Before You Start: Tools and Preparation
Gather these tools before you begin. Measuring every window in a room takes about 10-15 minutes once you have the process down.
Steel tape measure (25 ft)
More accurate than cloth tape. Rigid blade holds straight across the opening.
Notepad or phone
Record every measurement immediately. Label by room and window position.
Flathead screwdriver or pry bar
Needed to carefully remove interior stops or trim for accurate pocket measurements.
Level (2 ft or 4 ft)
Used to check if the window opening is plumb and square.
Replacement vs. new construction: This guide covers replacement (pocket or insert) windows, where the new window fits into the existing frame. New construction windows require measuring the rough opening, which involves removing the entire window frame -- a different process. If you are unsure which type you need, call us at (703) 679-7741 and we can assess your situation during a free visit.
Measure the Width
Measure the horizontal distance between the inside faces of the window jambs (the vertical surfaces on each side of the window). Take three measurements:
Measure across the top of the window opening, from left jamb to right jamb.
Measure at the center of the window opening, from left jamb to right jamb.
Measure across the bottom of the window opening (along the sill), from left jamb to right jamb.
Record the smallest of the three width measurements.
The replacement window must fit through the narrowest point. If your three measurements are 35-7/8", 36", and 35-3/4", your width measurement is 35-3/4". Any gaps at wider points will be shimmed and sealed during installation.
Measure the Height
Measure the vertical distance from the top of the sill (where the window sits) to the underside of the head jamb (the horizontal surface at the top of the opening). Take three measurements:
Measure from the sill to the head jamb along the left side of the opening.
Measure from the sill to the head jamb at the center of the opening.
Measure from the sill to the head jamb along the right side of the opening.
Record the smallest of the three height measurements.
Same principle as width -- use the shortest measurement. If your three height readings are 53-1/2", 53-3/8", and 53-1/2", your height measurement is 53-3/8". The window must clear the shortest point.
Measure the Depth
The depth (also called the frame depth or jamb depth) determines what window profiles will fit in your opening. Measure from the inside face of the interior trim or stop to the outside face of the exterior trim or blind stop.
Common Window Frame Depths
Minimum for most replacement windows. Common in older DMV homes with single-pane windows.
Standard range for modern double-hung and casement replacement windows. Most common in DMV-area homes built after 1970.
Standard new construction frame depth. Allows room for thicker insulated glass units and deeper frames.
Depth matters for energy efficiency. If your frame depth is too shallow, you may be limited to thinner window profiles that have less insulating value. Many pre-1960s homes in Arlington, Alexandria, and Georgetown have shallow frame depths (under 3-1/4") that require either a special slim-profile replacement window or a full-frame replacement. We assess this during our free measurement visit.
Check for Square
An out-of-square opening means the window frame is not a true rectangle. This is common in older homes where the structure has settled over time -- and extremely common in DMV-area homes built before 1950.
How to Check Square
Measure diagonally from the top-left corner to the bottom-right corner. Write it down.
Measure diagonally from the top-right corner to the bottom-left corner. Write it down.
Compare the two diagonal measurements. If they match within 1/4 inch, the opening is square enough for a standard replacement window. If the difference exceeds 1/4 inch, the opening is out of square and may require shimming, a custom-sized window, or frame repair.
Within 1/4 inch
Opening is square. Standard replacement window will work with normal shimming.
More than 1/4 inch
Opening is out of square. Professional assessment recommended before ordering. May need frame work or a custom window.
Identify Your Frame Type
The existing window frame material affects how the replacement window is installed and what products are compatible. Identify your frame type before ordering.
Wood Frame
The most common frame type in pre-1980 DMV homes. Wood frames are found in virtually all historic homes in Georgetown, Old Town Alexandria, Capitol Hill, and similar neighborhoods. Look for visible wood grain, painted or stained surfaces, and corners joined with nails or dowels.
Installation note: Check for rot. Probe the sill and lower corners with a screwdriver -- if the wood is soft or crumbly, the frame needs repair or replacement before installing new windows.
Vinyl Frame
Standard in homes built after 1990 and in previous replacement windows. White or tan hollow plastic profiles with welded corners. Vinyl frames cannot be painted and may yellow or warp over time, especially on south-facing exposures in the DMV summer heat.
Installation note: Check for warping or cracking. Vinyl that has deformed may not provide a stable surface for a pocket replacement. A full-frame replacement may be needed.
Aluminum Frame
Common in 1960s-1980s homes and many DMV condominiums. Thin metal profiles, usually silver or bronze anodized. Aluminum conducts heat readily, making these frames the least energy-efficient option.
Installation note: Aluminum frames are often thinner than wood or vinyl, which limits the depth available for replacement windows. Measure depth carefully.
Composite / Fiberglass Frame
Found in newer or recently replaced windows. Looks similar to painted wood but is a fiberglass or wood-composite material. Stronger than vinyl, more stable than wood, and can be painted.
Installation note: These frames are typically in good condition and make excellent candidates for pocket replacement installations.
Common Measurement Mistakes to Avoid
These errors cause the most problems with replacement window orders. Every one of them results in delays, extra costs, or poorly fitting windows.
Measuring the glass instead of the frame opening
Always measure the frame pocket -- the space the entire window unit sits in. The glass is smaller than the frame opening by several inches in each direction.
Taking only one measurement per dimension
Windows settle and frames shift over time. Always take three measurements for width (top, middle, bottom) and three for height (left, center, right), then use the smallest.
Using the old window as a reference
The existing window may not be the correct size for the opening -- it may have been shimmed, modified, or was the wrong size to begin with. Always measure the opening itself.
Rounding measurements to the nearest inch
Measure to the nearest 1/8 inch. A 1/2-inch rounding error can mean the difference between a window that fits and one that does not.
Forgetting to check for square
An out-of-square opening looks fine to the eye but causes installation problems. Diagonal measurements take 30 seconds and reveal issues that save hours of frustration later.
Ignoring frame damage or rot
A replacement window installed in a rotted frame will fail. Check for soft wood, peeling paint, moisture damage, and visible gaps before ordering new windows.
DMV-Specific Window Measurement Considerations
The Washington DC metro area has housing stock that spans three centuries, from colonial-era rowhouses to modern high-rises. Each era has its own window challenges.
Historic Homes (Pre-1930)
Weight-and-pulley windows with non-standard sizes. Openings may be significantly out of square due to settling. Many have unusual proportions that require custom-sized replacements. Historic district regulations in Alexandria, Georgetown, and Capitol Hill may require specific window styles.
Mid-Century Homes (1940-1970)
Many have aluminum single-pane windows with shallow frame depths. Common in Arlington, Bethesda, and Silver Spring. The aluminum frames are often corroded and may need full-frame replacement rather than pocket inserts.
Late 20th Century (1970-2000)
First generation of vinyl and double-pane windows. Many are now failing with fogged glass or broken seals. Typically standard sizes that match current replacement window catalogs.
Condos and High-Rises
Often require association approval and may have specific window specifications. Measure carefully -- condo windows are frequently non-standard sizes. High-rise installations above the 3rd floor require commercial-grade frames rated for wind load.
When to Call a Professional for Measurements
DIY measurements work well for standard rectangular windows in good frames. However, some situations call for professional measurement to avoid costly errors.
The opening is significantly out of square (more than 1/4 inch difference in diagonals)
You have arched, round-top, or non-rectangular windows
The existing frame shows signs of rot, water damage, or structural failure
You are in a historic district with specific window requirements
The home is a high-rise condo with non-standard sizes or wind load requirements
You are replacing bay windows, bow windows, or multi-unit combinations
You are unsure whether you need a pocket replacement or full-frame replacement
Free Professional Measurement
Expert Glass Repair provides free on-site window measurements throughout DC, Northern Virginia, and Maryland. Our technicians measure every opening to 1/16-inch accuracy, check for frame damage, assess depth compatibility, and identify any issues that could affect your replacement window order. No obligation -- call (703) 679-7741 to schedule.
Measurement Recording Checklist
Use this checklist for each window. Label every measurement with the room name and window position (e.g., "Kitchen -- above sink" or "Master bedroom -- south wall, left").
| Measurement | Where to Measure | Record |
|---|---|---|
| Width -- top | Inside jamb to inside jamb, across the top | ______ in. |
| Width -- middle | Inside jamb to inside jamb, across the center | ______ in. |
| Width -- bottom | Inside jamb to inside jamb, across the sill | ______ in. |
| Height -- left | Sill to head jamb, along the left side | ______ in. |
| Height -- center | Sill to head jamb, at the center | ______ in. |
| Height -- right | Sill to head jamb, along the right side | ______ in. |
| Depth | Interior stop to exterior stop | ______ in. |
| Diagonal A | Top-left to bottom-right | ______ in. |
| Diagonal B | Top-right to bottom-left | ______ in. |
| Frame type | Wood / Vinyl / Aluminum / Composite | ________ |
| Frame condition | Good / Fair / Damaged / Rotted | ________ |
Frequently Asked Questions
Should I measure from inside or outside the house?
Always measure from inside the house. Remove the interior trim or stops if possible to access the window frame pocket. Interior measurements give you the actual opening size that the replacement window must fit into. Exterior measurements include trim and casing that are not part of the window pocket.
Do I need to deduct anything from my measurements for replacement windows?
For pocket (insert) replacement windows, most manufacturers recommend ordering windows 1/4 inch to 1/2 inch smaller than the measured opening in both width and height. This allows for shimming and leveling during installation. Your installer or manufacturer will specify the exact deduction -- do not deduct on your own without guidance.
Can I measure windows without removing the interior trim?
For a rough estimate, yes. Measure from jamb to jamb (the flat surfaces on each side of the window). However, for an accurate measurement, removing at least one section of trim or stop reveals the actual frame pocket dimensions, which may differ by 1/2 inch or more from a jamb-to-jamb measurement.
What if my window measurements are different at the top and bottom?
Always use the smallest measurement. If the width at the top is 35-3/4 inches and the bottom is 36 inches, order based on 35-3/4 inches. The gaps at wider points can be filled with shims and sealed during installation, but a window that is too large will not fit.
How accurate do my window measurements need to be?
Measurements should be accurate to 1/8 inch. A 1/4-inch error can usually be accommodated with shimming, but anything more can cause fit problems, air leaks, or the need to reorder. This is why we recommend professional measurement for custom and high-end window projects.
Related Services
Related Articles
Signs You Need New Windows
10 warning signs that your windows need replacement.
Window Buying Guide
How to choose the right replacement windows for your home.
Energy-Efficient Windows Guide
Understanding Low-E, double-pane, and argon-filled windows.
Window Tax Credits 2025-2026
Federal and local incentives for energy-efficient windows.
By the Expert Glass Repair Team
Window measurement and installation specialists serving the DMV since 2004
Expert Glass Repair provides free on-site window measurements and replacement installations throughout Washington DC, Northern Virginia, and Maryland. We measure to 1/16-inch accuracy and guarantee proper fit on every installation. Call (703) 679-7741 to schedule your free measurement.
Need Help Measuring Your Windows?
Skip the guesswork. Our technicians provide free on-site measurements with 1/16-inch accuracy throughout DC, Northern Virginia, and Maryland.