What this tool does, and why the numbers matter
Curtain measuring fails at the arithmetic, not the tape. People measure their window accurately and then order panels the same width, which produces flat, skimpy curtains that cover the glass and nothing else. The rules that prevent this (rod length before fullness, fullness by header style, length from the rod rather than the frame) are simple but easy to apply in the wrong order. This tool runs them in the right order. It's the working companion to our complete measuring guide, which explains every rule the calculator applies; the guide teaches, the tool computes.
Common measurement mistakes we catch on custom orders
Every order that comes through our workroom gets a human review before cutting, and the same five mistakes account for nearly everything we catch. Ordering panels at window width instead of rod width times fullness. Copying the dimensions of old curtains that never fit in the first place. Measuring one window and assuming its twin matches, which pre-war LA framing routinely disproves by half an inch or more. Forgetting what lives under the window, so a floor-length quote meets a radiator at install time. And rounding to the nearest inch when workrooms cut to the eighth. If your numbers come from this tool, you've dodged the first mistake automatically, and the photos we ask for with every order catch most of the rest.
When to override the defaults
The defaults suit most rooms; the advanced panel exists for the rest. Push fullness up toward 3x for sheers and down toward 1.5x to 2x for stationary panels that never close, since decorative-only drapery doesn't need traverse fabric. Widen the fabric setting if you're pricing wide-width goods (some sheers and linings run 110 inches or more, which can eliminate seams entirely). Deepen the return when drapery layers over a shade, so the panel wraps its bigger brackets. And drop to a single panel on narrow windows where a pair would stack into two skinny stubs. If you're not sure whether your situation is an exception, it usually isn't; the defaults are the numbers we'd use.
Frequently asked questions
How do I measure my window if I can't reach the top?
Measure what you can reach and borrow the rest from the architecture. In most rooms the window height plus the floor-to-frame distance can be taken at chest height with a steel tape run up the wall, and a photo of the full wall lets us sanity-check the numbers against proportions. For genuinely tall installations, a laser measure from the floor is a fifteen-dollar solution that beats any ladder. And in LA and Ventura counties you can skip the problem entirely, since our in-home measurement is free.
What if my window isn't perfectly square?
Welcome to the housing stock we work in every day; a 1920s opening that's a half inch out of true is normal, not broken. For outside-mount curtains it rarely matters, because the rod mounts level to the world and the fabric hides the frame's tilt. For inside-mount work, measure the width at the top, middle, and bottom and use the narrowest number, and if the spread exceeds about three eighths of an inch, switch to an outside mount rather than fighting the geometry.
Should I add fullness for linen versus velvet curtains?
Adjust by fabric weight, yes. Lightweight fabrics like sheers and open linens need more of themselves to read as full, so run them at 2.5x to 3x. Midweight linens and cottons sit happily at the 2x to 2.5x our header defaults assume. Heavy velvets and interlined drapery carry so much body that 2x is usually plenty, and pushing past 2.5x mostly buys bulk on the rod. The advanced panel in the tool lets you override the default for exactly this reason.
How much extra fabric do pattern-matched curtains need?
More than the 15 percent overage our yardage estimate includes, and the pattern repeat decides how much more. Every seam and every panel must land on the same point in the design, so as a working rule, add one vertical repeat of extra length per fabric width being cut. A 25 inch repeat across a two-width panel adds about a yard and a half. When you order from us we run the exact repeat math against the mill's specs before quoting, so the estimate here is for budgeting, not for cutting.
Can I use this tool to measure for shades?
For the inside-mount width and height, yes: the inside-mount option applies the same measure-at-three-points logic a shade order needs, and the numbers it returns describe your opening. But shades need two things this curtain tool doesn't handle: a mounting-depth check (roughly 2 inches of flat depth for most headrails) and the maker's own clearance deduction, which you should never apply yourself. Our measuring-for-blinds guide covers the shade-specific rules.
How do I measure for drapery hanging over an existing shade?
Layering changes one number: the bracket projection. The drapery needs to clear the shade's headrail and its operating path, which usually means 5 to 6 inch brackets instead of the standard 3.5, and the panel's return should deepen to match so the side profile stays closed. Width and length math stay the same. Set the return depth slider in the advanced panel to your projection and the tool folds it into the panel width automatically.