Window Seat Dimensions
The numbers
- Depth: 16 to 18 in for a perch, 20 to 24 in for a lounge you can get your legs up on (Today’s Homeowner)
- Height from floor including cushion: about 18 in (This Old House), dropping to 14 to 16 in for children (Today’s Homeowner)
- Length: match or slightly exceed the window width. Published range 3 to 8 ft
- Cushion: 2 to 4 in, 3 in minimum on a hard box
- Backrest where present: 10 to 20 in
- Bedroom windows are egress windows. Minimum net clear opening 5.7 sq ft, height 24 in, width 20 in, sill no more than 44 in above the finished floor (IRC R310)
Published window seat depths run from 16 inches to 30. That looks like disagreement. It is actually two different pieces of furniture wearing the same name.
A 16 to 18 inch window seat is a perch. Feet on the floor, back straight, dining posture. You sit on it to put your shoes on or to eat breakfast.
A 20 to 24 inch window seat is a lounge. Legs up, back against a side wall or a pile of cushions, sideways. You sit in it for an hour with a book.
Decide which one you are building before you look at another number, because everything else follows from it.
Depth
This Old House gives 16 to 20 inches and repeats the range in its build guide. Today’s Homeowner extends it, splitting by use: casual seating 16 to 18, reading nooks 20 to 24, bay windows 18 to 30, with the warning not to go under 18 inches if you want people to actually sit there.
Both are right. The narrow end is the perch, the wide end is the lounge.
One practical note on the lounge version. If you want to sit sideways with your legs up, the seat has to be long enough for your legs, not just deep enough for your hips. A 24 inch deep seat that is only 36 inches long is still a perch, because there is nowhere for your feet to go.
Height
About 18 inches from the floor, including the cushion. This Old House says about 18. Today’s Homeowner gives 16 to 20 and describes it as roughly what you would expect from a dining chair, which is exactly the right mental model.
Lower for children. Today’s Homeowner gives 14 to 16 inches for children’s bedrooms and hallway seats.
The same frame-versus-finished trap applies here as with bench seating. If a plan gives you a height, ask whether the cushion is in it. See bench and banquette dimensions for the compression arithmetic.
Length
This Old House gives an absolute range of 30 to 50 inches. Today’s Homeowner gives 3 to 8 feet, with bay windows at 4 to 8, and offers the only relative rule I could find anywhere: match or slightly exceed the window width.
That rule is worth following. A seat noticeably narrower than the window looks like a mistake. A seat that runs wall to wall under a window looks built in even when it is not.
Cushion
Two to four inches. This Old House says 2 to 4 in both articles. Today’s Homeowner says plan for 3 inches minimum on top of the base.
Backrest, where there is one, runs 10 to 20 inches per This Old House. That is lower than the 16 inch minimum Fine Homebuilding sets for a dining banquette, and the reason is that a window seat back is often the wall, the window frame, or a stack of pillows rather than a built element.
The bit nobody mentions
If the window seat goes in a bedroom, check the egress requirement first.
The International Residential Code, Section R310, requires emergency escape and rescue openings in sleeping rooms. The published minimums are a net clear opening of 5.7 square feet, a clear opening height of 24 inches, a clear opening width of 20 inches, and a maximum sill height of 44 inches measured from the finished floor. Grade-floor openings get 5 square feet.
Why this matters
A window seat built in front of an egress window is a permanent obstruction in front of a required escape route. The code datum does not move because you built furniture.
I checked several of the highest-ranking window seat guides and none of them mention this. If you are building rather than buying, and the room is a bedroom, this is the first thing to check and not the last.
Radiators, vents and moisture
Windows and heat sources live in the same place surprisingly often.
This Old House recommends lattice panels on the front of the seat to let heat escape, or a hinged top for radiator access, and leaving space between the window and the seat for air circulation to prevent moisture buildup. No dimension is given for that gap.
Today’s Homeowner does give clearances: 6 to 8 inches of unobstructed space behind vent grilles, at least 3 inches above radiators, and ventilation slots placed at least 4 inches above the floor.
The moisture point is the one that bites later. Cold glass, a warm room and a sealed box under the seat is a condensation problem waiting to happen. Ventilate the cavity.
Storage inside
If the seat lifts or has drawers, the usable interior is smaller than the outside by about an inch to an inch and a half in each direction, which is the carcass.
There is no published standard for this. The one manufacturer listing I could source gives a storage bench with exterior dimensions of 31.5 by 19 by 15.75 inches and an interior of 30.5 by 17.75 by 14.5. That is a loss of about 0.6 inches per side and 1.25 inches vertically.
So an 18 inch deep window seat gives you roughly 16 to 17 inches of usable interior. Treat that as arithmetic from one example rather than an industry rule, because it is not one.
This Old House’s build uses end cabinets at 12 inches deep by 24.5 tall by 30 wide, with intermediate cabinets 15 inches tall. Today’s Homeowner suggests at least 6 inches of height per drawer.
Quick reference
| Dimension | Perch | Lounge |
|---|---|---|
| Depth | 16 to 18 in | 20 to 24 in |
| Height from floor | 18 in incl. cushion | 18 in incl. cushion |
| Length | Match the window width | 5 ft or more |
| Cushion | 2 to 4 in | 3 to 4 in |
| Backrest | Often none | 10 to 20 in |
Where to go next
Bench seating dimensionsSeat height, depth and the cushion compression arithmetic
All measuring guidesCheck the corner before you buy anything
Questions
How deep should a window seat be?
16 to 18 inches if people will sit facing out with their feet on the floor. 20 to 24 inches if the point is to curl up sideways with your legs on the seat.
How high should a window seat be?
About 18 inches from the floor to the top of the cushion, which is dining chair height. Drop to 14 to 16 inches in a child’s room.
How long should a window seat be?
Match or slightly exceed the window width. Published ranges run from 30 inches to 8 feet, but matching the window is what makes it look intentional.
Can I build a window seat in front of a bedroom window?
Check the egress requirement first. Bedroom windows have to meet minimum escape opening dimensions with a sill no more than 44 inches above the finished floor, and a seat in front of one is a permanent obstruction.
What about the radiator under the window?
Vent the front of the seat with lattice or slots, or hinge the top for access. Leave 3 inches above a radiator and 6 to 8 inches behind a vent grille.
How much storage do I actually get inside?
Subtract about an inch to an inch and a half from each exterior dimension for the carcass. An 18 inch deep seat gives you roughly 16 to 17 inches inside.