Curtain Fabric Calculator

Written byOleh Soloninupdated 25 September 2026

Rail width, cm

Curtain drop, cm

Fabric width on the roll, cm

Fullness ratio

1.5 is a light gather, 2 is standard, 3 is full.

Top and bottom allowances, cm

Pattern repeat, cm

Leave at 0 if the pattern does not need matching.

Fabric price a metre

Result

Fabric, m8.55
Cost$0.00
Fabric width before gathering, with side hems, cm620
Cut height, cm285
Drops3
How to cut it3 drops of 285 cm

The fullness ratio decides the bill

Works out how many metres of fabric a pair of curtains needs from the rail width, the fullness ratio, the hems and the pattern repeat.

Curtains are not the width of the window. They are the width of the rail multiplied by a fullness ratio: two is the standard gather, one and a half is a light one, and three is the full theatrical look that eats fabric.

After that the question is which way the fabric runs. Wide upholstery-weight fabric is often railroaded — turned on its side so the roll width becomes the drop — and then you simply buy the gathered width in running metres. Narrower fabric has to be cut into drops and joined.

How to use

Measure the rail, not the window: the curtains hang along its full length.
Take the drop from the hooks to the floor or the sill, and enter the top and bottom allowances together in the one allowance field, as your heading tape and hem require.
Ask the shop for the roll width and the pattern repeat — enter 0 for a plain fabric.
Pick a fullness ratio and a price per metre. The result says how to cut the fabric and how many metres to buy.

Fullness and metres

The metres go up in steps, because fabric is cut in whole drops. Three drops of 280 cm cover up to 840 cm of gathered width, so a fullness of 2.5 takes exactly the same fabric as 2, while 3 needs a fourth drop.

The practical point: once you have the figure for a fullness of 2, try 2.5 — sometimes the extra fullness costs nothing.

Every row comes from the calculator on this page: a 300 cm rail, a 260 cm drop, 25 cm of allowances and a plain 280 cm fabric.
Fullness	Gathered width, cm	Drops	Fabric, m	Cost at 900 a metre
1.5	470	2	5.7	5,130
2	620	3	8.55	7,695
2.5	770	3	8.55	7,695
3	920	4	11.4	10,260

Roll width and pattern repeat

If the roll is at least as wide as the cut drop, the fabric can be railroaded and you simply buy the gathered width: 6.2 metres of a 300 cm roll, with no seams at all. A 280 cm roll is five centimetres short of the 285 needed — and the same curtains take 8.55 metres.

Narrow fabric is cut into drops, and there the repeat decides: every drop is rounded up to a whole number of repeats. A 64 cm repeat stretches each drop from 285 to 320 cm; a 90 cm repeat, to 360.

Every row comes from the calculator on this page: a 300 cm rail, a 260 cm drop, 25 cm of allowances and a fullness of 2.
Fabric width, cm	Repeat, cm	How to cut	Fabric, m
300	—	railroaded, one seamless panel	6.2
280	—	3 drops of 285 cm	8.55
150	—	5 drops of 285 cm	14.25
150	32	5 drops of 288 cm	14.4
150	64	5 drops of 320 cm	16
150	90	5 drops of 360 cm	18

What this calculator shows

The metres of fabric needed and the cost
The gathered width and the cut height with allowances
How many drops are needed and how to cut them

What to keep in mind

Twenty centimetres are added to the gathered width for the side hems of both curtains.
A pattern repeat rounds each drop up to a whole number of repeats, which is what makes patterned fabric noticeably more expensive.

FAQs

What fullness ratio should I choose?

Two for most rooms. One and a half for a heavy fabric that gathers on its own, three for sheers and for a deliberately opulent look.

How much fabric for a three-metre rail?

At a fullness of 2, a 260 cm drop and 25 cm of allowances: 8.55 metres of a 280 cm fabric, cut as three drops of 285 cm. A 300 cm roll needs 6.2 metres; a 150 cm fabric, 14.25.

What is a pattern repeat?

The distance after which the pattern starts again. Each drop has to begin at the same point in the pattern, so every drop is rounded up to a whole number of repeats.

What allowances should be left?

It depends on the heading tape and the hem — check the tape's instructions. The field takes the top and bottom allowances together; leave extra if you want to lengthen the curtains later.

What does railroading mean?

Turning the fabric ninety degrees so the roll width becomes the curtain drop. It removes all the seams, but only works if the fabric is wider than the drop and the pattern has no direction.

Why can curtains 10 cm shorter take a quarter less fabric?

Because the cut drop may now fit within the roll width. A 250 cm drop with 25 cm of allowances cuts at 275 cm, which a 280 cm roll covers, so instead of three drops of 285 cm (8.55 metres) you buy 6.2 metres.

Worked example

A three-metre rail

Input: 300 cm rail, 260 cm drop, 280 cm fabric, fullness 2, 25 cm of allowances

Output: 8.55 metres

Note: The gathered width is 620 cm and the cut height 285, so the 280 cm roll cannot be railroaded and the fabric goes into three drops. Drop the fullness to 1.5 and it becomes two drops and 5.7 metres.

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Result

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