What frame size fits a 18x24 poster?
The quick answer plus an interactive mat-border calculator, a to-scale preview, and a printable guide. All sizes are computed geometry — we sell nothing.
Quick answer
Frame without a mat
18x24 in
exact standard frame -- no mat
Frame with a mat
24x30 in
3.25 in side / 3.25 in top-bottom mat border
Print aspect ratio
3 : 4
18x24 in
Mat-border calculator
Change the frame size to see how the mat border and opening change for a 18x24 poster print.
inches
inches
Mat opening is cut about 0.25 in smaller than the print on each side so the mat holds the edges. Borders are the visible mat reveal.
Every standard frame for a 18x24 poster
Borders shown as side × top/bottom mat reveal.
| Standard frame | Mat border (side x top/bottom) |
|---|---|
| 18x24 in | borderless |
| 20x24 in | 1.25 x 0.25 in |
| 20x30 in | 1.25 x 3.25 in |
| 24x30 in | 3.25 x 3.25 in |
| 24x36 in | 3.25 x 6.25 in |
How these numbers are computed & sources
- Aspect ratio reduces the 18x24 in print to lowest terms (ISO sizes are ~1:1.41).
- "Frame without a mat" is the standard frame whose nominal size matches the print; if none matches, the nearest larger standard frame is shown.
- "Frame with a mat" is the smallest standard frame whose per-side mat border lands in the comfortable 1.5-3.5 in range. Mat opening = print minus 0.25 in per side.
- Standard frame sizes come from a fixed registry; we list no store inventory, prices, or product matches.
Sources for the size conventions:
- Standard US picture-frame sizes (4x6, 5x7, 8x10, 11x14, 16x20, 18x24, 20x30, 24x36, ...) are long-standing retail conventions.
- ISO 216 A-series paper/print sizes (A4 210x297 mm, A3 297x420 mm, A2 420x594 mm) converted to inches at 25.4 mm/in.
Last reviewed: 2026-06-21.