Compare portrait A4, 210 × 297 mm, with portrait A3, 297 × 420 mm, by dividing each target side by its original side.
Two ratios
- Width: 297 ÷ 210 × 100 = 141.4286%.
- Height: 420 ÷ 297 × 100 = 141.4141%.
The small difference comes from nominal millimetre dimensions. The ideal ratio between consecutive A-series formats is √2, approximately 141.4214%.
To fit the complete A4 rectangle in A3 at a uniform scale, choose the smaller factor: approximately 141.4141%. The resulting width is 296.9697 mm, slightly below 297.
Area and sides grow differently
Multiplying each side by √2 doubles the area. Doubling each side quadruples the area.
Letter and Tabloid are not A-series formats. Do not apply the A4/A3 percentage to them: calculate both side ratios for the actual paper and orientation.
At the printer
Fit may use the printer’s usable area rather than the whole sheet, giving a smaller scale. A menu value such as 141% does not certify an exact physical distance. Measure the print if that distance matters.
For cards or labels, arranging pieces with their intended physical sizes in the planner is often more useful than enlarging a whole document without checking final sizes.
References and scope
The numerical examples are our own. Check a physical print on your own equipment.