Hopefully you haven’t bought your valance yet. You should always measure your window first and only then determine what size you need for your valance. There’s no such thing as a “one size fits all” valance. There’s no such thing as a standard window, either.
So, how much wider should a valance be than the window then? Most valances should only be 1-1/2 to 5 inches wider than the window if the valance hangs by itself or over a Roman shade. It should be 10 to 30 inches wider than the window if it’s hung over a pair of custom drapes. If the valance is gathered, its total, flat width should be 2 to 3.5 times the width of the window.
Let’s take a look at some examples to help you pinpoint a more definitive answer.
If You’ll Install the Valance By Itself
If you’ll just install a valance on your window without any layering (other than maybe a Roman shade underneath), then your valance has to be a tight fit around the sides of your window. You should install the valance as high as possible to add height to the room. But as far as the width is concerned, you’ll need to be conservative here.
Example #1: The Simple Valance
If the style of the valance is simple or has modern, straight lines, then you should only add 1 to 3 inches to the width of the window. Don’t forget to measure the wooden frame as it’s also part of the window. Take a look at these examples and notice how they cover the sides of the window just enough to frame the window. All these valances are only about 1-1/2 to 2 inches wider than the actual window. A tight fit like this is especially important in modern and transitional rooms.
A poor fit becomes glaringly obvious whenever you have a narrow window. This 26-inch window has a valance that’s only 1-1/2 inches wider than it.
If you have several window sections like this double window does, you’ll want a conservative fit on the valance too in most instances. Notice how this valance has two distinct arched sections. If the valance didn’t fit the window the way that it does, the entire design would be off center and look awkward.
Example #2: The Simple Valance That Flares Out
Some valances will flare out at the bottom. These valances typically have small bells or trumpets on the sides. If this is the case, the top of the valance would just need to barely cover the window frame. Here are the examples. Notice how these valances seem to get wider at the bottom.
If you stand right next to these kinds of valances, be warned that the window frame will be visible from that angle. This is the way these valances are intended to fit, however, and it’s perfectly normal.
Example #3: The Elaborate Valance
Now, once you get past these simple valances and get to the heavier, more elaborate styles, your valance will need to be slightly wider. These valances will generally be about 2 to 5 inches wider than the window, depending on the window and valance style chosen. Before I explain this, let me first show you the kinds of valances that would fit in this category. 
There are a few reasons why you’d want just a dash of extra width on these valances.
First, these valances can be quite heavy sometimes. It isn’t uncommon for some of our board-mounted valances that use a lot of pleated fabric to each weigh over 8 pounds. Because of this heaviness, it’s important for the window to not get lost and be overwhelmed by the valance. Even though an extra inch of added width may not sound like much, it can make a big difference on your window if you choose an elaborate valance like this.
The second reason primarily has to do with wide jabots. Jabots are the long, cascading pieces on the sides of these valances. Because some of them can each be 8, 10, 12 inches wide or more, you can imagine that could take up quite a bit of width on a window that’s only 36 inches wide. If you had a snug fit on your valance in this situation, the valance would look more like it’s covering the window instead of framing it the way it’s supposed to.
Example #4: The Valance That Gets More Narrow At the Bottom
Next, there are valances that get more narrow at the bottom. These are usually London, balloon, or relaxed Roman valances. The reason they get more narrow at the bottom is because of the weight of the fabric.
(This is actually a sign of a good quality valance. It means the workroom that made the valance for you didn’t skimp on fabric.)
For balloon, London and relaxed Roman valance styles, you want the width to be determined by the way that the valance falls at the bottom. The bottom hem of the valance should sit on the window frame and barely cover it. It should look like this:
Example #5: The Cornice-Style Valance
Some call this a cornice-style valance, others a box-style valance. No matter what you call it, the idea here is that this valance continues to go around the curtain rod on each side. It makes the valance look attractive when viewed from the side and gives it that boxed, finished look. In simple terms, it means that this valance has a return on each side.
You may see it sometimes referred to as a clearance also. These valances can either be board-mounted, or they can be mounted on a continental rod as a rod pocket valance. 
You’d follow the same rules from above for a valance like this, making sure to add the returns to the overall width. Don’t forget to do this for both sides.
In this example, a 40-inch window is dressed with a 50-inch shaped valance. Why is the valance 10 inches wider, you might ask. It’s because the valance has a 4-inch return on each side.
Example #6: The Gathered Valance
Not all valances are flat like the ones I showed you. Some are gathered, of course. With these kinds of valances, it’s much harder to tell you exactly how wide the valance needs to be. It depends on many things, but generally speaking, the valance will need to be somewhere between 2 to 3-1/2 times the width of the window you’re trying to cover.
For example, if the window is 50 inches wide, the valance should be about 100 to 150 inches wide when laid flat. If the valance is unlined or uses a thin, flimsy fabric, you may need to multiply that number to as much as 3.5x your window width to get the proper fit and coverage.
Here’s a resource that can help you calculate the width needed in case you have a very wide window.
If You’ll Install the Valance Over Draperies
Hopefully you have a sense of how wide valances need to be in case you install them as the only window treatment on your window. But what if you want to layer a pair of draperies under your valance?
How wide should the valance be? (I’m talking about heavy draperies here, not just average, light-weight curtains.) If space permits, you ideally want the valance to be 10 to 30 inches wider than the window. In some instances, you can even go beyond 30 inches, allowing your valances and draperies to span across the entire wall of windows.
Take a look at the two examples below. Each window is barely 40 inches wide, yet the window treatments span 60 inches across or more.
Each of the two draperies here is a double width (100 inches wide when spanned out), for a total width of about 200 inches for the pair.
Each of the draperies in this case is a width and a half (about 72 inches when spanned out per drapery). This is a trick that designers often use to make small windows appear larger and to allow more light to come into an otherwise dark room. Try it in your home yourself and you’ll see the difference.
Measurement Fails
Here are a few examples to show you what NOT to do. Don’t get me wrong – these are absolutely gorgeous valances. They’re just not quite the right size. Hopefully you’ll be able to spot the problems.
