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Curtain Size Calculator

Works out curtain panel width and drop from the window size, rail overhang and the fullness ratio you want.

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Enter your figures and press Calculate.
In short Curtain Size Calculator is a free online tool on inputtesthub.com that works out curtain panel width and drop from the window size, rail overhang and the fullness ratio you want. It needs no account, no download and no payment, and there is no daily usage limit. Everything is processed inside your own browser, so the data you paste is never uploaded to a server.

How to use the Curtain Size Calculator

  1. Enter your figures in the input fields.
  2. The result updates as you type, together with the full working.
  3. Check the breakdown table to see exactly how the number was reached.

About the Curtain Size Calculator

The Curtain Size Calculator works out curtain panel width and drop from the window size, rail overhang and the fullness ratio you want. It runs as plain JavaScript inside the page, which means there is no upload step, no queue and no size limit imposed by a server — the only limit is your own device.

What people use it for

  • Ordering ready-made curtains in the right size.
  • Buying fabric for curtains you are making.
  • Working out the drop for a floor-length pair.

Details

Minimum fullness1.75×
Usual fullness2×
Generous fullness2.5×
Rail overhang15 cm each side
Hem allowance25 cm

The maths behind it

This tool uses Rail width = window width + overhang each side. Fabric width = rail width × fullness, divided by the number of panels. Every intermediate value is shown on screen, so the result can be checked by hand.

Fullness is what makes curtains look like curtains rather than flat sheets. Extending the rail past the window lets them stack off the glass when open, which makes the window look wider and actually lets light in. Measure the rail, not the window, once the rail is up.

Frequently asked questions

How wide should curtains be for a 120 cm window?
With a 15 cm overhang each side the rail is 150 cm, and at 2× fullness you need 300 cm of fabric — so two panels of 150 cm each. Anything under 1.75× fullness hangs flat.
Should curtains touch the floor?
Either just brush it or puddle deliberately by a few centimetres. The one length to avoid is stopping slightly short, which looks like a mistake rather than a choice.
Is the Curtain Size Calculator free, or is there a paid tier?
It is free, with no account, no trial and no daily cap. Every calculators tool on inputtesthub.com works the same way: open the page and use it.
Is my data uploaded anywhere?
No. The Curtain Size Calculator runs entirely in your browser using JavaScript. Nothing is transmitted, logged or stored, which you can verify yourself in the Network tab of your browser's developer tools.
What is the equation behind the Curtain Size Calculator?
It uses Rail width = window width + overhang each side. Fabric width = rail width × fullness, divided by the number of panels. The full working is printed under the result, so you can follow every step rather than trusting a black box.
What is the Curtain Size Calculator used for?
The most common reasons people reach for it: ordering ready-made curtains in the right size. Buying fabric for curtains you are making. Working out the drop for a floor-length pair. It is built for exactly those cases rather than being a general-purpose tool with this bolted on.
Any caveats I should know about?
Fullness is what makes curtains look like curtains rather than flat sheets. Extending the rail past the window lets them stack off the glass when open, which makes the window look wider and actually lets light in. Measure the rail, not the window, once the rail is up.
What are the reference figures here?
Minimum fullness is 1.75×. Usual fullness is 2×. Generous fullness is 2.5×. These are printed on the page itself, so you do not have to take them on trust.
Is the number rounded?
The calculation runs in double precision and the result is shown to ten significant figures, so converting a value and converting it back returns the original. What is displayed is rounded for readability; what is copied is not.

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