Mat & Frame Size Calculator
Work out the frame size to buy and the mat opening to cut or order for a finished watercolor painting, from image size and mat border width.
Mat opening to order or cut: 10.5" × 14.5"
That's not a common off-the-shelf frame size. Nudging the mat width by a half inch may land you on a standard size, which is usually cheaper than a custom frame.
How it works
Pick your image size (a quarter-sheet at 11×15", a half-sheet at 15×22", or a custom size for a smaller study or an oddly cropped piece) and a mat border width, and the calculator gives you two measurements: the outer frame size to buy, and the mat opening to order or cut. The frame size just adds the mat border to all four sides of the image, so a 9×12" painting with a 3" mat needs a 15×18" frame (9 + 3 + 3 by 12 + 3 + 3). The mat opening is a little smaller than the image itself, 1/4" per side, so the mat board overlaps the edge of the painting instead of leaving a gap the paper could slip through. For that same 9×12" piece, the opening comes out to 8.5×11.5".
Worked example: a 15×22" half-sheet painting with a 4" mat needs a 23×30" frame, and the mat opening to cut is 14.5×21.5". That 23×30" frame size doesn't land on a common stock size, so a framer would likely quote it as a custom order, while the same half-sheet with a slightly narrower mat might land on a standard size and cost less.
FAQ
Why does the mat opening overlap the painting instead of matching it exactly?
If the opening were cut to the exact size of the paper, the painting would rest against the cut edge with nothing holding it flush, and it can shift or slip forward over time. A 1/4" overlap per side means the mat covers a thin strip of the painting's edge and holds it in place, which is standard practice for float or window matting.
Should I mat my watercolor at all, or frame it without one?
A mat keeps the glazing or acrylic off the surface of the paint, which matters a lot with watercolor since humidity can make unprotected paint stick to glass over time. It also gives the eye a bit of breathing room around the image, which most people find reads as more finished and more formal than a painting framed edge to edge.
How wide should my mat be?
2" to 2.5" reads as a casual, contemporary presentation, while 3" to 4" reads as more traditional and formal. There's no wrong answer, though very small paintings usually look better with a wider mat since it gives them visual weight on the wall.
How do I avoid paying for a custom frame?
Standard ready-made frame sizes are things like 8×10", 11×14", 16×20", and 18×24". If your calculated frame size is close to one of those, try nudging your mat width up or down a half inch. Landing on a stock size usually costs noticeably less than a custom-cut frame of the same rough dimensions.
Once your painting is framed, it's worth thinking about how the composition and light read from across the room. See simple composition rules for better watercolor paintings, an easy watercolor still life for beginners, and a beginner's first watercolor landscape, step by step.