Curtain Overlap Allowance Calculator

There’s nothing more disappointing then newly installed curtains that don’t quite meet in the middle. And what do you do? With pride, you close the curtains. Then you notice a sliver of light dividing them in half. Why is that? Everyone assumes that width of each panel is simply double the width of the window plus some additional fabric for effect. What they overlook are that two different pieces of material will not naturaly line up. Enter the curtain overlap allowance calculator.

Plug in your desired amount of fullness and how far out your rod extends and let it do the math. That way there are no conversions or coefficients to guess at, just a definite cut to follow to achieve an otherwise vague look. That’s easy enough in theory, but also frequently forgotten. You’re not just trying to cover the window with overlap. Instead, you’re aiming to create overlap by bringing together two separate piece of panel. This means the fabric has to account for however far apart those panels are.

How to Calculate Curtain Overlap

If there’s an eight-inch gap between the window frame and your curtain rod on either side, that’s an extra sixteen inches of total span. That’s how much your fabric need to reach, before any overlapping occurs. Many people forget to include this, or simply measure from glass outward, as if the curtain rod didn’t exist! That’s what the calculator will automatically take into account, along with any other side return. You want all of that wall space between brackets covered at the end.

Another factor is that fabric weight matter. Soft, draping linen would be okay with only an inch of center overlap, the cloth would still collapse down onto itself. But heavier velvet (or thermal lined blackout) doesn’t do that; they has some life and body and don’t want to get compressed. If you fold them up just as tight as you did the sheer voile, the force of those stacked layers will squeeze the leading edges away from each other. The calc takes that into account, factoring in more overlap for stiffer fabrics (because physics says so).

This number goes hand-in-hand with fullness ratio. Fullness means how many times wider than the window opening you want your finished curtain to be. A regular drapery will typically run double the width of glass, which looks luxurius and makes nice folds. Selecting a greater fullness (i.e. Two point five times) mean that you’ll have a lot more fabric, which uses up some of the space on the rod. The calculator considers this when calculating lead per panel. It will tell you exactly where to place each panel off the true center line in order to account for the bulk.

Other variables come into play depending on the hardware you choose: Fixed waveforms like grommets on curtain tops don’t budge once they’re drawn, and if the overlap is too tight, those hard arcs will fall short at the center. Bulky track system carriers can also keep the fabric from adhering tightly to the wall, where the slightest bump, like a protruding window handle stuck smack in the middle, can stop the close. The app has fields where you enter dimensions of any obstructions or lead-edges. It’s the real world of the install, and it’ll help ensure the remaining width of fabric provides enough length to span some odd piece of hardware or misshapen ring.

A dedicated bedroom or media space will require a more airtight seal than a casual living room, where some daylight coming through may be desirable. The calculator enables users to choose how tight they want their final closure to be. From basic light-blocking to thermal draft control. Depending on your choice, you will need a certain amount of baseline overlap; for thermal, this is about six to eight inches to properly prevent airflow. At first glance, this appears excessive. But again: for function.

Each scenario is a preset button that quickly lets you try it out. It loads actual world configurations and shows you how those variables play together before you put in your own figures. Compare the nursery blackout preset to the cafe curtain preset. The difference between what width panels they require is drastic. One calls for 90 inches of finished fabric per panel; another gets away with 60. That’s the reason a generic sizing chart never realy applies to something custom made.

That’s all you need to know to order online or take to the tailor. What you’ve calculated here is the final width, including hem allowance on the leading edge, and the finished width of the face. Why is knowing this final measurement important? Many people order panels that are too narrow (i.e. Wide enough to cover the window, but not wide enough to close properly). Having done this math beforehand allows you to calculate a good overlap so your curtains meets securely; in other words, they stay shut, not just because of tension, but also because of gravity.

You don’t see the center seam anymore, just a nice neat unified drape that does exactly what it was hired to do. That’s the true reward for getting the math right before cutting anything. You should of checked this earlier.