Origami Square & Size Calculator
Turn any rectangle into the largest possible square, read its diagonal, and plan how big a model will finish — or work backward to the square you need for a target size.
Largest square & model size
From sheet to square to model
Almost every traditional model begins with a true square, and the cleanest square from a rectangle uses the shorter side, leaving a thin strip to trim away. Keep that strip — it is perfect for lucky stars and practice folds.
Once you have a square, its side and diagonal drive the two starting bases, and the finished piece lands at a predictable fraction of the square. Planning the paper size first means a card or a garland comes out the size you actually intended.
Frequently Asked Questions
How do I get a square from A4 or Letter paper?
The biggest square you can cut from any rectangle has a side equal to the shorter dimension. From A4 (210×297 mm) that is a 210 mm square, leaving an 87 mm strip. The calculator shows the square, the offcut strip, and how much of the sheet ends up as waste.
Why does the diagonal matter?
A square's diagonal is its longest straight fold and the reference length for the preliminary and waterbomb bases that start most models. It is always the side times the square root of two — about 1.414 — so a 15 cm square has a 21.2 cm diagonal.
How big will my finished model be?
Finished size depends on the model, not just the paper. Each design collapses the starting square by a characteristic factor, so the tool multiplies your square by an approximate size factor for a few classics. Use it to plan, then treat the number as a close estimate rather than an exact spec.
What size paper should a beginner use?
A 15 cm (6 inch) square is the standard teaching size — big enough for clean folds, small enough to finish quickly. Drop to 10 cm once your creases are sharp, or scale up for display pieces using the target-size box.
Planning aid only. Finished-size factors are practical approximations that vary with the exact folding sequence and paper thickness.