Size smartly.
Right window size for any room. Checks code-required egress, natural light minimums, and recommends rough opening dimensions.
How we calculated this
IRC 2021 (Section R310) requires every sleeping room to have at least one window or door that meets egress requirements - a path out of the room in case of fire. Egress windows must have at least 5.7 square feet of openable area, with minimum clear opening dimensions of 20 inches wide and 24 inches tall. The maximum sill height is 44 inches above the finished floor. Ground floor bedrooms can use 5.0 square feet openable area.
Openable area depends heavily on window type. Double-hung and single-hung: only about half the total glass area opens (the sash). Casement windows crank outward and open about 90% of the glass area. Sliding horizontal windows: half opens. Fixed windows don't open at all. Awning and hopper windows: about 70% openable. The calculator applies the appropriate factor automatically.
Natural light and ventilation (IRC R303.1): habitable rooms need total glazing equal to 8% of floor area for light. For an 150 sq ft bedroom, that's 12 sq ft of window glass. For ventilation (unless mechanical ventilation is provided), 4% of floor area must be openable - 6 sq ft for the same bedroom. Kitchens and bathrooms may have mechanical ventilation (exhaust fan) that exempts them from the 4% openable requirement.
Rough opening dimensions add about 1/2 inch on each side to the window's actual dimensions to allow for shimming and adjustment during installation. A 36 × 48 inch window needs a 37 × 49 inch rough opening. Manufacturer specs sometimes require more (up to 1 inch each side for specific brands) - always verify with the exact window you're buying before cutting the framing.
For basements, egress requirements are the same as bedrooms - basement bedrooms must have compliant egress windows or a direct exit door. Window wells are required for any egress window whose sill is below grade; the well itself has minimum size requirements (9 sq ft area, 36 inches projection) and must include a permanent ladder for wells deeper than 44 inches.
Not captured: energy performance ratings (U-factor, SHGC for ENERGY STAR compliance), impact-resistance ratings (required in hurricane zones), triple-pane upgrades, glazing type, tempered glass requirements (within 24" of doors, floors, or wet areas), and egress window cost ($300-1,000 per window installed depending on type and size).
A window is one of the few things in your house the code can force you to make bigger
Pick any style you like. Paint the frame any color. But three code rules set the minimum size of a window before taste gets a vote, and two of them exist because a firefighter has to be able to get in and you have to be able to get out. Egress, natural light, and ventilation. Miss the egress rule on a bedroom window and the room legally stops being a bedroom. This is the rare place in a build where the inspector can tell you the window is too small and mean it.
The calculator up top checks all three against your room. Give it the floor area and it tells you the minimum glass for light, the minimum operable opening for ventilation, and whether a given window clears the egress escape minimum. This page explains those rules, then covers the other window job people actually search for more than any of them: how to measure an existing window so the replacement fits.
Egress: the rule that turns a room into a bedroom
An egress window is an emergency exit. Every sleeping room and every habitable basement space needs at least one, and the numbers are specific because a person in full firefighting gear has to fit through it in the dark. The clear opening, the actual gap when the window is open all the way, has to be at least 5.7 square feet on a ground or upper floor, or 5.0 square feet at grade level. On top of that area, the opening has to be at least 20 inches wide and at least 24 inches tall, and the sill can sit no higher than 44 inches above the floor so you can actually climb out.
Here is the trap that catches people. Those three numbers work against each other. A window that is exactly 20 inches wide would need to be more than 41 inches tall to hit 5.7 square feet, and a window exactly 24 inches tall would need to be over 34 inches wide. Meeting the width minimum and the height minimum does not mean you meet the area minimum. All three have to be true at once, on the clear opening, which is why a casement window (which swings fully open) often passes where a double-hung of the same size fails, since the double-hung only ever opens half its height.
Egress requirement | The number | Measured on | |
|---|---|---|---|
| Min clear opening area, upper floors | 5.7 ft² | 5.7 ft² | The open gap, not the glass |
| Min clear opening area, at grade | 5.0 ft² | 5.0 ft² | The open gap |
| Min clear width | 20 in | 20 in | The open gap |
| Min clear height | 24 in | 24 in | The open gap |
| Max sill height above floor | 44 in | 44 in | Floor to bottom of opening |
Every number is measured on the clear opening with the window fully open. A window's catalog size is always larger than its clear opening, so never size egress off the nominal dimensions.
How to measure a window for replacement
This is the job most people are actually here for, and it is easy to get wrong in a way you do not discover until the new window shows up too big or too small. The golden rule: measure the opening, not the old window, and measure it in three places for each dimension, then use the smallest number. Frames go out of square as houses settle, and ordering to the widest point means the window will not fit the narrow point.
For width, measure the inside of the window frame from left jamb to right jamb at the top, the middle, and the bottom. Write down all three and take the smallest. For height, measure from the sill to the top of the opening on the left side, the middle, and the right side, and again take the smallest. Then check that the opening is square by measuring the two diagonals corner to corner. If those diagonals differ by more than a quarter inch, the opening is out of square and you will need to shim the new window to make it plumb.
Composite illustration based on typical project dimensions, regional contractor pricing, and 2026 material costs. Not a specific real project.
Standard window sizes
Windows come in standard sizes, and staying on the grid saves real money. Custom sizes cost more and take longer, so unless a custom opening is already framed, most people pick the nearest standard. Sizes are written width by height, and the trade uses a shorthand where a 3040 window is 3 feet 0 inches wide by 4 feet 0 inches tall. Here are the common ones.
Type | Common widths | Common heights | |
|---|---|---|---|
| Double-hung | 24 to 48 in | 36 to 72 in | |
| Casement | 18 to 36 in | 24 to 72 in | |
| Sliding | 36 to 84 in | 24 to 60 in | |
| Picture (fixed) | 24 to 96 in | 12 to 96 in |
Standard double-hung sizes step in 2-inch increments. When a room needs a specific glass area for light, it is usually cheaper to use two standard windows than one custom unit.
Natural light and ventilation
Beyond egress, the code wants habitable rooms to have daylight and fresh air, and it states both as a percentage of the floor. The glass area has to equal at least 8 percent of the room's floor area, and the operable opening, the part that actually opens, has to equal at least 4 percent. A 150 square foot bedroom needs at least 12 square feet of glass and at least 6 square feet that opens.
One catch worth knowing: the 8 percent is glass area, not window size. The frame and sash eat 15 to 25 percent of a window's overall dimensions, so a window has to be noticeably bigger than 12 square feet to deliver 12 square feet of actual glass. The calculator works in glass area to keep you honest. If mechanical ventilation is present, some jurisdictions relax the operable opening requirement, but the light requirement almost always stands.
Rough opening and the header above it
The rough opening is the framed hole the window drops into, and it is always larger than the window unit so there is room to shim it plumb and level. The standard is the window unit width plus half an inch on each side, and the unit height plus half an inch at the top. A 36 by 48 inch window wants a 37 by 48.5 inch rough opening. Manufacturers publish exact rough opening dimensions, so check the spec sheet, because a few brands want more or less than the half-inch rule.
Framing a new opening means a header over it to carry the load the missing studs used to. A 36-inch opening takes a doubled 2x6 header in a non-bearing wall, and a doubled 2x8 to 2x10 in a bearing wall depending on what sits above. The stud spacing calculator counts the king studs and jack studs each opening needs, and if you are residing around the new window, the siding calculator handles the squares of siding and the trim.
Egress window wells for basements
A basement bedroom needs an egress window like any other, and below grade that means a window well outside it. The well has to give a person room to climb out: at least 9 square feet of floor area and at least 36 inches of horizontal projection out from the foundation. Go deeper than 44 inches and the well needs a permanently attached ladder or steps, because at that depth a person cannot climb out unaided in an emergency.
Well covers are sized to the well, not the window, and they have to be releasable from inside without tools or special knowledge, which rules out anything bolted shut. Standard covers come in a handful of sizes such as 44 by 38, 49 by 37, and 56 by 38 inches, so measure the well opening at its widest before ordering. A cover that is too small leaves a gap, and one that is too large will not seat.
Where window jobs go wrong
The failures are almost all measuring and ordering mistakes. Sizing egress off the window's catalog dimensions instead of the clear opening tops the list, and it fails inspection because the actual gap is always smaller than the nominal size. Measuring the old window instead of the opening is next, and it produces a replacement that binds or rattles. Ordering to the widest of three measurements rather than the smallest guarantees the window will not fit the tight spot.
Then the mix-ups. Buying a new construction window with a flange for an opening that needed a frame-in replacement, or the reverse. Forgetting that the frame steals a fifth of the glass area and coming up short on the light requirement. And skipping the diagonal check, then fighting an out-of-square opening with a window that was never going to sit plumb. None of these are hard to avoid. Measure the opening in three places, take the smallest, check the diagonals, and size egress on the clear opening, and the window that shows up will be the window that fits.
Frequently asked
What's the minimum window size for a bedroom?
Per IRC egress rules: 5.7 square feet openable area, minimum 20" clear width, minimum 24" clear height. A 32 × 48 inch double-hung barely meets this (with 1-inch frame deduction). A 36 × 48 inch double-hung or a 30 × 36 inch casement gives plenty of margin. Ground-floor bedrooms can use 5.0 sq ft openable.
Why are casement windows better for egress?
Casements open ~90% of their glass area (the whole sash swings outward on a hinge). Double-hung windows only open ~50% because only one sash moves at a time. For the same egress opening, a casement can be much smaller overall - practical for tight spaces where you can't fit a large double-hung.
How do I calculate rough opening size?
Standard rule: add 1/2 inch to each side of the window's advertised dimensions. A 36 × 48 window needs a 37 × 49 rough opening. Some brands specify more - Andersen often calls for 5/8" each side, Pella varies. Always verify with the installation instructions for the specific window you're buying.
Do I need egress windows in the basement?
If the basement has any sleeping rooms, yes - those rooms need compliant egress windows. If the basement is purely storage/utility, egress is not required per IRC but may be required locally. Egress windows in basements need window wells (below-grade excavated space) with minimum 9 sq ft area and a permanent ladder for deep wells.
What does 'rough opening' mean?
The framed opening in the wall before the window is installed. The window fits inside, with shims adjusting position and flashing sealing the perimeter. Rough opening must be larger than the window: typically 0.5-1 inch on each side. Too small: window won't fit. Too large: more shimming required, weaker connection.
How much natural light do I need per room?
IRC requires 8% of floor area as glass for habitable rooms (bedrooms, living rooms, dining rooms, kitchens). For a 150 sq ft bedroom: 12 sq ft of glass minimum. A single 36 × 48" window is 12 sq ft - just meeting code. For comfortable light, double this. The calculator flags rooms where the window is too small.
Can I use a skylight for egress or light?
Skylights count for light (when overhead, they provide more lumens per sq ft than wall windows due to direct sky view). Skylights do NOT count for egress - you can't climb out of a skylight in a fire. Skylights can count for ventilation if they're operable. For bedrooms: always need at least one wall window or door for egress.
What about window wells?
Required for egress windows with sills below grade. Minimum well dimensions: 9 sq ft horizontal area, 36-inch projection from the foundation. Minimum 44" depth needs a permanent ladder. Wells over 44" need attached ladders or steps, never a removable ladder. Budget $500-2,000 for a pre-fab well, $1,500-5,000 for a custom well with ladder and drain.
Sources
- IRC 2021 Section R310: Emergency Escape and Rescue Openings - Egress clear opening, width, height, and sill height requirements
- IRC 2021 Section R303: Light, Ventilation, and Heating - 8 percent light and 4 percent ventilation minimums
- AAMA/WDMA/CSA 101: Window Performance Standard - Window unit and rough opening tolerances
- IRC 2021 Section R310.2.3: Window Wells - Basement egress window well size and ladder rules
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