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Every code and bench figure3 entries, with sources

Accessible bench, for comparison

Seat 17 to 19 in high, 20 to 24 in deep and at least 42 in long. The federal standard for accessible benches, given here as a yardstick (ADA 903.3 and 903.5, the Americans with Disabilities Act standards)

Bedroom escape window

An opening of at least 5.7 sq ft, at least 24 in high and 20 in wide, with the sill no more than 44 in above the floor (IRC R310, the International Residential Code)

Window fall protection

Where the bottom of the opening is under 24 in above the floor and more than 72 in above the ground outside, the window can’t open wide enough to pass a 4 in sphere, or it carries an ASTM F2090 fall-prevention or opening-control device that releases to the full escape opening (IRC R312.2.1 and R312.2.2)

How tall should a window seat be?

Aim for about 18 inches from the floor to the top of the cushion. That’s roughly the height of a dining chair seat.

With the cushion on, anywhere from 16 to 21 inches is comfortable, so you’ve got a little leeway when you build.

For a child’s room, drop it to 14 to 16 inches. And if you’re shopping for a window bench instead of building a seat, most benches stand 18 to 20 inches tall, right in the same range.

How thick should my window seat cushion be?

For a window seat, go 3 to 4 inches thick. Cushions run as thin as 2 inches, but that’s too little on a hard base. The thinner the cushion, the firmer its foam has to be, so it doesn’t bottom out.

Both cushions in the table up top land in that range. The one for just sitting is 4 inches thick, the top of the range, and the one for reading is 3, right at the minimum for a hard base.

If you’d like something plusher or a different fabric, Window Seat Cushions has plenty more to look through.

How deep should a window seat be?

Go 16 to 18 inches deep if you’ll mostly sit facing forward, and 20 to 24 inches if it’s where you’ll settle in to read. Kitchen benches and casual seating sit in that shallower range, so think of it as a perch. Reading nooks get the deeper one, a lounge.

A 12 inch seat is only enough for a quick perch, like a little nook on a stair landing where you’d sit for a minute. Hallway seats go a bit deeper, at 14 to 16 inches, and 16 is where comfortable sitting starts when you face forward.

A cushion is as deep as the seat it sits on, so each pick up top matches one kind of seat, the shallower one for just sitting and the deeper one for reading. Build the seat 18 or 20 inches deep, and there’s a ready-made cushion in that size.

If you can’t decide, go 20 inches deep. That’s deep enough to read in, and still comfortable when you sit up straight.

How wide should a window seat be?

Make it as wide as the window, or a little wider. Window seats run anywhere from 3 to 8 feet, and the window decides where yours lands.

Double-hung windows, the most common kind, are usually 24 to 48 inches wide. One person needs at least 32 inches to sit facing forward, so under a narrow window, run the seat past it.

For two people, 42 to 50 inches does it, and 60 or more gives you extra seating. If the wall lets you, go at least 48 inches, so you can sit sideways with your back against one wall.

That makes the 45 inch cushion for just sitting a seat you face forward on, while the 60 inch one for reading has room to spare.

Bay window seat dimensions

A bay window built for seating sticks out 24 inches or more from the wall. Smaller bays, at 12 to 18 inches, bring in light more than room to sit.

The seat inside a bay usually runs 18 to 30 inches deep, so it can go deeper than one under a flat window. The height stays the same, about 18 inches to the top of the cushion.

The angled sides are the tricky part, since a ready-made rectangle won’t follow them. Window Seat Cushions walks through measuring a bay and the cushions you can have cut to its shape, angles and all.

What is a good window sill height?

For a window seat, start the window at least 6 inches above the top of the cushion. Over an 18 inch seat, that means a sill 24 inches or more from the floor.

In a bedroom, there’s an upper limit too. The building code wants every bedroom to have a window big enough to get out through in an emergency, with its sill no more than 44 inches from the floor. So for a seat in a bedroom, a sill from 24 to 44 inches works on both counts.

If the bottom of the opening is under 24 inches from the floor and the ground outside is more than 6 feet down, the window needs fall protection. The two devices for it are a window guard and a limiter that keeps the window from opening all the way, and both release so it can open fully in an emergency.

Child-safety advice is to keep furniture away from windows, so children don’t climb up near them. A window seat is furniture right under the window. With young kids at home, it’s worth fitting a guard or a limiter even where the code doesn’t ask for one.

Window seat measurements, step by step

Start at the window. Its width tells you how wide the seat should be, and its sill height from the floor tells you how much room you’ll have above the cushion.

Next, decide whether you’ll just sit there or read there.

Then pick the cushion, before you build anything. Make the box 18 inches minus the cushion’s thickness, and the top of the cushion lands at 18 before anyone sits.

Last, look at what’s under the window, since a radiator, a vent or a plan for storage all change the box.

Radiators, vents and storage under the seat

Over a radiator, leave at least 3 inches of space above it. Lattice panels on the front of the seat let the heat out, and a hinged top lets you reach the radiator.

If the seat covers a heating vent, put vent grilles on its front, at least 4 inches off the floor, with 6 to 8 inches of open space behind them.

A lift-up top or drawers turn the seat into storage. The walls take up a little room, so an 18 inch deep seat holds a bit under 17 inches front to back inside. Drawers need at least 6 inches of height each, plus room for the slides and framing.

Some space between the window and the seat lets air move, which keeps moisture from building up back there.

Window seat height with cushion

A window seat’s height includes the cushion, so the 18 inches is measured to the top of the cushion you’ll sit on.

The easy mistake is building the box to 18 inches and setting the cushion on top. Then the seat sits 3 or 4 inches higher than you planned, by the thickness of the cushion. That’s why you pick the cushion before you build.

If a build plan or a bench listing gives a height and says nothing about the cushion, you can’t tell whether the cushion is counted. For a bench, that’s worth asking the seller about before you order.

Building a window seat to fit a ready-made cushion

Say your window is 45 inches wide, or a little narrower, and you’d like a perch under it. The ROFIEJOX tufted cushion in 45 x 18 x 4 inches fits that, so you’d build the box 45 inches long and 18 deep to match it. The box comes out 14 inches tall, which puts the top of the cushion at 18 before anyone sits.

If you’d rather have 18 inches with someone sitting on it, build the box to 16 inches. That allows about 2 inches for 4 inch foam to sink, so the seat starts at 20 and settles to about 18.

The 2 inch allowance is for plain 4 inch foam, and the listing doesn’t say how far this cushion sinks. So get it first and measure it with someone sitting on it before you set the box height.

The 60 x 20 x 3 inch lounge cushion goes on a box 60 inches long, 20 deep and 15 tall, for 18 inches to the top before anyone sits. Either way, the top of the cushion lands at dining chair height.

Where to go next

Window seat questions

I sold contract-grade furniture from 2016. More about who runs it.