Because ISO paper sizes are designed around a simple principle — halving always preserves the shape — folding is effortless. This is a short guide to the three most useful folds: the half fold, the quarter fold, and the tri-fold.
Half Fold: A4 → A5
The simplest fold. Bring one short edge to the other short edge. The result is A5 (148 × 210 mm), which fits a C5 envelope perfectly. Use this for greeting cards, small programmes, and quarter-page booklets.
Tip: Score the fold line lightly with a bone folder or the back of a spoon before folding — this prevents cracking on thick stock.
Quarter Fold: A4 → A6
Fold in half to A5, then in half again. The result is A6 (105 × 148 mm), which fits a C6 envelope. Use this for postcards, invitations, and small booklets.
Tri-Fold (DL): A4 → DL
The business standard. Fold A4 into three equal panels of 99 mm each. The result is 99 × 210 mm, slightly larger than a DL envelope (110 × 220 mm), leaving about 5 mm on each side.
There are three tri-fold styles:
- Letter fold — one panel folds in, the other folds over. The flap is on the outside.
- Roll fold — one panel folds in, the middle in, the top over. The flap is enclosed.
- Z-fold — panels fold alternately like a fan.
The Math Behind Folding
If you fold an A4 in half, the resulting area is exactly half of A4 — the same as A5. Fold it in half again and you get A6. This is because the ISO 216 A series is a binary geometric progression: A0 → A1 → A2 → A3 → A4 → A5 → A6 → A7 → A8 → A9 → A10.
Quick Reference
- A4 — 210 × 297 mm — the original sheet
- A5 — 148 × 210 mm — half of A4
- A6 — 105 × 148 mm — quarter of A4
- A7 — 74 × 105 mm — eighth of A4
- DL — 110 × 220 mm — A4 folded into thirds
Common Mistakes to Avoid
- Not scoring thick paper — 200+ gsm paper will crack along a fold without a score line.
- Folding by hand without a guide — small misalignments produce a crooked fold. Use a ruler or fold against a table edge.
- Using the wrong fold for the envelope — DL envelopes require tri-fold; C5 requires half-fold.
Why Folding Matters
Getting the fold right is the difference between a professional-looking mailing and an amateur one. Once you know the A-series relationships, folding becomes mechanical — you will never have to guess how many folds to fit a particular envelope again.