Curtain Size Converter

Curtains need to be wider than the window to look full and pleated.

Fullness ratio determines how gathered the curtains look:

• 1.5x = flat/casual look (minimal gathering)
• 2x = standard fullness (most common)
• 2.5x = full/luxurious look
• 3x = very full/formal (pinch pleats)

Rod length guidelines:

• Rod should extend 8 to 12 inches (20 to 30 cm) beyond each side of the window
• This makes the window appear larger and lets the curtains stack off the glass when open

Curtain width formula:

• Rod length = window width + (side extension × 2)
• Total curtain width = rod length × fullness ratio

Measure the fullness against the rod, not the glass. The curtains have to cover the whole rod when they are drawn, and the rod is a foot or two wider than the window, so sizing fabric off the window width leaves you short by exactly that much. It is the most common way to end up with panels that will not quite meet in the middle.

Example (metric):

• Window width: 150 cm
• Rod extends 25 cm each side, so rod length = 150 + 50 = 200 cm
• Fullness: 2x
• Total curtain fabric: 200 × 2 = 400 cm
• Two panels: 400 ÷ 2 = 200 cm each

Example (imperial):

• Window width: 60 inches (152.4 cm)
• Rod extends 25 cm each side, so rod length = 202.4 cm = 79.7 inches, call it 80
• Fullness: 2x
• Total curtain fabric: 79.7 × 2 = 159.4 inches
• Two panels: 159.4 ÷ 2 = 79.7 inches each

The calculator works in centimetres and converts, which is why the imperial figures land a fraction under the round numbers you would reach with a 10 inch extension. That fraction of an inch is well inside the tolerance of a stack-back allowance, so round it however you like. The two windows above are also not the same size: 150 cm is 59 inches, so the metric example is the narrower of the two.

For two panels, divide the total width by 2.

Drop and hem allowance:

The drop is measured from the rod, or from the bottom of the rings if you are using them, down to where you want the hem. On top of that, cut fabric needs a hem allowance at the bottom and a header allowance at the top, usually 15 to 25 cm (6 to 10 in) in total. Ready-made panels already include both, so the allowance only matters if you are sewing.

The most common curtain mistake is buying panels the same width as the window, which leaves them looking skimpy and stretched flat when closed. Fullness is the whole point: even a casual look wants 1.5 times the width to cover, and 2x is what reads as proper curtains. And you measure fullness against the rod, not the glass, because the rod runs wider than the window. Buy for the rod length times your fullness ratio, then split that across your panel count.

Length is the other decision, and it changes the whole feel of a room. Curtains ending at the sill look casual and work well over a radiator or a counter. Dropping to just below the sill is a safe everyday choice. Floor-length, with the hem just kissing the floor, looks the most finished, while letting fabric “puddle” an inch or two on the floor reads as formal and a touch dramatic. Whatever you pick, add hem and header allowance before cutting, usually around 15 to 25 cm in total, and remember that a bold pattern needs extra length so you can match the repeat between panels.

One thing worth deciding before you measure anything: mount the rod high and wide. Putting it 10 to 15 cm above the window frame rather than right on it makes the ceiling read as taller, and the wide extension means the open curtains sit against the wall instead of covering a third of the glass. Both cost you nothing but a longer rod and a slightly longer drop.