Curtain width is not window width. Panels have to gather, so you need roughly twice the width of the rod in flat fabric, and the rod itself runs wider than the window. Enter your window and we will give you the total panel width, how many ready made panels that is, and the length for the style you want.
Curtain width is not window width
This is the mistake, and it is close to universal. Curtains are meant to gather. A panel that lies flat against the wall looks like a bedsheet on a wire regardless of what the fabric cost, and the only cure is more width. The rule is that your total flat panel width should be about twice the length of the rod, and since the rod is already wider than the window, you end up needing far more fabric than the window measurement suggests.
Work it in that order and it stops being confusing. The rod runs 4 to 10 inches past the trim on each side, so the open panels stack against the wall instead of covering the glass. Then the fabric is a multiple of the rod: 1.5 times is the flat tailored minimum, 2 times is the working standard, 2.5 is generous, and sheers want 3 because you can see straight through thin gather.
The formula
rod length = window width + 8 to 20 in
total panel width = rod length × fullness (1.5 to 3)
panels = total width ÷ 50 in, rounded up to an even number
drop = measured from the ROD, not the window
Fifty inches is the near universal ready made panel width, and rounding to an even number matters because the pair has to meet in the middle. Length is always a drop from the rod: sill and apron are measured down past the trim and the glass, so they need the window height, while float and puddle are measured to the floor, so they need the rod height instead.
Worked example
Take a 36 inch window with standard 2x fullness. The rod should run 44 to 56 inches, extending 4 to 10 inches past the trim on each side. Multiply by two and you need 88 to 112 inches of total flat panel across it, which at the standard 50 inch panel width is 2 panels. Now the length: if the rod sits 96 inches off the floor and you want a float, the drop is 95.5 inches, so the panel to buy is the standard 96 inch. Note what happened to the width, though. Two panels sounds like it covers a lot and it covers one ordinary window, which is why wide windows and patio doors so often end up under-dressed.
Hang it high and wide
Two decisions cost nothing and change the room more than the curtains do. The first is height: put the rod at least 4 to 6 inches above the trim, and if you have the wall, go higher still, either halfway between the trim and the ceiling or 4 to 6 inches under the crown. The eye reads the whole run of fabric as the window, so a high rod makes a standard window look like a tall one.
The second is width. Running the rod well past the trim means that when the curtains are open they sit on wall rather than glass. That gives you more daylight and makes the window look wider, and it is the reason a rod bought to match the window exactly always disappoints. If you are choosing between spending on better fabric and spending on a longer rod hung higher, buy the rod.
Shower curtains are a different question
None of the arithmetic above applies to a shower curtain, because a shower curtain is meant to hang almost flat and to reach inside the tub rather than to gather. The sizes are simply standards. A standard shower curtain is 70 by 72 inches, which suits most tubs and stalls. Extra long is 70 by 84 inches, for high ceilings or a rod hung above the usual height. Stall curtains are narrower at around 54 by 78 inches.
The one thing worth knowing is that the liner and the curtain should be the same width, and the liner should hang inside the tub while the decorative curtain hangs outside. If your bathroom floor keeps getting wet, that is nearly always the reason rather than the size.
If you are between two lengths, buy the longer one
Ready made panels come in 63, 84, 95, 96, 108, 120, 132 and 144 inches, and almost no rod height lands exactly on one of them. When you are between two, take the longer panel. Shortening a curtain is a hem, which is an afternoon with an iron and hemming tape if you do not sew, and lengthening one is not possible. A curtain that stops three inches short of the floor is the visual equivalent of trousers that do the same thing.
The related economy is the liner. On anything pale or lightweight a liner does three jobs at once: it adds the weight that makes a panel hang in straight folds rather than billowing, it blocks the light the face fabric will not, and it absorbs the sun damage that would otherwise fade the curtains you chose. Buy it an inch or two shorter than the panel so it never peeks below the hem.
Frequently asked questions
What size curtains do I need for my window?
Measure the rod, not the window, then multiply. The rod should extend 4 to 10 inches past the window trim on each side, and your total panel width should be about twice that rod length. So a 36 inch window wants a rod of roughly 44 to 56 inches and about 90 to 110 inches of flat panel across it, which is two standard 50 inch panels.
How wide should curtains be compared to the window?
Between two and two and a half times the width of the rod, and the rod is already wider than the window. Panels that merely span the window hang flat and look thin no matter what the fabric cost. If you are hanging sheers, go to two and a half or three times, because you can see through the gaps and thin gather shows badly.
How many curtain panels do I need?
Take your total required width and divide by the panel width, which is 50 inches for most ready made curtains, then round up to an even number so the pair meets in the middle. Two panels is much less coverage than people expect: at standard fullness a pair of 50 inch panels properly dresses a rod of only about 50 inches, which is one average window.
How high should I hang a curtain rod?
At least 4 to 6 inches above the window trim, and higher if you have the wall. Halfway between the trim and the ceiling, or 4 to 6 inches below the crown moulding, makes the window read as much taller. Hanging the rod tight to the top of the trim is one of the two most common curtain mistakes and it costs nothing to fix.
What are the standard curtain lengths?
Ready made panels come in 63, 84, 95, 96, 108, 120, 132 and 144 inches. Those are drops measured from the rod, not from the window, so where the rod sits decides which one you need. If you fall between two lengths, buy the longer one, because taking a hem up is an afternoon and adding fabric is not possible.
Should curtains touch the floor?
It depends on the room rather than on taste. Float, stopping about half an inch above the floor, is the cleanest and stays cleanest since nothing drags. Puddle, running about 6 inches onto the floor, is deliberately soft and romantic and genuinely more work to keep tidy. Sill and apron lengths belong over counters, radiators and anywhere floor length fabric would be a nuisance.
What is the standard shower curtain size?
70 by 72 inches for a standard shower curtain, which suits most tubs and stalls. Extra long runs 70 by 84 inches for high ceilings or a raised rod, and stall curtains run narrower at about 54 by 78. None of the fullness arithmetic on this page applies, because a shower curtain is meant to hang nearly flat and to reach the inside of the tub rather than gather.
Do I need a liner?
For anything light coloured or lightweight, yes. A liner adds the weight that makes a panel hang straight instead of billowing, blocks the light the main fabric will not, and takes the sun damage that would otherwise fade your good curtains. Buy the liner an inch or two shorter than the panel so it never shows below the hem.