A window size calculator converts rough opening measurements to compatible standard window sizes, or tells you the exact rough opening needed for a specific window. Getting the right fit saves money by avoiding custom orders and ensures proper insulation, shimming, and a weather-tight seal.
Window Size Finder
Measure the framed opening width and height in inches.
Different window types may have different standard sizes available.
Standard Window Size Reference Chart
| Window Size (W x H) | Rough Opening (W x H) | Common Types |
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How to Use the Window Size Calculator
Choosing the right window for a rough opening — or framing the right rough opening for a window — is one of the most common tasks in new construction and renovation. Our free window size calculator matches your measurements to standard window sizes instantly, so you can order with confidence and avoid costly returns.
Step 1: Choose Your Mode
Select whether you already have a rough opening that needs a window, or you have picked a window and need to know the rough opening size. The calculator adjusts its inputs and results based on your selection. Most remodel projects start with an existing rough opening, while new construction starts with a desired window size.
Step 2: Enter Your Dimensions
Enter the width and height in inches. If you are measuring a rough opening, measure from stud to stud for width and from the sill plate to the header for height. Measure at three points (top, middle, bottom for width; left, center, right for height) and use the smallest measurement to ensure the window fits.
Step 3: Filter by Window Type
Optionally filter results by window type. Single-hung and double-hung windows are the most common for residential use. Casement windows crank open and provide better ventilation and a tighter seal. Sliding windows work well in wide openings, and picture windows are fixed panes used for large views where ventilation is not needed.
Step 4: Review Compatible Sizes
The calculator displays all standard window sizes that fit your measurement, ranked from best fit to acceptable fit. Each result shows the window dimensions, required rough opening, shim space on each side, and a fit rating. A perfect fit means 1/2 inch of shim space per side — the industry standard for proper shimming, insulating, and leveling.
Understanding Rough Openings
The rough opening is always slightly larger than the window itself. The standard formula adds 1 inch to the window width and 1/2 inch to the window height. This gap — filled with shims and expanding foam — allows you to plumb and level the window even if the framing is not perfectly square. Too little shim space makes installation difficult; too much compromises insulation and structural support.
Tips for Accurate Measurements
Always use a tape measure, not a ruler. Check for square by measuring diagonals — if the two diagonals differ by more than 1/4 inch, the opening is out of square and may need shimming adjustments. For replacement windows, measure the existing window frame (not the rough opening) since replacement units fit inside the existing frame.
FAQ
Is this window size calculator free to use?
Yes, the window size calculator is completely free with no limits or signup required. You can look up as many window sizes and rough openings as you need. All calculations run locally in your browser and nothing is sent to any server.
Is my data private when using this tool?
Absolutely. All calculations happen entirely in your browser using client-side JavaScript. Your measurements are never transmitted to a server or stored anywhere. Your data stays on your device.
What is a rough opening for a window?
A rough opening is the framed hole in the wall where a window is installed. It is slightly larger than the actual window to allow room for shimming, leveling, and insulating. Typically, the rough opening is 1 inch wider and 1/2 inch taller than the window unit.
How much bigger should a rough opening be than the window?
The standard rule is to add 1 inch to the window width (1/2 inch of shim space on each side) and 1/2 inch to the window height (1/2 inch at the top for shimming). Some manufacturers recommend slightly different tolerances, so always check the installation guide for your specific window.
What are the most common standard window sizes?
The most common residential window sizes in inches are 30x48, 36x48, 36x60, and 48x48. These sizes are widely stocked at home improvement stores and generally cost less than custom sizes. The first number is always the width and the second is the height.
What is the difference between single-hung and double-hung windows?
In a single-hung window, only the bottom sash slides up and down while the top sash is fixed. In a double-hung window, both the top and bottom sashes can move. Double-hung windows are easier to clean and offer more ventilation options but typically cost 10-20% more.
Can I install a smaller window in a larger rough opening?
Yes, but you will need to frame down the rough opening to the correct size for the new window. Use 2x lumber to build out the opening so the window fits snugly with the standard 1/2 inch of shim space on each side. Never leave excessive gaps — they compromise insulation and structural integrity.