How to Measure and Fit Security Bars for Unusual Window Shapes

Standard rectangular windows are straightforward to measure. Grab a tape measure, take three width readings and three height readings, use the smallest numbers, and you're done. But what about the arched window above your front door? The trapezoidal window in your stairwell? The oversized picture window that spans half your living room wall? Or that circular porthole window in the attic?

Unusual window shapes are more common than most people realize, especially in older homes, custom builds, and architectural styles that prioritize character over conformity. And while these windows add visual interest to your home, they also present real security vulnerabilities that are often overlooked simply because homeowners assume security bars won't fit them.

That assumption is usually wrong. With the right measuring approach and the right product, the vast majority of odd-shaped windows can be secured. This guide walks you through the process step by step, from the tools you'll need to the specific techniques for each unusual shape, including when a telescopic product like the SWB Model A can adapt and when you'll need a custom solution.

Tools You'll Need

Measuring unusual windows requires a few more tools than a standard rectangular window. Gather everything before you start so you don't have to climb up and down a ladder multiple times.

Essential Tools

• Steel tape measure (25-foot) — Retractable with a locking blade. Don't use a cloth tape; it flexes and gives inaccurate readings on curves.
• Torpedo level (9-inch) — For checking whether surfaces are plumb and level. Critical for trapezoidal windows where nothing is square.
• Pencil and notepad — Sketch each window shape with all measurements labeled. A phone photo helps, but a sketch with dimensions is more useful when you're ordering.
• Step stool or ladder — Many unusual windows are placed high on walls (arches above doors, stairwell windows).

Recommended Additional Tools

• Cardboard sheets and scissors — For creating full-size templates of irregular shapes. Appliance boxes work well for large windows.
• Flexible curve ruler or contour gauge — For tracing arches and curves accurately. A contour gauge (the kind with sliding pins) pressed against a curved frame copies the exact profile.
• Angle finder (digital or manual) — For measuring the exact angle of trapezoidal or angled window edges. A smartphone angle-measurement app works in a pinch.
• Laser distance measurer — Optional but helpful for oversized windows where stretching a tape measure across 6-8 feet solo is difficult.
• Blue painter's tape — For marking measurement reference points on the frame without leaving residue.

General Measuring Principles for Non-Standard Windows

Before diving into specific shapes, understand the principles that apply to all non-rectangular windows.

Principle 1: Find the Largest Rectangle That Fits Inside the Opening

Security bars are rectangular. Your window might not be. The fundamental approach to fitting bars on an unusual window is identifying the largest rectangle that fits within the window's shape. For an arch, that rectangle sits below the curve. For a trapezoid, it's the rectangle formed by the shortest side. For an irregular polygon, you're looking for the widest and tallest rectangular area that doesn't extend beyond any edge.

The SWB Model A telescopes horizontally, so it naturally adapts to the width dimension. Your job is to determine the width at the point where you want to mount the bars and verify that the height works.

Principle 2: Measure Multiple Reference Points

On a rectangular window, measuring at three points is sufficient. On an unusual shape, you might need five, seven, or more reference points to capture the geometry. For arched windows, measure the width at every 3-4 inches of height going up toward the curve. For trapezoidal windows, measure the width at both the top and bottom, plus every 6 inches between.

Principle 3: Document Everything Visually

Draw the window shape to scale on paper (graph paper helps). Label every measurement. Take photos from directly in front of the window (not at an angle, which distorts proportions). If you're ordering custom bars or consulting with an installer, these visuals are worth more than verbal descriptions.

Principle 4: Account for Tolerance

For frame-mount bars, you need the bars to fit snugly within the opening. Leave 1/8 to 1/4 inch of clearance on each side for the telescopic mechanism to expand. The Model A's adjustment range covers this naturally, but your base measurement should be the actual opening width, not a "rounded up" number.

For wall-mount bars, you have more flexibility because the bars mount to the wall surface surrounding the window. Bars can extend beyond the window's shape, covering the irregular portions with excess bar length that simply extends past the glass area.

Arched and Half-Round Windows

Arched windows (also called radius-top or round-top windows) have a flat bottom and sides with a curved top. Half-round windows (also called fanlights or transoms) are fully semicircular. Both are common above entry doors, in stairwells, and in gable walls.

Measuring an Arched Window

1. Measure the full width at the widest point (the bottom of the arch, which is the flat section where the curve begins). This is the same as the straight-sided width below the arch.
2. Measure the height from the sill to the very top of the arch (the apex). This gives you the overall window height.
3. Measure the "spring point" height — the point where the sides transition from straight to curved. Below this point, the window is rectangular. Above it, the window curves inward.
4. Measure the radius of the arch. For a true semicircle, the radius equals half the width. For a segmental arch (a shallower curve), the radius will be larger than half the width. Use a flexible tape or contour gauge to trace the curve.

Fitting Security Bars to an Arched Window

Rectangular bar approach (most common): Mount the Model A at or just below the spring point, covering the rectangular portion of the window. The arch above the bars remains unprotected but is typically too small for a person to fit through. If the arch portion is wider than about 10 inches across at any point, consider additional protection.

Wall-mount spanning approach: Mount bars on the exterior wall surface, spanning from one side of the window to the other. The bars extend above the rectangular portion and into the arch zone. Because bars are straight while the arch curves, there will be triangular gaps between the bars and the curved frame at the top. These gaps are usually too narrow for entry if bar spacing is 4 inches or less.

Custom curved bar approach: For complete coverage, custom-fabricated bars that follow the arch profile are available from specialty metalworkers. This is the most expensive option but provides the most complete protection and the best aesthetics. See our custom window security bars guide for more on this option.

Trapezoidal and Angled Windows

Trapezoidal windows have four sides, but the top and bottom (or left and right) are different widths, creating an angled profile. They're found in gable walls, stairwell sections, and contemporary architectural designs. Some windows have one angled edge and three straight edges; these are technically triangular remnants or parallelograms, but the measuring approach is similar.

Measuring a Trapezoidal Window

1. Measure both the wide edge and the narrow edge. If it's a horizontal trapezoid (wider at the bottom, narrower at the top), record both the bottom width and top width.
2. Measure the height on both sides. The left and right sides will be different heights if the top edge is angled.
3. Measure the angle of the sloped edge using an angle finder. Note whether the slope is on the top, bottom, left, or right.
4. Find the "inscribed rectangle." This is the largest rectangle that fits entirely within the trapezoid. Its width equals the narrow edge, and its height extends from the wider edge to the point where the narrow edge meets the sloped sides.

Fitting Security Bars to a Trapezoidal Window

Interior frame mount at the narrow dimension: The Model A can frame-mount at the narrow width of the trapezoid. The bars will leave uncovered gaps in the corners where the shape widens beyond the bar length. Whether these gaps are acceptable depends on their size; if the gap is under 6 inches wide at any point, it's too narrow for entry.

Wall-mount spanning the wide dimension: Mount bars on the wall surface, spanning the full wide dimension. The bars extend beyond the narrow edges at the angled end, providing complete coverage. The excess bar length beyond the window frame is aesthetically neutral on exterior installations and can be hidden by trim on interior installations.

Oversized Picture Windows

Oversized picture windows — typically 6 to 12 feet wide and 4 to 6 feet tall — are the centerpiece of many living rooms and great rooms. They're a security concern because the large glass area provides an easy entry point if broken, but their sheer width exceeds what a single bar unit can span.

Measuring an Oversized Window

1. Measure the full inside width at three points (top, middle, bottom). On large windows, variation between points can be significant; a 96-inch window that measures 95.75 at the top and 96.25 at the bottom is common in older homes.
2. Measure the full height at three points (left, center, right).
3. Check for mullions or dividers. Some large windows have interior mullions (vertical bars built into the window) that divide the glass into sections. Measure each section separately if you plan to mount bars within each mullion division.
4. Check the frame depth. Large windows often have deeper frames (3-4 inches) to accommodate thicker glass. Deeper frames provide a more secure surface for frame-mount bars.

Fitting Security Bars to an Oversized Window

The SWB Model A's modular stacking capability is designed for exactly this situation. Two or three units placed side by side cover the full span. Each unit telescopes to its portion of the width, and they're connected by shared mounting hardware to form a continuous barrier.

For a complete guide to securing wide openings, see our article on security bars for large windows.

Dividing the Width

When using multiple modular units, divide the total width as evenly as possible. For a 96-inch window using two units, each unit covers approximately 48 inches. For a 120-inch window using three units, each covers approximately 40 inches. Check the Model A's minimum and maximum telescopic range to ensure each section falls within the adjustment range.

Circular and Porthole Windows

Circular windows (also called oculus, round, or porthole windows) are found in attics, gable ends, bathrooms, and nautical-themed architecture. They range from 12-inch portholes to 36-inch decorative rounds. While small circular windows may not be large enough for a person to fit through, larger ones (24 inches or more in diameter) are potential entry points.

Measuring a Circular Window

1. Measure the inside diameter at three angles: horizontal, vertical, and diagonal. A truly circular window will have the same measurement in all three directions. If the measurements differ, the window is actually elliptical (oval), and you need both the major and minor axis measurements.
2. Measure the frame depth (how thick the frame is from front to back).
3. Note the frame material. Circular windows often have specialized frames (sometimes metal, sometimes wood with a circular routing) that may not provide the same mounting surfaces as rectangular window frames.

Fitting Security Bars to a Circular Window

Horizontal bar across the diameter: The simplest approach is mounting the Model A horizontally across the widest diameter of the circle. The bar covers the center of the window, and the remaining uncovered areas above and below the bar are crescent-shaped and too narrow for entry if the bar is positioned at the center.

Wall-mount cross pattern: For larger circular windows, mounting two bars in a cross pattern (one horizontal, one vertical) divides the circle into four quadrants, none of which is large enough for a person to pass through. This requires wall-mount brackets at the 12, 3, 6, and 9 o'clock positions on the surrounding wall.

Custom circular grille: A metalworker can fabricate a circular grille that matches the window's diameter exactly, with bars arranged in a sunburst, grid, or decorative pattern within the circle. This is the most complete and visually cohesive solution.

Creating Cardboard Templates

For any unusual window shape, a full-size cardboard template eliminates guesswork. Here's how to make one.

Step-by-Step Template Process

1. Get cardboard large enough to cover the window. Appliance boxes, shipping boxes, or poster board from a hardware store work well. For windows larger than a single piece, tape pieces together with packing tape.
2. Hold the cardboard against the window opening from the side where the bars will mount (interior or exterior).
3. Trace the window frame outline onto the cardboard using a pencil. For arches, press the cardboard against the curve and trace along the edge. For angled shapes, trace each straight edge.
4. Cut out the template along the traced lines. Use a utility knife for straight edges and scissors for curves.
5. Test-fit the template back into the window opening. It should fit snugly within the frame. Trim as needed until the fit is accurate.
6. Label the template with the window's location (e.g., "master bedroom west wall"), the date, and an arrow indicating "UP" and "INSIDE" or "OUTSIDE" so you don't get confused later.
7. Mark the mounting rectangle on the template. Draw the largest rectangle that fits within the shape; this is where your security bars will sit.

A cardboard template serves two purposes: it gives you a precise visual reference for ordering, and it lets you physically hold the template up to potential bar products at the store (or compare against the Model A's dimensions listed online) to verify the fit before buying.

When the SWB Model A Adapts vs. When Custom Is Needed

The SWB Model A's telescopic design handles a surprising range of unusual windows. But it has limits. Here's how to determine which approach your window needs.

Model A Adapts: These Situations

• Arched windows — The rectangular portion below the spring point is covered perfectly. The arch gap above is typically too small for entry.
• Trapezoidal windows — Mount at the narrow width; the slight uncovered corners are usually too small to matter.
• Oversized windows — Modular stacking covers any width in sections.
• Elliptical (oval) windows — Mount across the widest horizontal diameter.
• Windows that are slightly out of square — The telescopic adjustment compensates for frames that are up to 1/2 inch out of square.
• Any window where the uncovered area is under 6 inches wide at its widest point — Too narrow for a person to fit through.

Custom Is Needed: These Situations

• Large circular windows (30+ inches diameter) where a single horizontal bar leaves crescent openings large enough for entry.
• Severely angled trapezoidal windows where the difference between the wide and narrow edges exceeds 12 inches, leaving substantial uncovered triangular areas.
• Gothic pointed-arch windows where the arch narrows to a point, creating a tall triangular area above rectangular bars.
• Multi-curve or serpentine windows with S-curves, compound curves, or other organic shapes that have no usable rectangular section.
• Historically designated windows where visible bars must follow the window's profile exactly to satisfy preservation requirements.

For custom solutions, see our comprehensive guide on custom window security bars. A local metalworker or security fabricator can build bars that follow any shape, though expect 4-8 weeks of lead time and costs starting around $300-$500 per window depending on complexity.

Common Measuring Mistakes to Avoid

After years of helping homeowners secure unusual windows, these are the mistakes we see most often.

Mistake 1: Measuring the Glass Instead of the Frame Opening

The glass is smaller than the frame opening. Security bars mount in or on the frame, not on the glass. Always measure the inside dimensions of the frame, from jamb to jamb and sill to head. If you measure the visible glass area, your bars will be too small to seat properly in the frame.

Mistake 2: Not Accounting for Frame Irregularity

Unusual windows in older homes often have frames that have settled, warped, or shifted over decades. An arched window that was a perfect semicircle when built in 1920 may have a slightly flattened or asymmetric curve today. Always measure the actual current geometry, not what you think the shape "should" be.

Mistake 3: Forgetting to Check Clearance for Window Operation

Some unusual windows still open. Arched windows may have an operable lower section (a rectangular double-hung or casement below a fixed arch). Trapezoidal windows in stairwells sometimes have an operable lower section. Make sure your bar placement doesn't prevent the operable section from functioning. Reference our full window measuring guide for clearance specifics by window operation type.

Mistake 4: Taking a Single Measurement on a Curved Surface

Curves require multiple reference points. Measuring just the widest point of an arch tells you nothing about the shape of the curve above it. Take width measurements at every 3-4 inches of height as you move up into the curved section. This series of measurements defines the curve and tells you exactly where a rectangular bar will fit and where the gaps will be.

Mistake 5: Ignoring Wall-Mount as an Option

Homeowners with unusual windows often fixate on frame-mounting because that's the standard approach for rectangular windows. But wall-mounting is almost always simpler for irregular shapes because the bars attach to the flat wall surface surrounding the window rather than trying to fit within an oddly shaped frame. A rectangular bar wall-mounted over a circular window provides complete security even though it doesn't follow the window's shape.

Mistake 6: Not Creating a Template

For any window that isn't a standard rectangle, skip the template and you're guessing. Spend 15 minutes making a cardboard template and you'll save yourself the cost of returning a product that doesn't fit and reordering the correct size.

Frequently Asked Questions

Your Next Steps

Unusual windows don't have to remain unprotected. Here's what to do now:

1. Identify your unusual windows. Walk your home and note every window that isn't a standard rectangle.
2. Gather your tools — tape measure, level, angle finder, cardboard, and scissors.
3. Create a template for each unusual window using the cardboard method described above.
4. Determine your approach. For most shapes, the SWB Model A covers the rectangular inscribed area. Check whether any uncovered gaps exceed 6 inches.
5. Order or consult. If the Model A covers your needs, you can order directly. If custom fabrication is needed, bring your templates and measurements to a local metalworker for a quote.

For standard rectangular windows throughout the rest of your home, check our complete measuring guide and our best window security bars for 2026 roundup to find the right setup for every window you own.