Curtain Fabric Calculator & Fullness

The Calculation Method

The calculator determines how many fabric widths to sew together per curtain to achieve the right fullness, how long each drop must be (including hem allowance and pattern repeat rounding), and the total linear metres of fabric to buy. Fullness ratio controls the gathered look — pencil pleat uses 2.5×, meaning 2.5 times the track width in fabric. Eyelet headings use 1.5× because the metal rings space the folds differently. Each fabric width covers 1.37m (standard UK bolt width). If the gathered width per curtain exceeds one fabric width, you sew multiple widths together.

Pattern repeat complicates the cut length. Each drop must start at the same point in the pattern so the curtains match. The calculator rounds the cut length up to the next full pattern repeat, which can add significant waste — a 320mm repeat on a 2.6m base cut rounds up to 2.88m, wasting 280mm per drop. With four drops that adds over a metre of extra fabric. If you are also hanging blinds on the same windows, the blind dimension tool works from the same recess measurements.

Making Sense of the Output

Three outputs appear — fabric widths per curtain (panels you sew side by side), cut length per drop (what the fabric shop cuts from the roll), and total fabric in linear metres. The total already includes your waste factor. The cut length includes hem allowance and pattern repeat rounding, so it is the ready-to-cut figure.

If using a pattern repeat, the cut length may be noticeably longer than the actual drop — the excess is trimmed during making. While you have the room measurements out, you can also figure out how much paint you need for the walls and ceiling.

Practical Tips for Wall Coverings

Measure the pole or track end to end, not the window. Curtain tracks typically extend 15–20cm beyond the window frame on each side so curtains stack off the glass when open. Drop length depends on the style you want — sill length finishes 1–2cm above the windowsill, below-sill drops add 15cm past the sill, floor-length reaches 1cm above the floor, and puddle curtains pool 5–10cm on the floor.

Always measure the drop in three places — left, centre, and right — because floors and windowsills are rarely level. Use the longest measurement as your drop length. Fabric bolts in the UK are typically 137cm (54 inches) wide. If your fabric is wider or narrower, adjust the fabric width field in the calculator.

When buying patterned fabric, ask the shop for the exact repeat measurement — it is printed on the bolt label. Buy all fabric from the same dye lot to avoid colour variation between drops. Different dye lots of the same fabric can show noticeable colour shifts when hung side by side in full daylight. For a deeper explanation of how repeat matching works on walls, see the wallpaper pattern repeat guide — the maths is identical.

Factors That Change the Calculation

Bay windows need separate tracks for each panel, and each section should be calculated independently. Very heavy fabrics (velvet, interlined curtains) need lower fullness — 1.5–2× — because the weight creates drape naturally. Sheers and voiles need higher fullness (2.5–3×) to avoid looking flat and transparent.

If lining the curtains, buy the same total quantity of lining fabric but without the pattern repeat rounding — lining is plain, so no pattern adjustment is needed. For outdoor planting beds visible through the window, our plant spacing planner helps you layout a view worth framing.