How to Measure for a Replacement Window

Video Guide

Watch before you order — takes less than 5 minutes

Quick Reference

The golden rule: measure 3 times each direction, always use the smallest measurement, and subtract ¼" from each dimension.

WidthLeft jamb → Right jamb (3 spots: top, mid, bottom)

HeightSill → Head jamb (3 spots: left, center, right)

DeductionSubtract ¼" from both width and height

Order formatWidth × Height — always width first

⚠️ Important: Custom-sized windows cannot be returned. Measure twice — order once.

Step-by-Step Measuring Guide

Measure the Width

Measure inside the window frame from the left jamb to the right jamb. Take three measurements — top, middle, and bottom. Use the smallest number. Subtract ¼" for proper clearance.

💡 Pro tip: Always measure from jamb to jamb — not the glass or sash.

Measure the Height

Measure inside from the sill to the top of the frame. Take three measurements — left, center, and right. Use the smallest number. Subtract ¼" for clearance.

💡 Pro tip: Do not measure from the outside trim — measure the inside rough opening.

Check for Square

Measure diagonally corner to corner both ways. If the two diagonal measurements match (within ¼"), your opening is square and a standard replacement will fit perfectly.

💡 Pro tip: Openings more than ½" out of square may require shimming or custom sizing.

Record & Order

Write down Width × Height (always width first). These are your "rough opening" dimensions. When ordering, specify your rough opening — we build the window to fit.

💡 Pro tip: Double-check your numbers before submitting — custom windows cannot be returned.

Measuring FAQ

Do I measure from inside or outside?

Always measure from the inside of the frame. You want the distance between the side jambs (left to right) and from the sill to the head jamb (bottom to top).

What if my measurements are in fractions?

Round down to the nearest 1/8". For example, if your width measures 35 15/16", order a 35 7/8" window. It's always safer to order slightly smaller than the opening.

What is a rough opening vs. a window size?

The rough opening is the hole in your wall. The window size is the unit itself, which should be slightly smaller than the rough opening to allow for shimming and sealing.

My opening is out of square — can I still order?

Yes. Order based on your smallest width and smallest height measurements. When installing, use shims to plumb and level the window in the opening.

Do I need to remove the old window to measure?

No — for replacement windows you measure the existing frame opening with the old window still in place. Remove any interior trim or stops that might interfere with your tape measure.

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