Mastering the Perfect Fit: The Ultimate Curtain Length and Width Calculator Guide

Transforming a room often starts with the windows. Curtains are the mascara of interior design: they frame the view, soften the edges and add that essential layer of texture and warmth. However, nothing ruins a high-end look faster than “high-water” curtains that are too short, or panels so skimpy they look like flat sheets when closed.

Calculating the right length and width of your curtains decides the final feel of the room. Whether you are aiming for a crisp, modern look or a dramatic, romantic puddle, the maths must be right before you ever pick up scissors or click “buy” online. This guide works as a manual curtain length and width calculator, with every measurement in centimetres first (and inches in brackets), so each window in your home looks professionally tailored.

1. Use a metal tape measure

The foundation of any successful decor project is the tool you measure with. Don’t rely on a fabric tape used for dressmaking; these can stretch over time and be out by as much as 2.5 cm (an inch), which is the difference between a curtain that skims the floor and one that drags awkwardly.

Use a good-quality metal retractable tape measure. When measuring for curtains you are often dealing with drops of 213–305 cm (84–120 in), and a metal tape stays stiff, so you can measure heights accurately without it sagging. Measure twice and write each figure down straight away. Even a difference of about 1 cm (half an inch) can affect how the fabric hangs, especially for a precise floor-length finish.

2. Measure the window width

To calculate the width you need, start with the “base width”. This is not just the glass: it is the whole window, including any wooden casing or architrave. If your pole or track isn’t up yet, measure the window opening from outer edge to outer edge.

A well-dressed window usually looks larger than it really is, so your calculation should allow for the “stack-back”: the space the curtains take up when they are drawn open. As a default, let the pole extend about 15 cm (6 in) beyond the frame on each side, which adds 30 cm (12 in) to the window width in total. You can go up to 25 cm (10 in) each side for wide windows or bulky fabrics. This keeps the open curtains off the glass, so they don’t block light or the view, and the window feels more generous.

3. Calculate curtain fullness

If you buy curtains that are exactly the width of your window, they will look like a flat sheet when closed. To get soft, professional folds you need to calculate “fullness”.

The usual rule is 2 to 2.5 times the pole width:

• Standard fullness (2×): multiply your pole width by 2. This suits most everyday rooms and heavier fabrics such as velvet.
• Generous fullness (2.5×): multiply by 2.5. This gives a richer, tailored look that works beautifully with linen or cotton.
• Sheer fullness (3×): sheer fabrics are so fine that they need more volume to look intentional, so multiply your width by 3.

For example, if your window plus overhang measures 150 cm (60 in) and you want standard fullness, you need 300 cm (120 in) of total curtain width, usually as two panels of 150 cm each. Note that some UK curtain makers quote the width of a pair rather than a single panel, or quote the pole width the pair will cover rather than the flat fabric width, so always check the maker’s sizing guide before ordering.

4. Sill-length curtains

Sill-length curtains are practical and charming, and are often chosen for kitchens, bathrooms or windows with furniture or a radiator directly beneath. To calculate this length, measure from the pole position down to the window sill.

Then subtract about 1 cm (half an inch). This stops the fabric dragging on the sill, where it collects dust and bunches up. Sill-length curtains give a clean, casual look that frames the view without overwhelming a small space.

5. Apron-length curtains

The “apron” is the horizontal trim beneath the window sill. Measuring to below the apron is a traditional way to make small windows look slightly taller while staying practical.

To calculate apron length, measure to the sill and then add 10–15 cm (4–6 in). The curtain covers the trim and finishes a few centimetres below it. This style works well in bedrooms where you want to block more light than a sill-length curtain allows, without the formality of floor-length curtains.

6. Floor-length curtains

Floor length is the “Goldilocks” of curtain measurements: just right for almost every room in the house. It creates a long vertical line that draws the eye upwards and makes ceilings feel higher.

To calculate it, measure from where the pole will sit down to the floor. For a “hover” or “kiss” finish, subtract about 1 cm (half an inch) from that height. The curtains then hang straight without breaking at the hem. This is the most popular choice for modern, clean-lined interiors because it stays tidy and makes vacuuming easy. If there is a radiator or heater under the window, keep floor-length curtains clear of it: hang them so they sit to either side when drawn, or choose sill-length or a blind for that window. Our guide to living rooms with awkward radiators has more ideas.

7. The puddle effect

For a look of opulence, romance or old-world European charm, calculate for a “puddle”, where the fabric pools on the floor.

For a slight “break” (like trousers resting on a shoe), add 2.5–5 cm (1–2 in) to your floor-length measurement. For a true, dramatic puddle, add 10–20 cm (4–8 in). Puddling suits formal rooms with little foot traffic, as the fabric collects pet hair and dust more easily, and it works best with fabrics that drape well, such as silk, velvet or heavy linen.

8. Decide on rod placement

Where you hang the pole matters as much as the window size. The modern rule is to “hang high and wide”: instead of fixing the pole just above the frame, mount it 10–15 cm (4–6 in) above the frame, or even halfway between the frame and the ceiling.

This changes your length calculation, because your vertical measurement should always start from where the pole or track will sit, not from the top of the window. Mounting higher tricks the eye into reading the window as taller, giving the room a grander, airier feel.

Fix poles and tracks into suitable wall structure, such as masonry or timber studs, using fixings rated for the weight of your curtains; lined and blackout curtains are heavier than they look. If you rent, get your landlord’s permission before drilling. Our guides to curtains for awkward and bay windows when renting and choosing a curtain rod for a window with no wall space cover the alternatives.

9. Curtain calculator formulas

To act as your own curtain calculator, use these two formulas.

The width formula

• Window width + 30 cm overhang (15 cm each side) = pole width.
• Pole width × fullness (2, 2.5, or 3 for sheers) = total fabric width.
• Total fabric width ÷ number of panels = width per panel.

The length formula

• Height from pole to floor − 1 cm floor clearance = finished length for a floor-skimming curtain.
• For a puddle, add the puddle allowance instead of subtracting the clearance.

Remember the heading. With curtain rings, the curtain hangs about 2.5–5 cm (1–2 in) below the pole, so measure from the bottom of the ring eye instead. With a slot (rod pocket) heading, the curtain starts at the pole. Adjust your starting point to suit the hardware you choose.

A worked example

Your window measures 150 cm wide. Add 30 cm of overhang for a pole width of 180 cm. At standard 2× fullness you need 180 × 2 = 360 cm of fabric, which is two panels of 180 cm each. The pole sits 240 cm above the floor, so for a floor-skimming finish the drop is 240 − 1 = 239 cm. If you are using rings, deduct the ring drop the maker gives (often 2.5–5 cm) from that 239 cm. Before ordering, check whether the maker sizes curtains per panel or per pair.

10. Consider fabric weight

The final variable is the fabric itself. Heavy fabrics such as velvet or lined blackout curtains take up more space when drawn back. With a very heavy fabric, you may need a longer stack-back of 30–38 cm (12–15 in) each side so the window isn’t covered when the curtains are open.

Lightweight fabrics such as linen or sheer voile stack more slimly, so you can be more conservative with the width. Natural fibres like pure linen can also drop or shrink slightly with changes in humidity, so round your length up a little if you are working with unwashed linen, and check the maker’s advice.

Final thoughts

Good curtains are mostly good measuring. Start with a metal tape, add 15 cm of overhang each side, multiply by the right fullness, and measure the drop from where the pole will actually sit. Choose your length by the room, from a neat sill length in the kitchen to a soft puddle in a formal sitting room, and always check how your curtain maker quotes widths and headings. Get those numbers right and even ready-made curtains can look made to measure.

Frequently asked questions

Should curtains touch the floor?

In most modern schemes, yes. Curtains that stop a few centimetres above the floor (unless they are sill-length) often look unfinished. For a tailored look, let the fabric just kiss the floor or hover about 1 cm (half an inch) above it.

How do I measure for eyelet (grommet) curtains?

With eyelet curtains, the fabric rises above the pole. Measure from the top of the pole to the floor, then add the distance from the top of the eyelet to the top edge of the fabric, usually 2.5–4 cm (1–1.5 in), to find the total panel height. Many makers quote eyelet drops from the top of the eyelet, so check their guide.

What is the standard curtain width?

Many ready-made curtain panels are about 127–137 cm (50–54 in) wide, though UK sizes vary and are often sold as pairs. If your calculation calls for around 255 cm of fabric, two such panels will do. For very wide windows, you may need four panels, two on each side.

Can I use two different lengths in the same room?

Consistency usually looks best. If you have a standard window and a glazed door in the same room, keep the pole height and the hem level the same for both, even if the openings are different sizes.

How much extra fabric do I need for a puddle look?

For a subtle break, add 2.5–5 cm (1–2 in). For a moderate puddle that looks intentional without overwhelming the room, add about 10 cm (4 in). For a very dramatic, editorial look, add 15–20 cm (6–8 in).

For help choosing the curtains themselves, read our guide on how to choose curtains for your living room, see curtains for awkward and bay windows if you rent, and pair your fabrics with the floor using our living room rugs and curtains guide. Explore our curtains and window treatments service, or book a consultation to get every window measured and dressed properly.