You've measured the width twice, checked the height, and still worry that your custom shade won't follow the curve. That concern is justified. Learning how to measure arched windows is less about taking one big measurement and more about describing the opening's geometry and installation conditions accurately.

The measurements that matter are the base width, center height, leg height, curve rise or radius, and opening depth. Each one tells a manufacturer where the arch starts, how it develops, and whether a finished shade, shutter, or frame will clear the opening without binding.

Why Arched Windows Trip Up Even Careful DIYers

A homeowner can record every visible dimension and still receive a treatment that doesn't fit. One common failure happens when the curve begins higher on one side than the other. The numbers may look reasonable, but the shade's spring line sits out of position, leaving an obvious gap or forcing the frame against the jamb.

That problem comes from treating an arch like a rectangle with a decorative top. It's really a geometry problem disguised as a simple tape-and-pencil task. Width tells the fabricator how far the treatment must span, while height and leg measurements show where the curve begins and how it meets the straight sides.

The measurements that control the fit

You'll need to identify:

• Base width: The horizontal distance across the bottom of the arched opening.
• Center height: The vertical distance from the relevant spring line or base reference to the highest point, called the apex.
• Leg height: The height of each straight side before the curve begins. Measure the left and right sides separately.
• Rise or radius: The geometry of the curve. A perfect arch has a predictable relationship between its base and height, but irregular arches don't.
• Opening depth: The space available for an inside-mounted treatment, including room for brackets, fabric, frames, or cassettes.

Confusing radius with width is a frequent mistake. So is measuring the drywall edge instead of the inside frame. An older opening may also be slightly out of square, which means assuming symmetry can produce a treatment that looks correct on paper but binds during installation.

Practical rule: Every measurement should answer a construction question. If you can't explain what a number controls, don't send it as an unexplained figure on a custom order.

The reliable approach is to build a clear picture of the opening before ordering. You'll start with tools and reference points, then test the shape against basic geometry. If the arch refuses to behave like a standard half-circle, a physical template becomes more dependable than a long list of guesses.

Tools and Prep Before You Touch the Window

A steel tape measure gives you a straighter, steadier line across the opening than a soft cloth tape. Record each dimension to the nearest 1/8 inch, so a custom treatment has usable numbers rather than rounded guesses. For broader measuring habits, see this window measurement guide.

Gather the equipment before reaching toward the apex:

• Steel tape: Use one tape for the complete set of measurements.
• Notepad and pencil: Write the reference point beside every number.
• Level: Check horizontal and vertical references, particularly in older homes.
• Step stool: Work from a stable platform instead of stretching.
• Flashlight: Light the upper frame, trim, and bracket-clearance areas.
• Cardboard or builder's paper: Have material ready if the curve looks irregular.
• Measurement sheet: Sketch the opening so left and right values do not get reversed.

Prepare the opening and your reference system

Move furniture and décor away from the window, and clear anything sitting on the sill. Confirm whether the supplier wants inches or millimeters, then use that system throughout. Consistent units and fixed reference points prevent more ordering errors than expensive equipment can correct.

Check the lower opening by measuring its diagonals. You do not need to diagnose the whole installation yet. Flag any visible difference, because a skewed opening may call for a template or a special allowance rather than a standard order.

A phone laser measure can help on a flat wall, but a deep recess creates a trap. The device may read to the glass, trim, or a shadow instead of the treatment's mounting plane. For related glass-construction decisions, consult these windows double paned repair tips. Joey'z Shopping also provides a practical reference for consistent window measuring methods before you measure the curved portion.

Measuring a Standard Perfect Arch

A true half-round arch gives you a useful geometry check, but only if every measurement uses the same reference line. Record the base width, center height, and both left and right leg heights. For a perfect arch, the center height should equal half the base width. That relationship helps separate a genuine half-round from an opening that only looks symmetrical.

Find the spring line first

The spring line is the horizontal line where the straight sides become curved. Establish it at the two curve-start points before placing the tape. Measuring from the sill can combine the rectangular lower section with the arch and produce the wrong height for an arch-only treatment.

Measure across the spring line from the left inside edge to the right inside edge. Check the width again at the middle and near the top of the straight section. If the opening changes, use the smallest width for an inside mount. A wider number can leave the treatment binding or prevent it from seating in the recess, a point also covered in manufacturer measurement documentation.

From the center of the spring line, measure straight up to the apex. Take the reading three times without shifting the center point. Record the repeatable result. A tape held slightly to one side can make the arch appear taller or shorter than it is.

Measure both leg heights from the same lower reference, such as the sill or the bottom of the arch unit, to the point where each straight side meets the curve. Do not average a difference away. Unequal legs can indicate a skewed opening, and that difference may affect how a custom treatment sits.

Use geometry as a sanity check

For a perfect arch, the radius is half the base width, and the center height is also half the base width. A 48-inch base corresponds to a 24-inch center height, while a 60-inch base corresponds to a 30-inch center height. These examples provide a quick check against a misplaced spring line or an off-center tape position.

If the numbers do not follow the relationship, stop before ordering. The opening may have an eyebrow, swept, or otherwise irregular profile, or the measurements may use different starting lines. Ordering from an assumed half-circle can lead to a visible gap, binding at the sides, or a custom piece that must be remade.

Finish by recording the frame depth and any handles, locks, trim, or protruding hardware. Sketch the opening, label left and right, mark the spring line and apex, and place each number beside its measurement line. The sketch preserves your reference system, so the manufacturer can distinguish a width, rise, leg height, and depth instead of interpreting a loose list of figures.

How Arch Shape Changes What You Measure

The arch profile determines which measurements control the fit. A half-circle offers a useful geometric check, but an eyebrow, quarter, gothic, or swept-head opening needs a different reference system. Choosing the wrong formula can produce a shade or insert that binds at the sides, leaves a gap at the apex, or requires a costly custom reorder.

An eyebrow arch is shallow, so its rise is small relative to its width. Record the center height carefully because a small apex error remains visible across the top edge. A quarter arch follows a different geometry. Measure its width and diagonal rise toward the corner, then confirm the corner radius instead of applying a half-round assumption.

A Palladian arch has a rectangular lower section and a curved upper section. Measure the base width and each lower leg height, then measure the upper arc separately. A gothic arch narrows toward a point, so record the apex and check both diagonals. Do not assume the point is centered.

Match the method to the profile

A swept-head arch has no clear spring line because the curve flows directly into the jamb. Trace the profile or use a template. Selecting the transition point by eye changes the effective curve and can leave a custom treatment tight on one side.

Trade guidance separates forms including perfect arch, non-perfect arch, Paladian arch, eyebrow arch, quarter arch, true arch, swept head, and gothic arch, with shape-specific instructions for spring lines, radii, and height conventions in the Rose Collection arches measuring guide. The same broad categories, width, height, and curve transition, still apply. Their practical meaning changes with the profile.

After recording the geometry, Joey'z Shopping's guide to curtains for half-moon windows provides treatment ideas for keeping the curve visible or covering it. For broader home-project planning, Arcs laundry pickup Singapore is a separate household service and is unrelated to the window measurements.

Using a Template for Irregular and Custom Arches

The short answer is that a template wins when the opening won't describe itself cleanly in numbers. Older masonry, hand-built frames, asymmetrical curves, and swept heads can all defeat a width-and-height-only order.

Use butcher paper, builder's paper, or lightweight cardboard large enough to cover the complete opening. Tape it across the inside face, pressing it flat against the frame rather than allowing it to bridge across the curve. The paper should follow the actual mounting edge, not the surrounding drywall.

Trace the opening without changing the profile

Hold a marker perpendicular to the frame and trace the inner edge. Keeping the marker square to the surface helps prevent the line from drifting inward or outward as the curve changes direction.

Mark the following directly on the template before removing it:

• Top or apex: Identify the highest point.
• Spring line: Mark where the curve begins on each side.
• Left and right: Label orientation clearly.
• Mounting face: Note which side faces the room.
• Reference dimensions: Add the base width and a center-height check.

Remove the template carefully and place it back against the opening. The edges should align without forcing the paper into a different position. If it doesn't fit when you return it to the same frame, don't send it. The template is the order, and a bad template is a full-size bad measurement.

For a tall opening, create two sections and join them with the seam clearly labeled. Photograph the template in place before removal, with the frame and orientation visible. That photo helps a fabricator understand whether the traced line followed the frame, trim, or another surface.

Mail a full template in a rigid tube. Folding creates creases that distort the curve and can make a fabricator interpret a bent line as part of the window. Put the measurement sheet and the photograph with the template, not in a separate package that can lose its context.

Depth, Tolerance, and Final Numbers for Your Order

A custom order needs more than a shape. It needs a mounting description. The manufacturer must know whether the treatment will sit inside the opening, over the frame, or against another surface, because depth determines whether brackets, shade cassettes, shutters, or fabric can clear the glass and trim.

One consumer guide identifies 3 1/4 inches of minimum opening depth for certain inside-mount arched-window applications, as stated in Lowe's measurement guidance. Treat that figure as product-specific, not as a universal rule for every covering. Check the supplier's mounting requirements before finalizing the order.

Build a manufacturer-ready sheet

Record the base width at the spring line, center height to the apex, and both leg heights. Use the nearest 1/8 inch throughout, and don't switch between inches and another unit halfway down the page. Add the clear opening depth, mounting choice, frame condition, and any obstruction that could affect hardware.

Include the spring-line height and radius when you can calculate or identify them. If the left and right sides differ, show both figures rather than averaging them. For an irregular arch, send the template with the numbers, because the physical profile may carry information that a table cannot.

Don't invent a tolerance yourself. Fabricators may apply different allowances for fabric treatments, rigid shutters, cellular shades, and replacement frames. A width reduction that works for one product can create an unwanted light gap in another. Ask the supplier which direction and amount they require, then write that instruction on the final sheet.

A general roller-shade measuring habit can help, but an arch still requires shape-specific information. Joey'z Shopping's guide on how to measure for roller shades is a useful reference for mounting language and clear dimensions.

Before submitting, attach a photograph of the opening with the tape visible. The picture should show the measuring plane, not just the finished window from across the room. That small piece of evidence often resolves whether a number refers to the glass, frame, trim, or wall opening.

The following video can provide a visual supplement to the written process:

Quick Recap and Common Measurement Mistakes

A dependable workflow follows the installation, not the shape alone. Prepare the tools, identify the mounting surface, establish the spring line, measure the width, find the center height, check both legs, decide whether the opening needs a template, and finish with depth and supplier-approved allowances.

The mistakes that cause reorders

• Ignoring leg differences: Left and right transitions may not sit at the same height.
• Rounding too early: Keep the original eighth-inch readings until the supplier confirms the final format.
• Choosing the wrong mounting reference: Inside-frame and outside-mount orders use different starting points.
• Skipping depth: A treatment can match the curve and still collide with trim or hardware.
• Calling every arch a half-circle: Eyebrow, quarter, gothic, Palladian, and swept-head profiles need different checks.
• Sending numbers without orientation: Always label the left, right, apex, spring line, and mounting face.

The most useful final check is physical. Put the template back on the opening, confirm that the traced edge follows the same frame edge used for the measurements, and compare the marked apex and spring lines with the actual window.

Frequently asked questions

What if my arched window isn't symmetrical?

Measure both legs and both diagonal paths to the apex. If the shape still looks irregular, create a full template and send it with the measurement sheet.

When should I call a professional?

Call a professional when the opening is difficult to access, the frame has pronounced irregularities, the treatment requires complex inside hardware, or the supplier can't interpret your measurements confidently. A professional measurement is cheaper than guessing on a nonstandard custom order.

How do I verify a paper template?

Place it back against the same inside frame, align the labeled sides, and check the spring-line marks and apex. The traced edge should sit against the opening without folding, stretching, or being forced into place.

Can I order from width and height alone?

Only for a standard profile and a supplier that explicitly accepts those dimensions. Most custom arches benefit from leg heights, depth, shape notes, and a sketch or template.

The measurement process works when every figure has a job. Width controls span, height defines the rise, leg measurements locate the transition, and depth protects the installation. Treat the arch as geometry with hardware consequences, and you'll avoid the shortcut that causes the most expensive mistake, ordering a beautiful treatment for the wrong opening.

Joey'z Shopping offers curtains, blinds, and valances for homeowners updating window treatments, along with practical measuring guidance for unusual openings. Visit Joey'z Shopping with your finished sketch or template notes in hand, then compare treatment options that match the arch's shape and your mounting plan.