Curtain Maths

Curtain Maths

Curtains do not look good or block heat and light if they are flat. As a result curtains are usually 1.5 to 2.5 times wider than the space they are covering so that they can be pleated or gathered in some way. But how do you decide which size curtain to buy or to get made?

There are actually some pretty simple rules that will help you. You need to decide on three things – what style of curtain you want, how wide the curtains need to be and finally how long you want your curtains to hang.

Curtain & Pole/Rail Style

Three main curtain types, eyelet, pleat and tab tops.

There are three main types of curtain design which describe the top of the curtain and effect how they attach to a rail or pole – see right; the pleat, tab and eyelet.

The tab and eyelet curtains fit over a simple curtain pole and do not need any curtain hooks or rings. Note that eyelet curtains actually start above the curtain rail where as tab top curtains hang below them. So for the same window and curtain pole, eyelet curtains would need to be longer that tab tops.

There are lots of different sorts of pleats such as double box pleat, pinch pleat and goblet pleat but in terms of your measurements essentially you treat them all the same.  Pleat curtains can be fitted either to a pole or a curtain rail. Depending on how you attach them, and what rail you use, their vertical position will be different. If you choose a curtain pole and rings the curtains hang lower so they will need to shorter than if you were using a rail, see later.

Rail Position

The first job is actually choosing a post or rail and fitting it. A curtain pole/track/rail should be about 15cm above the window and at least 15cm wider each side before the ends (or finials) if you have space. Once you have fitted the rail/pole measure its total width, excluding the ends or finials and then add on 20cm. This additonal 20cm gives room for an air of curtains to overlap in the middle. It is important, if you do not add this on you will have an air and light gap. If you only want a single large curtain rather than two narrower ones you do not need to add this extra 20cm on.

You have now calculated the width you need to cover, but your curtains will need to be 1.5 to 2.5 times wider than this so that they gather. The guide below gives you the minimum width by curtain type.
If you want pencil pleat curtains on a pole, or track multiple your width by two.
If you want eyelet curtains on a pole, multiple your width by two.
If you want tab top curtains on a pole, multiple your width by 1.5.
If your fabric is really thin (voile or net), or if you want a very rich look or you need good thermal insulation then you should increase each of the multiplications by 0.5 times (2 times the width for tab tops and 2.5 times the width for eyelet or pencil pleat).

Worked Example

So look at the example below. The curtain pole is shown as A – it is the width of the pole excluding its ends. We want a pair of overlapping curtains so if A is 120cm, we need to add 20cm for our overlap giving 140cm.
For normal eyelet curtains we need to multiply this by 2. So we would need curtains that are 140 x 2 = 280cm wide. If we wanted an extra thick look we would multiply our width by 2.5 times, 140 x 2.5 = 350cm.
When you buy ready made curtains the packet normally states the width of one of a pair. So if the packet said 1.2m wide it means there are two 1.2m wide panels included.

Measuring for Curtains
Measuring for Curtains

Deciding on the Height

The image above illustrates the three usual curtain lengths – window sill length, below the window sill or floor length. If you want your curtains to skim the window sill then measure from the top to 1.5cm above the window sill. If you need to exclude light and cold air you should measure 15 cm below the window sill. If you decide on floor length curtains make sure they finish 1.5 cm above the floor so they do not get stained or frayed at the bottom. Make sure all your curtains finish at least 3cm above radiators.

In the explanation above I told you to measure from the top. Exactly where the top is varies depending on which curtain type and rail you have used. For eyelet and tab top curtains measure from the top of the curtain pole to where you want the curtain to finish. If you have pleat curtains with a track measure from the bottom of the track. If you have pleat curtains with a pole and rings measure from the bottom of the rings.

Related posts