Quick answer: An A4 page fits inside a portrait US Letter sheet at about 94.07% when no printable margins are reserved. With 5 mm reserved on every edge, the geometric limit becomes 90.71%. Your printer’s usable area can require a smaller scale.
Why 100% can cut off the page
A4 measures 210 × 297 mm. US Letter measures 8.5 × 11 inches, or 215.9 × 279.4 mm. Letter is wider but shorter. At 100%, a full A4 page extends 17.6 mm beyond Letter’s height, even before considering printer margins.
This is a page-size comparison. An A4 document with generous blank margins may still have all its visible content inside the printable area at 100%. A border or a measurement-sensitive template needs a more careful check.
Work out the limiting dimension
Divide the available width by the source width, then divide the available height by the source height. Use the smaller result so that neither dimension overflows.
Scale = min(215.9 ÷ 210, 279.4 ÷ 297) × 100 = 94.07%
At that scale, the source page becomes approximately 197.56 × 279.4 mm. Centering it leaves about 9.17 mm on each side and no spare height. This is a theoretical page fit, not a promise of borderless printing.
Allow for the printer’s margins
With 5 mm on each edge, the target area is 205.9 × 269.4 mm. The same calculation gives 90.71%. The scaled page is approximately 190.48 × 269.4 mm. If your print dialog accepts only whole percentages, round down to 90% to stay within this calculated area.
Use the print scale calculator to substitute your actual margins. The calculator assumes equal margins; for an asymmetric printer, reserve at least the largest margin on all sides as a conservative estimate.
Choose how much precision you need
Adobe offers automatic fitting and percentage-based scaling. Choose automatic fitting when the goal is simply to keep the content on the sheet. Use a calculated percentage when dimensions matter, and measure one test print before using the result.
Keep the original digital file. Scaling a print changes the output, so a 100 mm reference line at 90% becomes 90 mm. Do not use a resized pattern as though it were full size.
Use the same assumptions when comparing results
A display of 90.71% is the two-decimal rounding of 90.707…%. If the usable boundary is strict, a rounded-up entry may technically exceed it by a tiny amount. Round down to the precision your dialog accepts, or leave a little additional space. The whole-number recommendation is 90% for this case.
Use the Letter test sheet to check your printer at 100% before attributing every discrepancy to paper conversion. The reference PDF is already Letter-sized, so it tests the printing path without the A4 conversion.
Paper dimensions reference: Adobe’s A-series size table. Inch-to-millimeter conversions and worked examples are calculated for this guide.
Print-setting reference: Adobe’s PDF scaling guide.