An A3 poster frame fits artwork printed to A3 size — 11.7 × 16.5 inches (297 × 420 mm) — though frames with mats show a slightly smaller opening.
Buy the wrong size and your print either rattles behind the acrylic or won’t slide in. This guide pins down the exact numbers, the mat trap that trips up most buyers, and how to match a frame to your artwork on the first try.
A3 is an ISO 216 paper size, not a US standard like Letter or Legal — which explains most of the confusion. A US listing might call the same frame “A3” or “11.7 × 16.5 in,” and both describe the identical sheet. Knowing which number you’re looking at — the paper it holds or the frame’s outer edge — decides whether your poster fits.
How Big Is A3 In Inches? The Exact Numbers
A3 measures 297 × 420 mm, or 11.7 × 16.5 inches. ISO 216 defines the A-series so every size is half the area of the one before it, all sharing a 1:√2 ratio. That ratio means scaling A3 up to A2 or down to A4 keeps the shape identical. For print planning, A3 at 300 dpi is 3,508 × 4,961 pixels.
| Frame Detail | Measurement | What It Means For You |
|---|---|---|
| A3 paper size | 11.7 × 16.5 in (297 × 420 mm) | The artwork size the frame is built to hold |
| Typical mat opening | About 28.7 × 41 cm | Your visible image shrinks behind a mat |
| Common alternative outer size | 40 × 50 cm | Often paired with an A3 mount; frame edge is larger than the art |
| US-inch alternative | 16 × 20 in | Works only with a custom-cut mount for A3 art |
| A3 at 300 dpi | 3,508 × 4,961 pixels | The export size for a crisp print |
| A3 area | About 0.125 m² | Roughly double an A4 sheet |
| Proportions | 1:√2 ratio | Same shape as every other A-series size |
Does A Mat Change The Fit?
Yes — a mat shrinks your visible artwork. A typical A3 mat opening is about 28.7 × 41 cm, a bit under the full sheet; the paper still measures 11.7 × 16.5 inches, but the mat overlaps its edges. This is the most common A3 mistake: reading “frame size” as “visible artwork size.” Decide which you want — the full sheet edge to edge, or a matted border covering part of the print. For edge to edge, skip matted frames or buy a mat with an A3 opening.
When a listing gives outer dimensions, check those against your wall and the artwork opening separately. A 40 × 50 cm frame and a 16 × 20 in frame can both be sold as A3-friendly, but each gets there differently — one via an A3 mount, the other via a custom-cut mat. Comparing our roundup of tested A3 poster frame picks makes that side-by-side easier.
Why The Metric And Inch Labels Keep Confusing Buyers
The same frame gets described in millimeters on one listing and inches on the next. A3 frames are frequently sold as 11.7 × 16.5 in for US shoppers, while the ISO standard stays metric at 297 × 420 mm.
Listings also vary in what that number means. Some frames are sold by internal aperture — the opening the art sits in — and others by outer frame size, the full footprint including the border. Two listings can both say “A3” and differ by an inch or more. Confirming whether it’s aperture or outer size saves a return. The International Organization for Standardization maintains the A-series paper sizes that define A3 worldwide, so paper dimensions don’t drift between countries — only seller descriptions do. Whether a frame includes shatter-resistant acrylic is a product detail, not part of the standard.
Aspect Ratio And Regional Sizing Caveat
A-series sizes hold a 1:√2 aspect ratio, so a poster designed for A3 still looks correct at A2. One caveat for US buyers: A3 is an ISO 216 international size, not an American standard. US Letter and Legal follow different dimensions, so an A3 print won’t match a standard US frame. Treat the metric and inch figures as the same size stated two ways, and match the frame to your print by those numbers rather than the “A3” label alone.
FAQs
Will an A3 print fit a standard US frame?
Not cleanly. US frames follow Letter and Legal sizing, which use different proportions than A3. You’ll need an A3-specific frame or a custom-cut mat. Check the listed dimensions in inches and match them to 11.7 × 16.5 rather than trusting a “standard” label.
What size is the opening in a matted A3 frame?
A typical A3 mat opening is about 28.7 × 41 cm, slightly smaller than the full 297 × 420 mm sheet. The mat overlaps the paper’s edges, so you’ll see less of the artwork. If you want the entire image visible, choose a frame without a mat or one with a full-size opening.
Is A3 the same as 11 × 17 inches?
Close, but not identical. A3 is 11.7 × 16.5 inches, while 11 × 17 is a separate US size. They differ enough that a frame built for one may not seat the other snugly. Measure your print and compare it to the frame’s stated opening before buying.
References & Sources
- Papersizes.org. “A Paper Sizes.” Confirms A3 dimensions of 297 × 420 mm and the ISO A-series standard.
- Wikipedia. “International Standard Paper Sizes.” Documents ISO 216, the 1:√2 ratio, and A-series halving rule.
- Adobe. “A3 Format.” Supports common A3 uses in posters, prints, and design work.