How to Measure Window Glass for Replacement
Measure the glass you can see, width first and then height, to the nearest 1/16 inch, and note whether it is a single sheet or a sealed double pane unit and how thick it is. That is enough for a quote, but not for cutting: the real glass is larger than what you see, because its edges sit inside the frame. Here is how to take each measurement, what else to note, and why the final size is best taken with the old glass out.
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Measuring the visible glass
Use a steel tape measure, not a cloth one, and measure from inside the house.
1. Width
Measure across the glass from the edge of the frame on one side to the edge on the other. Do it at the top, middle and bottom, and write down the smallest.
2. Height
Measure from the top edge of the visible glass to the bottom, on the left, middle and right. Again, keep the smallest.
3. Each pane on its own
A double hung window has two sashes, and each has its own glass. Measure only the broken one, and say whether it is the top or bottom sash.
4. Write it width by height
Glass sizes are always written width first, so 24 by 36 means 24 inches wide and 36 inches high.
Why the real glass is bigger than what you see
The edges of the glass sit in a groove in the frame, hidden behind glazing putty, a bead or a rubber gasket. That hidden part is called the bite, and it is on all four sides. If you order glass at the visible size, it will fall straight through the opening.
How much bigger depends on the window: an old wood sash with putty, a vinyl window with snap in beads, and an aluminum frame with a gasket all hold the glass differently. That is why the cutting size is measured with the old glass out, or taken from the frame groove itself, not guessed from the visible size.
What to note besides the size
| What to note | How to check | Why it matters |
|---|---|---|
| Single or double pane | Look at the edge: two sheets with a strip between them is a sealed unit | A sealed unit is built to order, not cut from a sheet |
| Thickness | For a single pane, measure a broken edge. For a sealed unit, the full depth from outer face to inner face | The new glass has to fit the same groove in the frame |
| Stamp in a corner | A small etched mark in one corner | Usually means tempered glass, so the new glass must be tempered too |
| Print on the spacer | Text on the strip between the panes of a sealed unit | Can show the maker and date, which helps match the unit |
| Grids or tint | Bars between the panes, frosted, tinted or patterned glass | The new glass has to match the rest of the window |
One sheet, cut to size and set in fresh putty or new beads. The usual case in older wood sashes, storm windows and basement windows.
The whole unit is replaced, made to the size and depth of your frame, with low-E and gas fill chosen to match or improve on the old one.
Glass in doors and near tubs must be safety glass
If the glass is in a door, a storm door, a sliding patio door or a tub or shower enclosure, the replacement has to be safety glass, tempered or laminated, even if what was there before was not. The U.S. Consumer Product Safety Commission sets a federal safety standard for architectural glazing that covers glass in those products. Building codes add other spots, such as large low windows and glass next to doors.
Tempered glass can’t be cut after it is made, so the size has to be exact before it is ordered. See our tempered glass page for more on how it is made.
Send us your numbers, we measure the rest
For a quote, your visible size, the thickness and a few photos are plenty. Before anything is cut or ordered, we measure the opening ourselves, so a small mistake on your tape never turns into glass that doesn’t fit. Cut glass can’t be returned, which is the main reason to let the installer take the final size. For what moves the price, see what sets the cost of replacement glass. Read more on our window glass repair page.
- Visible width and height, smallest readings
- Single or double pane, and the thickness
- A photo of the whole window and a close photo of one corner
- Which sash or pane is broken, and which floor it is on
Measuring window glass questions
More questions? Browse our glass repair FAQ or contact us.
How do I measure window glass for replacement?
Measure the visible glass width first, then height, at three points each, and keep the smallest reading to the nearest 1/16 inch. Note whether it is single or double pane and how thick it is.
Is the replacement glass the same size as what I can see?
No. The edges of the glass sit inside the frame, so the real glass is larger on all four sides. The cutting size is best measured with the old glass out.
How do I measure the thickness of a double pane window?
Measure the full depth of the sealed unit, from the outer face of the outside sheet to the inner face of the inside sheet. The new unit has to fit the same groove.
Do I need tempered glass for my replacement?
If the glass is in a door, near a tub or shower, or in a spot the building code lists, yes. A small stamp in one corner usually means the old glass was tempered.
Can you give me a quote from my measurements?
Yes. Your visible size, the thickness and a few photos are enough for a free estimate. We measure the opening ourselves before anything is cut or ordered.
Measured your window?
Send us the size and a photo, or call (202) 600-6220 for a free estimate.