How to Measure Skylights and Specialty Shape Windows for Shades

Quick Answer: To measure a skylight for shades, use a steel tape and measure the inside of the skylight frame or well: width at the top, middle and bottom, length at the left, center and right, and the depth of the opening. Record the smallest width and length to the nearest 1/8 inch, check both diagonals for square, and note any cranks or sensors. Arched and trapezoid windows need extra measurements: the arch's base width and center height, or the trapezoid's bottom width, both side heights and the sloped edge.

Shaped and overhead windows are custom-made from your numbers, so a measuring mistake costs more here than on a standard window. This guide covers skylights, arched windows and trapezoid windows step by step, with worked examples and the checks that catch errors before you order from our specialty shapes shades collection.

Tools and Ground Rules

• A steel tape measure marked in 1/8-inch increments. Cloth tapes stretch and give false readings.
• A stable ladder or platform for skylights and gable windows. Skylights in most homes sit 9 feet or more off the floor, so clear a safe working space first and have a second person hold the tape end on high glass.
• Paper and pencil, or a photo of each window with numbers written on it.

Three rules apply to every shape:

1. Measure every window separately, even if two look identical. Frames that look the same often differ by 1/4 inch or more.
2. Record width first, then height or length, and never round. Write 34 5/8, not "about 34 1/2."
3. Do not subtract anything yourself unless the measuring guide tells you to. Custom shades are usually made from exact frame measurements, with clearance deductions taken at the factory.

How to Measure a Skylight for Shades

Skylight shades almost always mount inside the frame or the drywall well (the light shaft between the ceiling and the roof window). You are measuring the opening the shade will sit in, not the glass.

Step 1: Width of the window frame at three points

Measure the inside width of the opening from side to side at the top (nearest the roof peak), middle and bottom. Write down all three and circle the smallest. If the shade is built to the widest number, it will bind in the narrowest spot.

Step 2: Length at three points

Measure from the top inside edge to the bottom inside edge at the left, center and right. On a sloped skylight, "length" runs along the slope, not straight up and down. Circle the smallest.

Step 3: Check the diagonals

Measure corner to corner both ways. If the two diagonals differ by more than about 1/4 inch, the opening is out of square, and a rigid shade may leave a tapered gap on one side. Note the difference on your sheet.

Step 4: Measure the depth

Measure from the frame edge to the room-side face of the well. The shade's headrail and side rails need a flat mounting surface of sufficient depth, and the guide for your product lists the minimum.

Step 5: Look for obstructions

Venting skylights may have a crank, handle, motor or rain sensor inside the frame. Note its position and how far it projects, because hardware inside the opening is one of the most common reasons a skylight or arched shade will not fit.

Step 6: Check the mounting surface and durable materials

Note whether the skylight well is drywall, wood, aluminum or vinyl. Shades are made from durable materials, but their durability depends on what the brackets screw into. Wood and solid drywall corners are easy; thin aluminum or vinyl can be difficult and may need different fasteners or a slightly different bracket position. Two factors to check before you select a mount: enough flat space for each bracket, and nothing (trim, a sensor, a hinge) where you plan to add a screw. A quick look now will ensure the shade goes up the first time.

Measurement	Where	Use
Width	Top, middle, bottom	Smallest
Length	Left, center, right	Smallest
Diagonals	Both corners	Flag if difference is over 1/4"
Depth	Frame edge to well face	Confirm minimum mount depth
Obstructions	Cranks, handles, sensors	Note location and projection

Our skylight shades use the cellular measuring method described on the cellular shade measuring page. For help choosing fabric and power once you have your numbers, see our sibling post on motorized skylight shades, or go straight to the Motorized Blackout Skylight Cellular Shades.

How to Measure an Arched Window

Arched shades are built to the curve, so you need the base width, the height of the curve, and a quick check of what kind of arch you have. Getting these right lets the shade create a clean, built-in line that will enhance the shape instead of hiding it.

1. Base width: measure straight across the bottom of the arch, inside the frame, where the curve meets the straight sides or the lower window's head.
2. Center height: measure from the middle of that base line up to the highest inside point of the curve.
3. Identify the arch type: divide the width by two. If the center height equals that number (within about 1/4 inch), it is a standard half-round arch. If the height is noticeably less, it is an eyebrow or segmented arch, which needs a different template.

Example: a base width of 48 inches with a center height of 24 inches is a true half-round, since 48 / 2 = 24. A 48-inch base with a 15-inch rise is an eyebrow arch.

If you also need the glass area, for example to compare heat loss, the area of a half-round is 3.14 x radius x radius / 2. A 48-inch arch has a 24-inch radius, so 3.14 x 24 x 24 / 2 = about 904 square inches, or roughly 6.3 square feet. Weffort's arched shade is made for standard (perfect) arches. Follow the arch cellular shade measuring guide for its exact steps, and see our arch window shades guide for how to treat the lower window.

How to Measure a Trapezoid Window

Trapezoid windows have one sloped edge, so you need four numbers instead of two.

1. Bottom width inside the frame.
2. Left height inside the opening along the left edge.
3. Right height inside the opening along the right edge. On a triangle, the short edge is zero.
4. Sloped edge length from the top of the short side to the top of the tall side.

Also note which side is taller as you face the window from inside the room. That orientation is easy to get backwards, and it decides which way the shade is built.

How to find a trapezoid's side length (and check your work)

The sloped edge is the hypotenuse of a right triangle whose sides are the bottom width and the difference between the two heights. Use it to check your tape reading:

Example: bottom width 48 inches, left height 24 inches, right height 60 inches. The rise is 60 - 24 = 36 inches. The sloped edge should be the square root of (48 x 48 + 36 x 36) = the square root of 3,600 = 60 inches. If your tape says 60 1/8, you measured correctly. If it says 57, re-measure, because one of the heights is off.

The angle of the slope is the arctangent of rise / width: 36 / 48 = 0.75, which is about 37 degrees. Most people never need the angle, but it is a quick way to confirm that two matching trapezoids on a gable really are mirror images.

To figure the square footage of a trapezoid window, average the two heights and multiply by the width: (24 + 60) / 2 x 48 = 2,016 square inches, or 14 square feet.

Use the trapezoid shade measuring guide for the exact form fields, and see our sibling post on trapezoid window shades for angled and gable windows for fabric and control choices.

Why Precise Measurements Matter for Energy Efficiency

A custom shade that fits its window frame tightly does more than look tidy. Every gap between the shade and the frame lets warm air reach cold glass in winter and lets sunlight and heat slip past in summer. On skylights, which face the sky and collect heat at the top of the house, those gaps add up. Measuring to the nearest 1/8 inch and choosing the smallest width ensures the shade sits close to the frame on every side without rubbing, which is how a cellular shade delivers the insulation it is designed for. Many homes have a variety of window styles and sizes on one wall, so it is worth taking slightly more time on each one rather than assuming they match.

Inside Mount or Outside Mount?

For skylights and shaped windows, an inside mount is the default, because the shade needs the frame to hold its shape and rails. An outside mount is only practical on a standard rectangle below an arch or gable, where you may want to cover trim or hide a shallow frame. If you do outside-mount a lower window, measure the total area you want covered, including overlap past the frame on each side. Our guide to inside vs outside mount explains the trade-offs.

Common Measuring Mistakes

Mistake	What happens	Fix
Measuring the glass instead of the frame	Shade is too small and leaves light gaps	Measure inside the frame or well
Using the widest width	Shade binds or will not fit	Use the smallest of three widths
Rounding to the nearest half inch	Visible gap or rubbing edges	Record to the nearest 1/8 inch
Swapping trapezoid sides	Shade slopes the wrong way	Note the taller side from inside the room
Ignoring skylight cranks	Shade hits hardware	Measure obstruction position and depth
Assuming identical windows	One shade fits, the other does not	Measure every window separately

Not sure your numbers are right? Read how the MeasureFit guarantee works before ordering. To understand how cell size changes the stack and look of the finished shade, see the cellular shade cell size guide.

Frequently Asked Questions

How do I measure a skylight for blinds?

Measure the inside width at three points and the length at three points, use the smallest of each, check both diagonals, and measure the depth of the opening. Record every number to the nearest 1/8 inch and note any cranks or sensors.

How to measure skylight for shades if the opening is not square?

Measure both diagonals. If they differ by more than about 1/4 inch, still record the smallest width and length, and note the diagonal difference so the fit can be checked before the shade is made.

How do I find a trapezoid's side length?

Subtract the short height from the tall height to get the rise, then take the square root of (width squared + rise squared). Compare that result with your tape measurement of the sloped edge to catch mistakes.

How do I calculate the area of an arch?

For a half-round arch, the area is 3.14 x radius x radius / 2, where the radius is half the base width. A 36-inch arch has an 18-inch radius and an area of about 509 square inches.

How do I figure the square footage of a window?

Multiply width by height in inches and divide by 144. For a trapezoid, multiply the width by the average of the two heights first.

What are the standard sizes of skylights?

Skylight manufacturers sell a range of standard sizes, usually named by the roof opening. The finished opening inside your house depends on the well, drywall and trim, so never order a shade by the size printed on the skylight label. Measure the actual opening the shade will sit in.

Can you get shades for skylights?

Yes. Skylight shades are made to order for sloped and flat skylights, usually as cellular shades that run in side rails, and can be motorized for skylights out of reach.

Ready to Order?

Once you have every window measured and double-checked, you are ready to select fabric and control. Browse skylight, arched and trapezoid options in the Weffort specialty shapes shades collection and add your exact measurements on the product page. Keep your measuring sheet and photos: they are easy to share with an installer and make any later question quick to answer.