Design & Layout
Photo Frame Size Calculator
Calculate the exact mat (passe-partout) opening size and border widths for any photo frame and artwork combination. Enter the frame size and photo size, set your mat border, and get precise cutting dimensions.
Last updated: June 2026
Formula used
Horizontal border = (Frame width − Mat opening width) ÷ 2
Top border = Total vertical space ÷ (1 + bottom_weight)
Bottom border = Total vertical space − Top border
The weighted-bottom rule: make the bottom mat border slightly wider (10–25%) than the top so the artwork does not appear to sag visually.
Example calculations
Example 1 — 30 × 40 cm frame, 15 × 20 cm photo, equal borders:
Mat opening = 156 × 206 mm. Borders: horizontal = (300−156)÷2 = 72 mm, vertical = (400−206)÷2 = 97 mm.
Example 2 — 20 × 25 cm frame, 10 × 15 cm photo, weighted bottom (+15%):
Opening = 106 × 156 mm. Total vertical = 94 mm. Top = 94÷2.15 = 43.7 mm. Bottom = 94−43.7 = 50.3 mm.
How to use this calculator
- Enter the frame inner size (rabbet opening) in mm.
- Enter the photo or artwork size in mm.
- Set opening overlap — 3–5 mm covers the photo edge to hold it in place.
- Choose equal borders or weighted bottom.
- Read the mat board cutting dimensions and border widths.
Standard frame and photo sizes (mm)
| Frame name | Frame (mm) | Typical photo |
|---|---|---|
| 4R / 4×6 in | 102 × 152 | 102 × 152 |
| 5R / 5×7 in | 127 × 178 | 127 × 178 |
| 8R / 8×10 in | 203 × 254 | 203 × 254 |
| A4 frame | 210 × 297 | 148 × 210 (A5) |
| A3 frame | 297 × 420 | 210 × 297 (A4) |
| 30 × 40 cm | 300 × 400 | 200 × 300 |
Notes & limitations
This calculator handles a single flat mat board. Double mats require separate calculations per layer. Always cut the mat board slightly smaller and test-fit before final cutting. Mat board thickness (1.5–3 mm) is not accounted for. For framing coloring page artwork, a standard A4 mat in a 30 × 40 cm frame works well. Browse coloring pages at ColoringBuds.com.
Frequently asked questions
How do I calculate mat border size for a picture frame?
Subtract the mat opening from the frame inner size, then divide by 2 for equal borders. Mat opening = photo size minus overlap on each edge (3–5 mm). This calculator does it automatically.
What is the weighted bottom rule for framing?
Tradition says the bottom mat border should be 10–25% wider than the top, because artwork centred by measurement appears to sag visually. A 10:12 (top:bottom) ratio is a common starting point.
What is passe-partout in framing?
Passe-partout (also called a mat or mount board) is a cardboard border between the frame and the artwork. It creates a visual gap and prevents glass from touching the artwork.
How much of the photo should the mat overlap?
3–5 mm per side is standard. This holds the photo in place without hiding too much of the image.
What size mat do I need for an A4 photo in an A3 frame?
A3 frame inner = 297 × 420 mm. A4 photo = 210 × 297 mm. With 3 mm overlap, opening = 204 × 291 mm. Borders: horizontal = 46.5 mm, vertical = 64.5 mm.