A double mat is one package with two windows, not two projects. The inner mat’s window shows the artwork (minus overlap). The outer mat’s window is larger by twice the reveal, so a thin line of the inner board becomes a second border. If you calculate those windows independently, they will not share a centre and the reveal will look drunk.
Reveal is usually 3/16" to 3/8". A 1/4" reveal is the default here because it reads from across a room without eating the image. Black inner / ivory outer is the classic photography stack. Two ivories with a 1/8" reveal is quieter and often better for drawings.
Bottom weighting still applies to the outer borders. The reveal can stay even while the outer mat is heavier at the bottom. That is the look most people want and the one this calculator produces. If you drag in the studio, snap hints will say Bottom weighted when the extra lands on a 1/2" or 1" step.
Thickness doubles. Two 4-ply boards plus glass plus backing is why cheap ready-made frames spit the stack back out. Use the stack inspector after you open the studio. If remaining depth goes negative, you need a deeper rabbet, thinner glazing, or a single mat.
Cut the inner mat’s window first if you are on a manual cutter, then the outer, then hinge the art to the inner board. The calculator does not teach hinging, but it gives both window sizes so you can mark them before the board is on the cutter.
Do not confuse reveal with overlap. Overlap hides the artwork edge under the inner window. Reveal shows inner board around that window. Mixing them up produces either a crop you did not want or a stripe of art peeking beside the inner board.
Double mats on tiny pieces (4×6) can look overbuilt. Consider a single mat or a fillet. The tool will still do the math; taste is your job. On 16×20 and up, a double mat earns its keep by stepping the eye inward.
Because both boards share the same outer size, glass stays one rectangle. That is the practical win: one acrylic order, one backing, two windows. The workshop lists both mats with their own window diagrams.
Color picking belongs on the canvas. Come here for millimetres, go to the studio to see whether that oxblood inner is too loud against a green print. Local image preview never uploads the file.
Search traffic lands here because “double mat calculator” is a specific job. The site’s real product is still the visual studio. Use the button when you want to drag the reveal instead of typing it.
Frequently asked questions
What is a mat reveal?
The strip of inner mat visible around the window, created because the outer window is larger.
Should both mats be the same outer size?
Yes for a standard double mat. They sit as a stack in one rabbet.
Does a double mat change glass size?
No. Glass follows the outer board / package size.
How much extra rabbet depth do I need?
Add the second board’s thickness. Check the stack inspector so the leftover is not negative.