I've never fancied myself as a professor, or a teacher of any kind. I never considered a career in torture either, so it’s all the more surprising I've elected to share some details on how we calculate the fabric quantities for curtains.
Anyway... here goes.
Why might you care about calculating fabric quantities?
I'm often asked why a certain amount of fabric is required or if we can make with what the customer has already. As the answer isn't always what they want to hear I thought a few pointers would help so you at least get the gist, even if you decide not to do the full calculations yourself - and why would you? Surely you'd prefer a gin and tonic instead, and in my experience gin and maths are rarely complimentary. Curtains use lots of fabric, obviously, but sometimes much more than people think especially when the cost of the fabric tends to be the biggest expense when having curtains made.
For starters, we need to allow enough fabric to give the curtains fullness. This is what we call the allowance that stops them being flat sheets when closed and is affected by the curtain heading you want.
For triple pleats this goes up to 2.75 to 3 x as we need more fabric to form the 3 pleats. Either way, this gives us the number of drops of fabric we need.
For a 2m-wide pole we will need:
200cm x 2.5 fullness (for double pleat) = 500cm
This is then divided by the width of the fabric. Most fabric comes 137cm wide, so...
500 ÷ by 137 = 3.6. They don't sell it by parts of widths so we have to round it up to 4 widths.
Still with me? Its not so hard after all.
Now we need to account for the drop (or height) of the curtains. If it’s a plain fabric - the maths is easy. Just add the hem allowance - to allow for turnings top and bottom. We use 23cm. So if my curtains are 230cm high then the total drop I need is 253cm per drop.
Remember we had 4 widths (above) so the calculation is... 4 widths x 253cm (height) = 1012cm. You can usually buy fabric in 10cm increments (above 1m) so you'd need 1020cm or 10.2m. Now you can see why us curtains makers don't like working in millimetres, like you clever designers like to.
Fabric Quantities in summary:
1. Calculate how many widths of fabric you need to cover the pole, taking into consideration the heading type and stack back.
2. Work out the total length of each drop, remembering to include the heading and hem.
3. Multiple them together to get the total metres required.
It's more complex if you have a patterned fabric - just like with wallpaper you need to allow for pattern matching.
You will need to find the vertical pattern repeat of the fabric. As before you take your finished drop + the turnings. We had 253cm. You now need to know how many pattern repeats fit into that drop. Divide 253 by the vertical pattern repeat - lets say its 60cm: 253 ÷ 60 = 4.22. To match everything when you join the fabrics you will need full pattern repeats so we round up to 5 repeats. 5 x 60cm = 300cm, and that's you drop per width.
4 widths x 300cm = 1200cm or 12m. Its an extra 2m just to allow for the pattern in this example.
BUT... Don’t assume a big pattern repeat always means more fabric – so many people shy away from them because of this, but it all depends on the window height and the size of the repeat and you need to do the calculations to know for sure… If the pattern repeat above had been just 9.5cm less at 50.5 cm, you have needed exactly the quantity as for plain fabrics.
Drop patterns and pleating to pattern add another level of complexity and I don't think it's fair to add this here too, For our purposes now, assume that it may well mean even more fabric.
Roman blinds are much easier, and for plain fabrics you need to add 20-30 cm
to the drop of the blind, and if its over around 130cm wide you’ll need an extra width (or 2).
If there are patterns on multiple blinds its good practice to match the patterns across the top of each and again this takes a little calculating.
A word on standard fabric widths vs room high fabrics
The vast majority of fabric is standard width and sold approximately 137cm wide and by the linear metre. Sheer fabrics and a few other fabrics are sold room high – this means that they come approx. 3m high and sold by the metre wide. So, standard width fabric is sold by 1.37 sq/m, whereas room high is around 3 sq/m. The great benefit of room high is that there are no joins. The bad news is that this doesn’t usually translate into lower making costs as the fabric is harder to control and keep flat so its swings and roundabouts for the maker.
Can you cheat on fabric quantities?
Yes, you can! You can shorten the hem allowance if a pattern repeat is killing your budget. Sometimes when its really tight we add false hems to save that extra few metres – this is fabric added to the face fabric that gives us enough for turnings, yet you won’t see it as its hidden away in the finished curtain. You can reduce the fullness ratio and end up with less drops… meaning less widths of fabric. But, you are compromising, and if you go too low in your fullness it starts to hurt and you’ll regret it - the pleats start to look rubbish. As a rule of thumb, we rarely go below 2x to ensure a great look.
Wave curtains are different – of course they are. Nothing is ever that easy.
Rather than making up in widths of fabric you make these to the exact length of the wave tape.
There are a few options on how to configure wave curtains, but our most popular set up uses 2.1 x fullness.
If you are still with me, remember, curtains makers are here to help you work all this out, but having a least the basics in mind will be a great start for you when planning ahead and outlining budgets.
If you’ve found this helpful, please share.
Next time… Embellishments