How the curtain size calculator works
Curtains should cover the rod, not just the window. The rod extends past the frame on each side so open panels stack off the glass. Fullness is how much wider the fabric is than the rod; 2× means the panels together are twice the rod length.
The calculator divides the fabric width you need by your panel width, rounds up, and makes the count even so curtains hang as a pair. Curtain length is the rod height minus the clearance you want at the floor.
Formula and assumptions
- extension
- how far the rod passes the frame on each side
- fullness
- fabric width ÷ rod width
Assumptions and limits
- Panel width is the flat width of each panel before gathering.
- Ring and header style can change the finished length; check the product.
Worked example
A 70 in window, 8 in extension each side, 2× fullness, 50 in panels (example), rod 84 in from the floor, 0.5 in clearance.
- Rod = 70 + 16 = 86 in.
- Fabric = 86 × 2 = 172 in.
- 172 ÷ 50 = 3.44; round up to 4 panels (2 pairs).
- Length = 84 − 0.5 = 83.5 in.
These are the calculator’s default values, so the live result above matches this example until you change an input.
How to measure correctly
- Measure the window outside the trim.
- Decide the rod height before measuring length; many people hang it several inches above the frame to make the window look taller.
- Measure from the rod (or ring bottom) to the floor in more than one place.
Rod and fabric width
| Window | Rod length | Fabric at 2× | Fabric at 2.5× |
|---|---|---|---|
| 36 in | 52 in | 104 in | 130 in |
| 48 in | 64 in | 128 in | 160 in |
| 60 in | 76 in | 152 in | 190 in |
| 72 in | 88 in | 176 in | 220 in |
| 96 in | 112 in | 224 in | 280 in |
Curtain Size Calculator FAQ
How wide should curtains be for my window?
Measure the rod, not the window. Together the panels are usually 2 to 3 times the rod length, depending on how full you want them to look.
How far should a curtain rod extend past the window?
Home Depot’s guide suggests 8 to 10 in on each side for an outside mount, so open curtains clear the glass. Wall space and furniture may limit it.
How long should my curtains be?
Measure from the rod to the floor and subtract the clearance you want: a small gap to hover, 0 to touch, or extra length to puddle.
What does 2x fullness mean?
The total flat width of the panels is twice the rod length, so they gather into soft folds when closed.
Why are panels rounded to an even number?
Most windows use a pair that meets in the middle. If you want a single panel that draws to one side, use the fabric width figure.
Do blackout curtains need different sizing?
Extending the rod further past the frame and above the window reduces light gaps at the edges. The panel math is the same.