There is no single standard marble window sill size, because every window opening and wall is different. The right sill is as long as the opening, plus any ears you want past the trim, and as deep as the distance from the window frame to the finished wall face, plus a small overhang. Marble is a good choice for interior sills; for exterior sills, granite holds up better. If your window does not match a stock piece, a sill cut to order is the normal route.
What sizes do marble window sills come in?
Marble sills are usually 3/4 inch thick, the same as our standard thresholds. Length and depth are where sizes vary. A stock piece only works if it happens to match your opening, so check the sizes listed in our marble window sills category against your own measurements.
Our stock thresholds come in 24, 30 and 36 inch lengths and 3 to 6 inch widths. A shallow sill in a narrow window can sometimes use one of those sizes. Many windows are wider than 36 inches, though, and deep walls need deeper stone. Longer, deeper or thicker pieces are custom work, cut at our Jersey City shop.
How do you measure a window sill for marble?
Measure in inches, to the nearest 1/16 inch, and write down every number. You need three things: length, depth and the shape of the ends.
- Measure the opening length. Measure from side to side inside the window opening, where the sill will sit. Take it at the front and the back, because old openings are often out of square. Use the larger number and note the difference.
- Decide on ears. Ears (also called horns) are the ends of a sill that extend past the opening and sit under or past the side trim. If you want them, measure how far past the opening each side should go, and add both to the length.
- Measure the depth. Measure from the face of the window frame, where the sill butts against it, out to the finished wall face. Do this after drywall or plaster is up, not before.
- Add the overhang. Most sills project a little past the wall face. Add the overhang you want to the depth.
- Note anything in the way. Window latches, tracks, radiators and outlets below the sill can all change what will fit.
Our threshold measuring guide uses the same method, and the same tools work for both jobs.
How deep should a marble window sill be?
As deep as your wall needs, plus the overhang. There is no fixed number. Sill depth follows wall construction, so a thin framed wall gives a shallow sill and a thick masonry wall gives a deep one. Here is how each measurement adds up:
| Measurement | Where to take it | What to add |
|---|---|---|
| Length | Side to side inside the opening, front and back | Ears past the opening on each side, if you want them |
| Depth | Window frame face to finished wall face | A small overhang past the wall |
| Thickness | Height available under the window frame and trim | 3/4 inch is standard; thicker stone is custom |
| Ends | Shape of the side trim or returns | Square ends, or notched ears to wrap the trim |
Keep the overhang modest. A small lip looks finished and sheds dust. A deep, unsupported overhang is a lever, and a hard knock on it can crack the stone.
Should you use marble for an interior or exterior sill?
Use marble indoors. Marble is a calcium-based stone, so acids etch it, including acid rain. Outdoors it also faces freeze and thaw cycles, which are hard on any stone with fine veins and fissures. An exterior marble sill tends to dull and weather over time.
For exterior sills, granite is the better choice. It is harder and far less sensitive to acids. Our granite pieces, such as Black Absolute, follow the same sizing approach. For a side-by-side look at the stones, see marble vs granite vs quartz.
Indoors, marble is a classic sill material. It handles sun, plant pots and the odd coffee cup well, as long as it is sealed and wiped up promptly.
Polished or honed: which finish suits a sill?
Both work indoors; choose by the room and the look you want.
- Polished is glossy, shows the veining clearly and wipes clean easily. It is the usual pick for living rooms and bedrooms.
- Honed is matte and hides scratches and water spots. It suits bathroom and kitchen windows, where condensation and splashes are common.
If the sill should match a threshold or vanity in the same room, order the same stone and finish. You can compare stones by color, from White Carrara and Calacatta Gold to Crema Marfil and Nero Marquina.
What edge profile does a marble sill have?
Our window sills are beveled the same way as our thresholds. The front edge, the one facing the room, gets a small 1/4 inch chamfer. The back edge, which sits against the window frame, stays square so it butts tight. That is our standard bevel: one long edge only, never both.
The ends are square unless you ask otherwise. If your sill has ears, tell us whether the exposed ends should be finished too, and include a sketch with your order.
Should you buy a stock sill or order a custom one?
Buy stock when a listed size matches your opening and depth, or when you can trim length on site with a wet saw. Order custom when the window is wide, the wall is deep, you want ears, or the sill must be thicker than 3/4 inch.
Custom pieces are priced from your measurements. As a rough guide, custom runs about the stock price plus 50%, and our custom cost guide explains how. Custom-cut orders cannot be returned, so double-check every number. Processing takes 3 to 4 business days, then UPS Ground ships crated pieces at a flat $70 each.
Have your length, depth, overhang and stone choice ready? Send them over and we will quote a sill cut to fit. Get a custom sill quote
Frequently asked questions
What is the standard thickness of a marble window sill?
Interior marble sills are usually 3/4 inch thick, which is our standard thickness. Thicker stone, such as 1-1/4 inch, is available as custom work when you want a heavier look or the sill must span a wider gap.
Can I use a marble threshold as a window sill?
Often, yes. A threshold and an interior sill are the same kind of stone part, a flat strip with one beveled edge. If one of our stock lengths and widths matches your opening and depth, it can serve as a sill.
Is marble a good material for an exterior window sill?
Not really. Marble etches from acid rain and weathers outdoors, especially with freeze and thaw. Granite is the better choice for exterior sills, and marble is best kept indoors.
How much should a window sill overhang the wall?
Keep it small. A modest lip past the finished wall face looks clean and protects the edge of the drywall or plaster. A deep, unsupported overhang is easier to crack with a knock.
How do I order a marble window sill in a custom size?
Send your length, depth, overhang, thickness and stone through our contact form, with a photo or sketch if the ends are notched. We quote from your numbers and cut the sill at our Jersey City shop. Shipping details are on our delivery page.