Mat Board Margin Calculator

How It Works

This calculator computes mat board margins — the border of cardboard surrounding artwork in a picture frame. A well-proportioned mat draws the eye to the artwork and provides visual breathing room.

The tool operates in two modes:

• Given Frame — You already have a frame. Enter artwork and frame dimensions to calculate how much margin you get on each side.
• Recommend Frame — You have artwork but no frame yet. Choose a style preset to get recommended frame dimensions with aesthetically pleasing margins.

Both modes support bottom-weighting — a traditional framing technique where the bottom margin is slightly wider than the top to counteract the optical illusion that makes centered artwork appear to droop.

How to Use This Calculator

Mode A: Given Frame

1. Enter artwork dimensions — Measure the piece to be framed (width × height in mm)
2. Enter frame inner dimensions — The inside measurement of your frame (the space the mat must fill)
3. Set overlap — Typically 3mm; the mat will cover this much of the artwork edge
4. Toggle bottom weight — Recommended for most artwork; adds ~10% more to the bottom margin
5. Read margins — The calculator shows exact margin widths for each side

Mode B: Recommend Frame

1. Enter artwork dimensions — Same as above
2. Choose a style preset — Museum (wide), Gallery (balanced), Minimal (narrow), or Custom
3. Set overlap and bottom weight — Same as Mode A
4. Get frame recommendation — The calculator suggests the ideal frame inner dimensions

Understanding Bottom Weight

When artwork is placed at the mathematical center of a mat, it appears to be slightly below center — a well-documented optical illusion. Professional framers compensate by making the bottom margin 10-20% wider than the top.

This calculator uses a 45/55 split (top/bottom) in Given Frame mode and a 1.2× multiplier in Recommend mode when bottom weight is enabled. The effect is subtle but makes a noticeable difference in how "right" the framing looks.

Style Presets Explained

Museum (22.5% of artwork short side) — Generous margins that give artwork maximum breathing room. Common in fine art galleries and museum exhibitions where each piece commands wall space.

Gallery (15% of artwork short side) — A balanced, professional look suitable for most artwork. The standard choice for prints, photographs, and mixed media.

Minimal (10% of artwork short side) — Narrow margins that keep the focus tight on the artwork. Works well for contemporary pieces, salon-style gallery walls, or when frame size is constrained.

Tips for Framers

• Double mats add depth and elegance. Use a 3-6mm reveal between the outer and inner mat. Calculate margins for the outer mat, then add reveal width to each side for the inner mat window.
• Acid-free mats are essential for valuable artwork. Regular mat board contains acids that yellow and damage paper over time.
• Bevel-cut the window opening at 45° for a professional finish that reveals the mat's white core.
• Color choice — White and off-white are safest. Colored mats can complement artwork but should never compete with it.
• Float mounting — For artwork with deckle edges or where you want to show the full sheet, calculate zero overlap and use a float-mount technique instead.

FAQ

Why is the bottom margin wider than the top?

This is called bottom-weighting. Due to an optical illusion, artwork centered mathematically in a frame appears to sink below center. Adding 10-20% more margin at the bottom creates the perception of visual centering. It's a standard practice in professional framing.

What is mat board overlap?

The overlap is the amount of artwork edge hidden behind the mat window opening. Typically 3-6mm, it prevents raw artwork edges from showing and helps secure the piece against the backing board. Too much overlap hides image area; too little risks the edge showing.

How do I choose the right mat width?

Museum style uses wide margins (20-25% of artwork short side), Gallery is moderate (12-18%), and Minimal is narrow (8-12%). Larger artwork generally needs proportionally narrower mats. A 200mm artwork with Gallery preset gets ~30mm margins, while an 800mm piece might only need ~50mm.

Can I use this for double mats?

Yes. Calculate margins for the outer mat normally, then add your desired reveal width (3-6mm) to each margin for the inner mat's window opening. The inner mat window will be slightly larger, exposing a thin border of the inner mat color around the artwork.

What overlap should I use for canvas or thick media?

For stretched canvas or board-mounted prints thicker than 3mm, reduce overlap to 1-2mm or use zero overlap with a float-mount technique. The standard 3-6mm overlap is designed for paper and thin prints that lie flat behind the mat.

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Last reviewed: June 2026