Before You Start: Tools and Preparation
Before taking any measurements, gather the right tools and understand what you are measuring. For replacement windows (also called insert or pocket windows), you are measuring the inside dimensions of the existing window frame -- not the rough opening in the wall framing. The existing frame stays in place, and the new window unit fits inside it.
You will need a steel tape measure (25 feet minimum), a notepad, a pencil, and a level. If the window has interior stops or trim that can be removed, take them off to expose the full frame. If you cannot remove the stops, measure between them and note this on your worksheet.
Important Note on Measurement Accuracy
Always measure to the nearest 1/16 inch. Round down, never up. A window that is 1/16 inch too small can be shimmed easily, but a window that is 1/16 inch too large cannot be installed without modifying the frame. Take each measurement at least twice and verify your numbers before ordering.
Step 1: Measuring the Width
The width measurement is taken from the inside face of one side jamb to the inside face of the opposite side jamb. Take this measurement at three points: the top of the window opening, the middle, and the bottom. These three measurements often differ because frames settle, shift, or warp over time.
Top Width
Place your tape measure at the top of the window opening, about 1 inch below the head jamb. Measure from the inside face of the left jamb to the inside face of the right jamb. Record this measurement.
Middle Width
Measure at the horizontal center of the opening. This is the most commonly used measurement, but it should not be the only one you take.
Bottom Width
Place your tape measure about 1 inch above the sill. Measure from the inside face of the left jamb to the inside face of the right jamb. This measurement is often the smallest because sills can bow upward.
Use the smallest width measurement.
Of the three width measurements, record the smallest one as your ordering width. The replacement window must fit through the narrowest point of the opening. Any gaps at wider points are filled with shims and insulation during installation.
Step 2: Measuring the Height
The height measurement is taken from the highest point of the window sill (not the stool, which is the flat interior ledge) to the underside of the head jamb at the top. Like width, take three measurements: left side, center, and right side.
Left Height
Place your tape measure on the sill at the left side of the opening and measure straight up to the underside of the head jamb. Make sure the tape is plumb (vertical). A level held against the tape can help verify this.
Center Height
Measure from the sill to the head jamb at the horizontal center of the opening. This is the most commonly referenced height measurement.
Right Height
Measure from the sill to the head jamb on the right side of the opening. If this differs from the left side by more than 1/4 inch, the opening is noticeably out of square.
Use the smallest height measurement.
Of the three height measurements, record the smallest one as your ordering height. The window must clear the shortest point during installation.
Step 3: Measuring the Depth (Frame Depth)
Frame depth is the measurement from the inside face of the window frame to the outside face. This determines whether a replacement window will fit within the existing frame without protruding past the interior or exterior wall surface. Most replacement windows require a minimum frame depth of 3-1/4 inches.
To measure depth, open the existing window sash and place your tape measure against the inside face of the frame, extending it outward to the exterior stop or blind stop. If the frame has interior stops or trim, measure from behind them (from the jamb surface). Record the depth measurement at multiple points because older frames can vary in depth due to settling or repair work.
Depth Considerations for Energy-Efficient Windows
Triple-pane and some high-performance double-pane windows are thicker than standard replacement windows. If your frame depth is less than 3-1/2 inches, verify that your chosen window product will fit. Insufficient depth is one of the most common reasons a replacement window installation fails during the fitting stage. In some cases, frame extensions or a full-frame replacement may be necessary.
Step 4: Check if the Opening Is Square
A square opening has corners that form true 90-degree angles. Over time, house settling, foundation movement, and frame deterioration cause openings to shift out of square. An out-of-square opening affects window operation and sealing, so it must be identified before ordering.
Measure the diagonals of the opening: from the top-left corner to the bottom-right corner, and from the top-right corner to the bottom-left corner. If the two diagonal measurements match within 1/4 inch, the opening is acceptably square. If the difference exceeds 1/2 inch, the frame may need to be rebuilt or adjusted before installation.
Within 1/4 Inch
The opening is square enough for a standard replacement window installation. Shims will compensate for the minor difference.
1/4 to 1/2 Inch Difference
The opening is moderately out of square. Installation is still possible with careful shimming, but mention this to your installer so they can plan accordingly.
Over 1/2 Inch Difference
The opening is significantly out of square. This indicates frame damage, structural settling, or both. A professional assessment is recommended before ordering windows.
Level Check
Hold a level against the sill and each side jamb. The sill should be level and the jambs should be plumb. Deviations greater than 1/4 inch per foot suggest structural issues.
Common Measurement Mistakes to Avoid
These are the mistakes we see most often from homeowners who attempt to measure for replacement windows on their own. Avoiding these errors will save you time, money, and frustration.
Measuring the glass instead of the frame opening
The glass size is not the window size. You need the frame-to-frame dimensions, which are always larger than the glass. Measuring the glass will result in a window that is too small.
Taking only one measurement per dimension
A single width or height measurement can miss variations caused by settling or warping. Always take three measurements for each dimension and use the smallest.
Measuring from the exterior
Exterior measurements include trim, casing, and brick mold that will remain in place. Always measure from inside the house, from jamb to jamb.
Rounding measurements up
Always round down to the nearest 1/16 inch. A window ordered even slightly too large cannot be installed without cutting the frame, which is costly and time-consuming.
Forgetting to check the frame condition
Rotted, warped, or damaged frames will not support a replacement window properly. Probe the frame with a screwdriver to check for soft spots, especially at the sill and lower corners where water collects.
Ignoring frame depth
If the new window is deeper than the existing frame, it will protrude past the interior wall. This is especially common when upgrading from single-pane to double or triple-pane windows.
Measuring for Special Window Types
Not all windows are standard rectangles. Bay windows, bow windows, arched windows, and custom shapes require additional measurements and considerations.
Bay and Bow Windows
Measure each section of the bay or bow individually, including the angle between sections. Record the projection distance from the wall. Bay and bow windows typically require professional measurement because the angles and structural support must be precisely calculated.
Arched and Circle-Top Windows
For arched windows, measure the width at the widest point (the spring line where the arch begins) and the height from the sill to the peak of the arch. Also measure the height from the sill to the spring line. The arch radius is typically half the width, but verify this -- some arches are elliptical rather than semicircular.
Casement and Awning Windows
Measure the same way as double-hung windows: width at three points, height at three points, and depth. Additionally note the hinge side (left or right for casement, top for awning) and the crank or handle location. These details affect the ordering specifications.
Sliding Windows
Measure width and height as with any window. Note which panel slides (left, right, or both) and whether the window has a fixed center panel. For three-panel sliders, measure the overall opening and the position of the fixed panel.
DIY Measurement vs Professional Measurement
While homeowners can certainly measure their own windows using this guide, professional measurement offers several advantages. A trained installer will identify frame damage, structural issues, and installation complications that measurements alone do not reveal. Most window companies in the DC metro area, including Expert Glass Repair, offer free professional measurement as part of the estimate process.
If you are ordering windows through a full-service installer, the company will typically send a technician to take final measurements after you accept the estimate. These measurements are guaranteed -- if the window does not fit due to a measurement error, the installer remakes it at no cost. This guarantee does not apply to homeowner-supplied measurements.
When Professional Measurement Is Essential
Always use professional measurement for custom shapes (arched, circular, trapezoidal), bay and bow windows, historic homes with non-standard frames, windows with visible frame damage, and any situation where the opening appears significantly out of square. These scenarios have too many variables for reliable DIY measurement.
Frequently Asked Questions
What tools do I need to measure for replacement windows?
You need a steel tape measure (at least 25 feet), a notepad and pencil, a level (2 to 4 feet long), and a straightedge or square. Avoid cloth tape measures as they stretch and provide inaccurate readings. A digital tape measure can also work but verify its accuracy against a known measurement first.
Should I measure from the inside or outside of the house?
For replacement windows (insert windows that fit into the existing frame), always measure from the inside. You need the dimensions of the existing window frame opening, not the exterior trim or casing. Measure from jamb to jamb for width and from the sill to the head jamb for height. Outside measurements include trim and casing that will remain in place.
How much gap should I leave for replacement windows?
Standard practice is to order replacement windows 1/4 inch smaller than the opening in both width and height. This provides 1/8 inch of clearance on each side for shimming and insulation. Some manufacturers specify their own clearance requirements, so always check the installation guide. Too much gap creates insulation problems; too little makes installation impossible.
What if my window opening is not square?
Out-of-square openings are common, especially in older homes. Always take three measurements for both width and height, then use the smallest measurement when ordering. The installation team will use shims and foam insulation to fill any gaps. If the opening is more than 1/2 inch out of square, the frame may need to be rebuilt before installation.
Can I measure for replacement windows myself or should I hire a professional?
Homeowners can take preliminary measurements using this guide, and many do so accurately. However, professional measurement by the installing company is strongly recommended before ordering. Professionals check for structural issues, verify the window type, assess the frame condition, and account for factors homeowners often miss, such as sill angle and frame depth. Most reputable window companies offer free measurement as part of the estimate process.
Related Guides
By the Expert Glass Repair Team
Serving the DMV since 2004 -- DC, Northern Virginia & Maryland
Expert Glass Repair provides professional window measurement and replacement services throughout the Washington DC metro area. Our technicians measure every window before ordering to guarantee a perfect fit. Call (703) 679-7741 for a free measurement and estimate.
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