Curtain Size Calculator

The Curtain Size Calculator turns your window measurements into the two numbers that actually matter when you shop or sew: the flat width of each curtain panel and the finished length (drop). It applies a proper fullness ratio so your curtains gather into rich folds instead of hanging flat, accounts for the classic "hang high and wide" designer trick, and shows a true-to-scale diagram of the rod, window, and panels. You get a ready-made buying recommendation and, for DIY sewers, the exact cut dimensions including hem allowances.

How to Measure for Curtains

1. Width: measure the width of your curtain rod (or track), not just the glass. If you have not put the rod up yet, use the window width plus how far you plan to extend the rod on each side.
2. Height: measure your window height and how high off the floor the sill sits, so the tool can work out the drop for any style.
3. Rod position: decide how far above the window the rod will go and how far it will extend past each side. Higher and wider makes a room feel bigger.

Curtain Width Formula

Curtains look best when there is far more fabric than the space they cover. That extra fabric is set by the fullness ratio.

Curtain Fullness Ratio Guide

Curtain Length (Drop) Styles

Why Hang Curtains "High and Wide"?

Mounting the rod well above the window frame and extending it past each side is the single most effective trick for making a room feel larger. Hanging high draws the eye upward and makes ceilings feel taller, while hanging wide lets the open panels sit against the wall instead of covering the glass — so more light comes in and the window looks bigger than it is. This calculator lets you set both the mount height and the extension, and shows the result on the scale diagram.

How Much Fabric to Buy or Sew

If you are buying ready-made panels, the calculator tells you how many standard panels you need to reach your target fullness and which standard length to choose. If you are sewing, it adds a header allowance at the top, a hem allowance at the bottom, and a side hem allowance to give you the exact cut width and length per panel. Always buy a little extra to match any pattern repeat.

How to Use This Calculator

1. Measure your window: choose inches or centimeters and enter the window width and height.
2. Set the rod position: enter the rod height above the window, the extension per side, and the floor-to-sill height.
3. Choose style and fullness: pick where the curtains fall, a fullness ratio, and the number of panels.
4. Read your results: see the flat width per panel and finished drop, the scale diagram, the ready-made size to buy, and the cut dimensions for sewing.

Frequently Asked Questions

How wide should curtains be for my window?

Curtains should be much wider than the window so they gather into folds instead of hanging flat. Multiply the rod width (window width plus the extension on each side) by a fullness ratio of 1.5 to 3. For a standard tailored look use 2 times the rod width, then divide by the number of panels to get the flat width of each panel.

What is curtain fullness ratio?

Fullness ratio is how many times wider the total fabric is than the rod it covers. A ratio of 1.5 gives a relaxed, economical gather, 2.0 is the standard tailored look, 2.5 is luxurious, and 3.0 is used for sheers and voiles which need extra fabric to look full.

How long should curtains be?

Curtain length depends on where you want the hem to fall. Sill length stops at the windowsill, apron length falls a few inches below the sill, floor length just touches the floor, and puddle length adds extra fabric that pools on the floor. Measure the drop from the top of the rod down to the chosen end point.

How high above the window should I hang a curtain rod?

Hanging the rod higher than the window frame makes the ceiling feel taller. A common guideline is 4 to 6 inches (10 to 15 cm) above the frame, or roughly halfway between the frame and the ceiling. Extend the rod 8 to 12 inches (20 to 30 cm) beyond each side so open curtains clear the glass.

How many curtain panels do I need?

Most windows use two panels, one drawn to each side. Wide windows or sliding doors may need three or four panels. Add up the flat width of your panels and make sure the total is at least 1.5 to 3 times the rod width so the curtains still look full when closed.

How much fabric do I need to make my own curtains?

For each panel, take the finished drop and add a header allowance at the top and a hem allowance at the bottom (about 8 inches or 20 cm each) to get the cut length. Add a side hem allowance to the flat panel width to get the cut width. Multiply the cut length by the number of panels for the total fabric length before matching any pattern repeat.

Additional Resources

• Curtain - Wikipedia
• Window Treatment - Wikipedia