Measuring an arch window for replacement requires three core measurements: the base width, the total height at the tallest point, and the height of the straight sides before the curve begins. Getting these right determines whether your replacement window fits cleanly or leaves gaps that need shimming, trimming, or worse, a reorder.
Identify Your Arch Type First
Not all arched windows curve the same way, and the shape affects how manufacturers build the replacement. There are three common types you’ll encounter:
- Equal leg arch: A rectangular window with a curved top. The two straight sides (legs) are the same height, and the curve sits on top like a cap.
- Elliptical arch: A straight bottom edge, two short vertical sides, and a wider, flatter curve across the top. The curve is more oval than circular.
- Eyebrow arch: Similar to elliptical, but the curve meets the bottom corners at sharp angles rather than flowing smoothly into vertical sides. These have little to no straight leg height.
Knowing which type you have helps the manufacturer match the curve profile. If you’re unsure, take a photo of the full window from straight on and send it along with your measurements when ordering.
Tools You’ll Need
A sturdy 25-foot metal tape measure with a locking feature is essential. You’ll be measuring overhead, often from a ladder, and a lock keeps the blade rigid while you record numbers. A measuring square helps you check whether the window opening has true 90-degree corners at the base, which tells you if the frame has shifted or settled over time. A standard level (or laser level) confirms that the sill is flat and the sides are plumb.
For windows that are high up or particularly large, a laser distance measure speeds things up and reduces the chance of reading a flexing tape wrong at an awkward angle.
Measuring the Base Width
Start with the width across the bottom of the arch opening. Place your tape measure inside the frame, from one side jamb to the other, at the widest point along the sill. This is your base width, sometimes called the span or chord length.
Take this measurement in three places if the window has straight legs: once at the sill, once at the midpoint of the legs, and once just below where the curve starts. Use the smallest of the three numbers. If the opening is slightly out of square, the smallest measurement ensures the replacement unit will fit through the tightest point. Record to the nearest 1/8 inch.
Measuring the Total Height
Measure from the bottom of the sill straight up to the highest point inside the arch opening. This is the center height, and it captures the full dimension the replacement window needs to fill. Keep the tape perfectly vertical. If you’re not confident it’s straight, hold a level against it or have someone sight the tape from a few feet away.
As with width, precision matters here. A measurement that’s off by even a quarter inch can mean a window that’s too tight to install or too loose to seal properly.
Measuring the Leg Height
The leg height (also called shoulder height) is the vertical distance from the bottom of the sill to the point where the curve begins. This is where the straight side of the window transitions into the arch, sometimes called the spring line. On an equal leg arch, both sides should be the same height, but measure both anyway to confirm.
To find the spring line, run your eye or a straightedge up the side jamb. The point where the frame stops being vertical and starts bending inward is your spring line. Measure from the sill up to that point on each side. Round down to the nearest 1/8 inch if your measurement falls between markings.
Some window manufacturers ask you to add roughly 2 inches to the leg height measurement to ensure the bottom rail of the replacement sits correctly. Check with your specific manufacturer before adjusting, since this varies by product line.
Calculating the Arch Rise
The rise is the height of just the curved portion, measured from the spring line up to the peak of the arch. You can get this by subtracting the leg height from the total center height. So if your total height is 72 inches and your legs are 48 inches, your rise is 24 inches.
Manufacturers need the rise to determine whether your arch is a true semicircle, a partial circle, or an ellipse. For a true semicircle, the rise equals exactly half the base width. If your base width is 36 inches and your rise is 18 inches, you have a perfect half-round. If the rise is less than half the width, you have a segmental or elliptical arch.
If you want to confirm whether the curve is a segment of a perfect circle (which simplifies manufacturing), you can calculate the radius. Take the rise (H) and the base width (W) and plug them into this formula: radius equals (H squared plus W squared) divided by (8 times H). A consistent radius across the curve means the manufacturer can use a standard circular template rather than a custom mold, which typically costs less and ships faster.
Checking for Square and Plumb
Before you finalize your numbers, use a measuring square at the two bottom corners where the sill meets the side jambs. You’re checking for 90-degree angles. If the corners aren’t square, note the degree of deviation. A small amount (less than 1/4 inch across the full width) is normal in older homes and can be corrected with shims during installation. Anything larger may need frame repair before a replacement window goes in.
Hold a level against each side jamb to check that it’s plumb (perfectly vertical). Then lay the level on the sill to check that it’s flat. If the sill slopes noticeably, measure the height on both the left and right sides and note both numbers. The installer needs to know about any tilt to plan the fit correctly.
Rough Opening vs. Ordering Size
The measurements you take inside the existing frame are your rough opening dimensions. The actual window you order will be slightly smaller to allow clearance for installation. Standard deductions vary by window type: for picture windows and double-hung styles, manufacturers typically subtract 1/4 inch from the width and 1/2 inch from the height. Casement and awning styles usually get 1/4 inch taken off both width and height.
Some manufacturers apply these deductions automatically when you submit rough opening sizes. Others offer “tip to tip” ordering, where they build the window to the exact dimensions you provide with no deductions. Confirm which system your manufacturer uses before placing an order. Getting this wrong means your window arrives either too large to fit or too small to seal.
Recording Your Measurements
Write down every measurement as you take it, not after you climb down from the ladder. A simple sketch of the window shape with dimensions labeled at each point prevents confusion later. Your final set of numbers should include:
- Base width: the narrowest of your three horizontal measurements
- Total center height: sill to peak
- Left leg height: sill to spring line on the left
- Right leg height: sill to spring line on the right
- Rise: total height minus leg height
- Arch type: semicircle, elliptical, or eyebrow
Measure everything twice on separate occasions if possible. Arch windows are among the most expensive window types to manufacture, and a mismeasurement often means a complete reorder since curved glass can’t be trimmed on site.