Short answer: a frame is sold by the picture (inside) size it holds, and the outside is larger by the moulding width. This calculator adds the moulding on each of the four sides to estimate the outside size, using the published front widths of our four profiles, and it works out a mat window from an artwork size and a mat overlap of 2 to 4 mm per side. Both results are estimates: a sample settles the exact sizes.
Estimate the outside size of a frame
Work out a mat window and board size
How the two sums work
| Result | Sum | Where the rule comes from |
|---|---|---|
| Estimated outside size | Picture size + 2 x moulding width, in each direction | Our frames page says the outside of the frame is larger by the moulding width; we read that as the width on each of two opposite sides. Front widths: Fine 15, Slim 19.5, Classic 21, Broad 28 mm. Our own arithmetic. |
| Mat window | Artwork size - 2 x overlap, in each direction | The Canadian Conservation Institute says mat edges should cover the artwork edges by at least 2 mm to 4 mm, as reported on our conservation guide. Our own arithmetic on that rule. |
| Mat board outside size | Window + 2 x border, in each direction | The border is your choice. The 50 mm used above is an invented figure for the arithmetic, and a wider bottom margin is usual. |
Worked examples
| Case | Sum | Result |
|---|---|---|
| 8 x 10 in picture, Slim profile | 203.2 + 2 x 19.5 = 242.2 mm wide; 254 + 2 x 19.5 = 293 mm high | 242.2 x 293 mm, about 9.54 x 11.54 in |
| 16 x 20 in picture, Classic profile | 406.4 + 2 x 21 = 448.4 mm; 508 + 2 x 21 = 550 mm | 448.4 x 550 mm, about 17.65 x 21.65 in |
| 300 x 400 mm artwork, 2 mm overlap | 300 - 4 = 296; 400 - 4 = 396 mm window | Window 296 x 396 mm |
| 300 x 400 mm artwork, 4 mm overlap | 300 - 8 = 292; 400 - 8 = 392 mm window | Window 292 x 392 mm; with a 50 mm border the board is 392 x 492 mm |
Limits to keep in mind
- The sources do not say whether a published front width includes the part of the moulding that overlaps the picture, so check the estimate on a sample before you plan shelf space, cartons or a wall layout.
- Conventions vary between sellers. State whether a size is the picture (inside) size, the outside size or the mat size in your brief.
- A mat adds thickness as well as size, and the rabbet must hold the glazing, mat, artwork and backing together, so check the moulding depth too.
- The mat sums follow conservation guidance written for works on paper; a photo mount is an option inside our frames, and it changes the visible picture area, so give the picture size and the opening you want.
For all 31 standard sizes at once, see the outside size estimates (data). Related: inside size vs outside size, photo frame profiles compared, standard photo frame and paper sizes, the photo frame order brief.
Start an enquiry when your brief is ready; we reply within 1 business day, with questions or a quote.
Sources: photo frames [S5]; photo frame inside size vs outside size [S18]; photo frame profiles compared [S19]; conservation matboard and mounting for framed prints [S38]; standard photo frame and paper sizes in India [S44]. As of 2026-11-12. Terms are typical and are confirmed in your written quote.