FramingMath says frames are sold by the size they hold, the inside dimension, set by the rabbet [4]. Sellers differ, so confirm with your supplier which dimension a quoted size refers to, and write it in your brief. The rabbet is the channel that holds the glazing, mat, art and backing [1]. Sight size is the smaller visible opening [1]. Outside size is the whole frame including moulding [1].
Buyer guide
Photo frame inside size vs outside size: rabbet, sight size and mat size explained
FramingMath, a framing glossary, says a frame is sold by the size it holds, its inside dimension [4]. The outside size is larger because it adds the moulding on every side [1], and sight size is the smaller visible opening [1]. Here is how the four terms differ and how to state them in a brief.
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Short answer
The short answer
The four size terms in plain words
Most confusion comes from using one word, "size", for four different measurements. Here they are, using the definitions in the sources we read.
- Rabbet size: The rabbet is the L-shaped channel cut into the back inner edge of the moulding. It holds the glazing, mat, art and backing (Custom Picture Frames [1]). FramingMath says the inside dimension of a frame is set by the rabbet, and gives the UK term as rebate [4].
- Sight size: The visible opening of the frame, measured inside the lip [1]. Web Picture Frames defines window size as the visible opening inside the mat or frame where the artwork shows through, and says it is also called sight size [3].
- Outside size: The total external measurement of the finished frame, including the moulding width on all four sides [1].
- Mat outside size: The overall dimensions of the finished mat, meaning the opening plus all four borders [4]. FramingMath says the frame size you buy is approximately your mat outside size [4].
Quoted size versus outside size: a worked example
Custom Picture Frames gives this example: a 16 by 20 frame with 2 inch wide moulding has outside dimensions of 20 by 24 inches, and when you order a 16 by 20 frame the sight size is roughly 15 3/4 by 19 3/4 inches [1]. By simple arithmetic (ours, not the glossary's), 2 inches of moulding on each of two opposite sides adds 4 inches to each dimension, which matches the 16 by 20 to 20 by 24 step. So allow for the outside size, not the quoted size, when you plan wall space.
Rabbet size versus sight size
The rabbet is the pocket at the back. The sight opening is what you see from the front. Sight size is measured inside the lip, and Custom Picture Frames says it is slightly smaller than the rabbet size because the lip overlaps a small portion of the contents [1].
The same idea applies to a mat. The mat window is the hole cut in the mat that shows the artwork [4]. Web Picture Frames says the window is typically slightly smaller than the artwork, and advises subtracting a small margin, usually 1/8 to 1/4 inch, from the artwork dimensions [3].
So when someone says "window size", check whether they mean the mat window or the frame's own opening. Web Picture Frames says the term covers the opening of a frame or a mat [3].
Fit: the rabbet needs room, not a tight squeeze
The rabbet also has a depth. Custom Picture Frames says a standard frame has 3/8 to 1/2 inch of rabbet depth and that shadow boxes need much deeper rabbets, sometimes 2 inches or more [1].
Conservation guidance shows why a tight fit is a poor idea. The Canadian Conservation Institute says a painting should not fit tightly in its frame. For a canvas painting it advises 3 to 6 mm (1/8 to 1/4 in.) on all sides between the frame and the painting [2]. It gives the reason that wood components need space to expand when relative humidity rises [2]. It also advises, as a general guideline, 3 mm (1/8 in.) between the glazing and all edges of the rabbet [2]. That guidance is written for paintings, so treat it as a reference point and not as a specification for a printed photo frame.
The same note says frames with wood mouldings wider than 10 cm may need more space [2].
As noted above, FramingMath treats the frame size you buy as approximately the mat outside size. The sources do not say how much clearance a mat and backing stack needs in a printed photo frame, so ask the manufacturer. In your brief, state whether the mat and backing stack must clear the rabbet by a set amount.
Quick comparison of the four terms
The table below sets the terms side by side. It is a simplification.
| Term | What it measures | Also called | Relative size |
|---|---|---|---|
| Rabbet size | The pocket at the back of the moulding that holds glass, mat, art and backing [1] | Rebate (UK) [4] | Larger than sight size [1] |
| Sight size | The visible opening, measured inside the lip [1] | Frame window size; also used for the mat window [3] | Slightly smaller than the rabbet [1] |
| Outside size | The whole finished frame including moulding on all four sides [1] | Outside dimensions [1] | Larger than the visible opening, because it includes the moulding width on all four sides [1] |
| Mat outside size | Mat opening plus all four borders [4] | - | Roughly matches the frame size you buy [4] |
Conventions can vary between sellers. Confirm which dimension a quoted size refers to.
How to state sizes in a frame brief
- Write the unit and say which dimension you mean, for example (illustrative only) "inside size, 8 x 10 inches, as the artwork or mat the frame holds".
- Give the mat outside size separately if a mat is included, and confirm it fits the frame.
- Give the visible window size if the look matters.
- Ask for the outside size when you plan shelf space, shipping cartons or wall layouts.
- Give the moulding choice, because the outside size depends on moulding width. Our guide to frame profiles explains how profile width changes the look.
Moulding material can also change which profile widths are available; our guide to polystyrene, wood and aluminium frame materials covers the options. For common size names, read standard photo frame and paper sizes in India.
How this applies when you work with us
ArtzFolio is a private-label manufacturer in Noida, serving businesses and brands only. Minimum order and lead time are confirmed in the quote, so write down inside size, outside size and mat size in your brief. Samples are paid, and the cost may be reimbursed or adjusted by agreement. Payment is 50% advance, with the balance as agreed, and our shipping term is EXW Noida. See pricing, minimums and lead times or contact us. We reply within 1 business day.
Sources
Sources
- Frame anatomy glossary, Custom Picture Frames. custompictureframes.com (read 2026-11-08)
- CCI Notes 10/8: Framing a Painting, Canadian Conservation Institute, Government of Canada. canada.ca (read 2026-11-08)
- Frame window size, Web Picture Frames. webpictureframes.com (read 2026-11-08)
- Matting & framing glossary: the terms explained, FramingMath. framingmath.pages.dev (read 2026-11-08)
FAQ
Questions buyers ask
Is a frame's advertised size the inside or outside size?
FramingMath says frames are sold by the size they hold, the inside dimension [4]. Sellers can differ, so confirm which dimension a quote refers to.
What is the difference between rabbet size and sight size?
The rabbet is the channel that holds the glass, mat, art and backing, while sight size is the visible opening measured inside the lip. Sight size is slightly smaller than the rabbet size [1]. If the look of the opening matters, state the sight size in your brief.
What is mat outside size?
It is the overall size of the finished mat: the window opening plus all four borders [4]. Give it separately in your brief when a mat is included.
How much smaller is a mat window than the artwork?
A small margin, usually 1/8 to 1/4 inch, is subtracted from the artwork dimensions to get the window size [3]. State the window size separately in your brief so it is not guessed.
Why is the outside size bigger than the size on the label?
The outside size includes the moulding width on all four sides [1]. In one worked example, a 16 by 20 frame with 2 inch wide moulding has outside dimensions of 20 by 24 inches [1]. Ask for the outside size when you plan shelf space or cartons.
Should artwork fit tightly in the rabbet?
No. Conservation guidance says a painting should not fit tightly and suggests 3 to 6 mm (1/8 to 1/4 in.) clearance on all sides for a canvas painting [2]. That advice is for paintings, so ask the manufacturer what clearance a printed photo frame design allows.
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