How to Measure for Replacement Windows

Step 1: Understand What You Are Measuring
For a standard insert replacement - where the new window fits into the existing frame - you measure the inside of the existing frame, not the rough opening in the wall. If the frame is damaged and you are doing a full-frame replacement, you will measure the rough opening instead. This guide covers the more common insert replacement scenario.

Step 2: Measure the Width
Extend your tape measure across the inside of the frame from jamb to jamb (the two vertical side pieces). Take this measurement at three heights - near the top, at the middle, and near the bottom. Record all three numbers. The smallest of the three is your working width. This accounts for any bowing or settling in the frame.

Step 3: Measure the Height
Now measure from the top of the sill (the flat horizontal piece at the bottom of the frame) up to the underside of the head jamb (the top horizontal piece). Take this measurement at three positions - left side, center, and right side. Again, use the smallest number as your working height.

Step 4: Apply the Manufacturer Deduction
Most window manufacturers require you to deduct a small amount from both dimensions to allow for proper shimming, squaring, and insulation during installation. A common deduction is 1/8 inch on each side (1/4 inch total per dimension), but this varies by brand and installer. Always confirm the exact deduction with your window supplier or contractor before finalizing your order.

Step 5: Check for Square
Place a level diagonally across each corner of the frame. If the bubble is significantly off, the frame is out of square. Note this and share it with your installer - a skilled crew can often compensate during installation, but extreme racking may affect which window style fits best.

Step 6: Record and Double-Check
Write your final width and height (after deductions) clearly, width first. Re-measure at least once to confirm. Even a 1/4-inch error can require a reorder, so take your time and verify before submitting any order.

• Use a sturdy ladder on a flat surface when measuring windows above the ground floor - never lean out of an open window to reach a corner
• Keep children away from open windows, especially on upper floors, during measurement
• Watch for rotted or soft wood around old frames, which can crumble or give way unexpectedly
• If the window is painted shut with lead-based paint (common in homes built before 1978), avoid sanding or scraping it without proper EPA Lead-Safe precautions
• Wear gloves when handling old aluminum frames, which can have sharp edges