PDF Aspect Ratio Check: The Fast, Accurate Methods (and Fixes)

• Goal: confirm each PDF page’s ratio, then decide whether to accept it (e.g., A-series for print) or normalize it (e.g., to Letter for forms or KDP).
• Scope: We’re checking the page aspect ratio (not embedded images).

Which page box to measure (TrimBox → CropBox → MediaBox)

A PDF page can define up to five “page boxes”: MediaBox, CropBox, TrimBox, BleedBox, and ArtBox. For production-accurate aspect ratio, use this order:

1. TrimBox — the final trimmed size (best for print deliverables).
2. CropBox — the viewing area (fallback if TrimBox is missing).
3. MediaBox — the full canvas (fallback if the others are missing).

Pro tip: Many viewers show CropBox by default. If you’re preparing for print, always prefer TrimBox when it exists.

Rotation & units (the two “silent” gotchas)

• Rotation: PDFs can declare a page rotation (0/90/180/270). If a page is rotated 90°, width and height may appear “swapped.” Always check the rotation value during your aspect ratio check.
• Units: PDF coordinates are in user units. By default, 1 unit = 1/72 inch. Some PDFs change this via the UserUnit entry. If UserUnit ≠ 1, multiply box dimensions by this value before computing aspect ratio.

Quick ways to check a PDF’s aspect ratio

Option 1 — Adobe Acrobat / Acrobat Reader (fastest GUI)

1. Open the PDF → File > Properties to see Page Size.
2. To keep it visible while browsing pages: Edit > Preferences > Page Display → enable Always Show Document Page Size. You’ll see page dimensions near the scroll bar.

Option 2 — Command line (Poppler pdfinfo)

Print sizes for all pages (and every page box) reliably:

# Install poppler-utils (macOS: brew install poppler | Linux: apt/yum)
# List sizes for all pages:
pdfinfo -f 1 -l 9999 file.pdf

# Include all page boxes (Media/Crop/Bleed/Trim/Art) for each page:
pdfinfo -f 1 -l 9999 -box file.pdf

Compute the aspect ratio instantly with awk:

pdfinfo -f 1 -l 9999 -box file.pdf |
awk '/^Page *[0-9]+ size:/ {
  w=$4; h=$6; gsub(/[^0-9.]/,"",w); gsub(/[^0-9.]/,"",h);
  r=w/h; printf("Page %s → ratio = %.6f (w/h)\n", $2, r)
}'

Option 3 — Python (programmable & cross-platform)

Use pypdf to prefer TrimBox→CropBox→MediaBox, account for UserUnit, and report rotation:

# pip install pypdf
from pypdf import PdfReader
import sys

def pick_box(page):
    for name in ("trimbox", "cropbox", "mediabox"):
        box = getattr(page, name, None)
        if box:
            return box, name
    return page.mediabox, "mediabox"

pdf = PdfReader(sys.argv[1])
for i, page in enumerate(pdf.pages, start=1):
    box, which = pick_box(page)
    uu = getattr(page, "user_unit", 1) or 1
    w = float(box.width) * uu
    h = float(box.height) * uu
    rot = getattr(page, "rotation", 0) or 0
    ratio = w / h if h else 0
    print(f"Page {i}: {w:.2f}×{h:.2f} pt | box={which} | rot={rot} | ratio w/h={ratio:.6f}")

Batch-checking many PDFs (CSV report)

# macOS/Linux: create report.csv with one line per page across all PDFs
echo "file,page,width_pt,height_pt,ratio_w_over_h" > report.csv
for f in *.pdf; do
  pdfinfo -f 1 -l 9999 -box "$f" |
  awk -v F="$f" '/^Page *[0-9]+ size:/ {
    w=$4; h=$6; gsub(/[^0-9.]/,"",w); gsub(/[^0-9.]/,"",h);
    r=w/h; printf("%s,%s,%s,%s,%.8f\n", F, $2, w, h, r)
  }'
done >> report.csv

How to fix or normalize aspect ratio

Fix in Acrobat Pro (no code)

1. Tools > Print Production > Preflight.
2. Click Single Fixups (wrench icon) → expand Pages.
3. Use Scale pages to specified size to set a target (A4, Letter, custom).

Fix with Ghostscript (fit to a new page size)

Force pages to A4 (595×842 pt), scaling/centering as needed:

gs -o out.pdf -sDEVICE=pdfwrite \
  -dFIXEDMEDIA -dPDFFitPage -sPAPERSIZE=a4 \
  -dCompatibilityLevel=1.7 in.pdf

Fix with pdfcpu (precise boxes & resizing)

# Set TrimBox on page 2 and mirror to BleedBox:
pdfcpu box add -pages 2 -- "trim:[10 10 500 800], bleed:trim" in.pdf out.pdf

# Resize all pages to A4:
pdfcpu resize -pages all -paper a4 in.pdf out.pdf

Special case: ultra-tall “scroll” PDFs

Split each page into tiles that better match standard ratios:

# Two vertical tiles per page:
mutool poster -y 2 in.pdf out.pdf

Common page sizes & aspect ratios (cheat sheet)

FAQ

Does “Page Size” equal aspect ratio?

Yes. Aspect ratio is just width ÷ height. Check “Page Size” in Acrobat and divide.

Which box should I trust for print?

TrimBox (finished size). If it’s missing, fall back to CropBox, then MediaBox.

Why do my numbers look swapped?

The page likely has a rotation flag (e.g., 90°). Read rotation along with width × height.

My PDF shows odd values like 0.35278 mm per unit—why?

You’re seeing user units (default 1/72″ per unit). If a file sets UserUnit ≠ 1, multiply dimensions by that factor before computing the ratio.