How do I fold a letter for a small envelope?

For small A2 or C6 envelopes, use a quarter-fold (double half-fold): fold the sheet in half horizontally from bottom to top, then fold it in half vertically from left to right.

Which end goes into the envelope first?

Insert the solid folded crease first, with the smooth edge pointing toward the bottom of the envelope.

What envelope size fits A4 paper folded in half?

An ISO C5 envelope (162 × 229 mm) is designed specifically to fit A4 paper folded in half once.

Can I mail a letter folded into a square?

Square envelopes require extra non-machinable postage surcharge from USPS. Rectangular #10 or DL envelopes are standard.

Interactive Paper Folding Tutor

Updated: September 20, 2026

Commercial Envelope Insertion Guide

How to Fold a Letter to Fit Any Envelope Size

Proper paper folding and envelope insertion ensure your mail looks professional, passes through postal processing machines smoothly, and protects sensitive document contents from damage.

Matching Paper & Envelope Sizes

Choosing the correct fold depends on your envelope: #10 and DL require tri-folds; C5 requires half-folds; C6 and A2 require quarter-folds.

Complete Envelope Compatibility Matrix

#10 Business (4.125 × 9.5 in) • ISO DL (110 × 220 mm) • ISO C5 (162 × 229 mm) • ISO C6 (114 × 162 mm)

Step-by-Step Insertion Guide

Choose Matching Fold: Select Tri-Fold for #10/DL, Half-Fold for C5, or Quarter-Fold for small A2/C6 envelopes.
Align Side Margins: Make sure paper corners line up evenly before creasing.
Check Thickness: Keep folded thickness under 1/4 inch (6mm) for standard letter rate postage.
Slide Fold First: Insert the solid top-folded edge into the envelope first to prevent catching.

Envelope TypeDimensionsBest Fold

#10 Business4.125 × 9.5 inTri-Fold

DL Envelope110 × 220 mmA4 Tri-Fold

#10 Window4.125 × 9.5 inZ-Fold

C5 / 6×9 in162 × 229 mmHalf-Fold

A2 / C6 Card4.375 × 5.75 inQuarter-Fold

Commercial Envelope Insertion Guide Technical Diagram

Pro Tips for Envelope Insertion

Always insert the folded crease edge first so loose pages don't catch on the envelope adhesive flap.
Check address window alignment before sealing window envelopes.
Avoid over-filling envelopes beyond 3 sheets of standard 20 lb paper.

Envelope Insertion Errors

Forcing a letter into a small envelope resulting in wrinkled margins.
Inserting loose open edges first which tear during mail machine sorting.
Using incorrect postage for envelopes thicker than 0.25 inches.

Envelope & Paper Size Compatibility Matrix

Match your sheet dimensions to standard commercial envelopes to prevent postage delays and bulky mailings.

Envelope TypeDimensionsBest Fold

#10 Business4.125 × 9.5 inTri-Fold

DL Envelope110 × 220 mmA4 Tri-Fold

#10 Window4.125 × 9.5 inZ-Fold

C5 / 6×9 in162 × 229 mmHalf-Fold

A2 / C6 Card4.375 × 5.75 inQuarter-Fold

Explore Related Folding Guides

How to Fold a Letter for an Envelope How to Fold a Letter Into an Envelope How to Fold Paper Into Thirds (Tri-Fold Guide) How to Fold A4 Paper for DL & C5 Envelopes Interactive Z-Fold Accordion Guide Origami Self-Mailing Letter Fold

Frequently Asked Questions About Folding Letters

Answers to common stationery, business mailing, and postal envelope folding questions.

Use a standard tri-fold: fold the bottom third up, then fold the top third down. The folded sheet will measure 3.66 × 8.5 inches, fitting comfortably inside the 4.125 × 9.5 inch envelope.