Mat Board Margin Calculator

How It Works

A mat is the paper or board frame around a picture, cut with a rectangular window that shows the artwork and margins on all four sides that separate the artwork from the frame. Getting the margin dimensions right involves three interacting numbers: the frame outer size, the mat window size (which must be smaller than the artwork's visible area to hold it in place), and the artwork visible area. From any two, the third is fixed.

The math from Vivian Kistler's Complete Photo Guide to Framing, Ch. 4:

• Mat outer = Frame inside dimension (rabbet) — the mat sits inside the frame's rabbet
• Mat window = Artwork visible area − reveal — the reveal is 1-3 mm of overlap that grips the artwork edge
• Side margin = (Mat outer − Mat window) / 2 (equal left and right)
• Top margin — usually equal to side margin
• Bottom margin — usually 10-30% larger than side margins for optical center (see below)

Optical center is the framing convention where the bottom margin is 10-30% larger than the top margin, because human perception places the visual center of a rectangle slightly above the geometric center. A picture hung with equal top/bottom margins looks bottom-heavy; adding weight to the bottom margin balances the visual composition. The Fine Art Trade Guild (FATG) framing standards and Larson-Juhl's framing manual (larson-juhl.com/framing-guide) both document this convention.

Different framing traditions use different bottom-weight ratios. Museum framing typically uses 30% bottom weight (top + side = 2.5 units, bottom = 3.25). Photo galleries often use 20%. Studio and personal use often use 10-15% or equal margins. The tool exposes the bottom-weight percentage as an adjustable slider from 0% (equal margins) to 40% (heavy bottom weight).

Double-mats add a second mat with a slightly smaller window (typically 3-5 mm) inside the primary mat, revealing a thin border of the second mat's color. The tool computes the inner mat window separately.

How to Use This Calculator

1. Enter mode — Frame-first (you have the frame size, compute mat margins) or Artwork-first (you know the artwork size and want to specify a preferred side margin, compute the frame size).
2. Enter dimensions — frame outer size (or rabbet size if you know it), artwork visible area (paper size or image area — pick one), and preferred reveal (1-3 mm typical).
3. Set bottom weight percentage — 0% for equal margins, 15% for informal framing, 20-30% for gallery-standard optical center.
4. Optional: enable double-mat — enter the inner mat reveal (typically 3-5 mm of visible inner mat).
5. Read the results — mat outer size, mat window size, top/side/bottom margin dimensions, and a scaled preview of the mat layout.
6. Export the cut sheet — a PDF with mat outer, window, and cut lines dimensioned for a mat cutter.

Real-World Use Cases

Standard 11×14 print in a 16×20 frame

A framer computes the mat for an 11 × 14 inch print in a 16 × 20 inch frame. With a 0.125 inch reveal on all sides, the mat window is 10.75 × 13.75 inches. Side margins are (16 − 10.75) / 2 = 2.625 inches; if the framer uses 15% bottom weight, top margin is 2.625, bottom is 2.625 × 1.15 = 3.02 inches. The tool converts to metric and reports 66.7 mm side/top and 76.7 mm bottom.

Museum-style triple-margin

An archivist frames a botanical print with equal top and side margins and a 30% bottom weight for gallery hanging. Artwork is 400 × 500 mm; preferred side margin is 80 mm. Bottom margin at 30% weight is 80 × 1.30 = 104 mm. Mat outer size is 400 + 160 = 560 mm wide × 500 + 80 + 104 = 684 mm tall. Frame inside dimension must match this, so the archivist orders a custom frame at that exact rabbet size.

Working backward from artwork

A gallery preparator has a 500 × 700 mm work and a preferred 80 mm side margin. Bottom weight 20% → bottom margin 96 mm. Mat outer: 500 + 160 = 660 mm wide × 700 + 80 + 96 = 876 mm tall. Round up to a standard 660 × 880 mm frame size (some frame shops offer this) or order custom.

Tips & Safety Notes

• Reveal is not the mat window size. The reveal is how much of the artwork edge the mat covers; the mat window is smaller than the artwork's visible area by twice the reveal.
• Use acid-free mat board for archival work. Regular cardstock will yellow the artwork over years due to acid migration. Museum-grade Bainbridge or Crescent mat board with an alpha-cellulose or cotton core is worth the premium for anything you want to keep.
• Cut mats with a bevel cutter, not a hobby knife. The 45° bevel is standard framing convention and cuts cleaner than any hand-held blade. A basic Logan or Fletcher mat cutter is under $50 new.
• Test-fit before final assembly. Insert the mat into the frame dry (no tape or hinges yet) to verify the fit. A 0.5 mm error is often visible against the frame's inside edge.
• Handle artwork with clean cotton gloves. Fingerprints on a print or drawing show up under raking light and cannot be removed once transferred.

Limitations & What This Tool Cannot Do

• Assumes a rectangular mat. Cutouts for oval, circular, or double-window mats use different geometry not covered here. For those, hand-lay-out with drafting tools is standard practice.
• Frame rabbet is not modeled. The tool reports the mat outer size; the frame's inside dimension (rabbet) must be equal or slightly larger — usually 3-6 mm oversized for tolerance, added manually depending on your frame supplier's spec.
• Optical center is a convention, not a physics. Different framing traditions use different bottom-weight ratios (10-30% typical). The tool exposes a slider from 0% to 40% but does not enforce any specific tradition. Match the ratio to the exhibition context or house style.

FAQ

What is "optical center" and why do galleries make the bottom margin larger?

Optical center is the convention of making the bottom margin 10-30% larger than the top and side margins. Human perception places the visual center of a rectangle slightly above the geometric center — a picture with equal margins looks bottom-heavy. Making the bottom margin larger visually balances the composition. Museum framing typically uses 30% bottom weight.

How much bottom weight should I add for a portrait vs. landscape frame?

Portrait orientation typically uses 25-30% bottom weight because the taller aspect ratio makes the bottom margin more visible. Landscape uses 15-20%. Square framing uses 20-25% or equal margins depending on style. The rule is not strict; check what looks right for your specific artwork.

What is a standard margin proportion (e.g., 3:3:3:4) and where does it come from?

The 3:3:3:4 ratio (top:side:side:bottom) gives roughly 33% bottom weight and is a common gallery-standard proportion. Larger margins overall (e.g., 4:4:4:5) apply to fine art in museum settings. The proportions descend from 17th-century framing conventions codified in modern gallery standards.

Should the visible mat window be sized to the artwork's paper edge or its image area?

Depends on the artwork. For photographs and prints with printed borders, size the mat window to the image area (covering the paper edge) — this hides the border and makes the image "float". For fine art prints, drawings, and archival work where the paper margin is part of the piece, size the mat window to cover just a few millimeters of the paper edge as a reveal.

Can I calculate a double-mat with two colors from this tool?

Yes. Enable the double-mat mode, set the inner mat's reveal (typically 3-5 mm of the inner mat visible inside the primary mat's window), and the tool computes the inner mat's window separately. The two mats are cut individually and stacked; the inner mat is typically 1-2 shades darker than the outer for a subtle depth effect.

How do I convert between the mat outer size, mat window size, and frame rabbet?

Mat outer = frame rabbet (they must match exactly). Mat window = artwork visible area − reveal × 2 (twice, one for each side). Given any two of the three, the third is fixed. The tool works in either direction — enter frame outer and artwork, get mat window; or enter mat window and preferred margin, get required frame size.

Related Tools

• Box Template Creator — Same-lab layout tool
• Spine Width Calculator — Same-lab paper-and-board layout tool
• Calligraphy Guideline Generator — Same-lab paper layout tool

References

• Vivian Kistler, Complete Photo Guide to Framing, Ch. 4: Mat Cutting and Design — mat window and margin math
• Larson-Juhl framing standards manual (larson-juhl.com/framing-guide) — commercial framing conventions and bottom-weight ratios
• Fine Art Trade Guild (FATG) framing best practices — museum-standard mat sizing and archival material selection

Reviewed by the Craft Calc Lab team on August 5, 2026.