pole, multiply curtain width by 1.5 times
  • For eyelet curtains on a pole, multiply width by two times
  • For pleat curtains on a pole, multiply width by two times
  • Increase the width by 2.5 times for extremely thin fabrics, such as net or voile

How to Decide Height

Buying curtains that possess the right width offer homeowners several benefits. Does this mean you’re done calculating curtain maths? You still must calculate curtain height, although many homeowners assume curtain height doesn’t matter. Homeowners have three curtain height options: floor length, below the window length, and window sill length.

Floor length curtains should flow until about 1.5 centimetres from the floor. Below the window curtains measure about 1.5 centimetres below a window sill to prevent light and cold air from entering a room. Window sill length curtains measure 1.5 centimetres from the top of a window sill. Remember to size all curtains to end at least three centimetres above every type of radiator in your home. Defining the top of a curtain depends on the type of curtain, as well as the type of pole used to secure the curtain.

Classic Curtain Appearance Example

Let’s take an example of a commonly used curtain design. The classic curtain design starts with a curtain rod mounted about 10 centimetres above a window opening. Use a ruler to calculate the 10-centimetre measurement and then measure to the curtain end at just a little more than one centimetre from the floor. Some homeowners want to create the stylish puddle effect, which means you have to add between 12 and 16 centimetres to the curtain length. You can also create the illusion of more room height by placing the pole or rail between four and six centimetres below a ceiling. This curtain measurement calculation works best for large rooms that contain several windows to enhance the greater height effect.

The ability to perform curtain maths not only ensures an aesthetically appealing curtain layout, it also provides functionality by reducing light and limiting exposure to cold air drafts. Moreover, measuring and hanging your own curtains eliminates the need to consult with a shop owner that sells drapes and curtains. You save both time and money measuring your own curtains.


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