Sewing Calculators
Pattern enlargement percentage.
Measure one line on your pattern, say how long you want it to be, and get the percentage to set on the copier or in the print dialog. It also shows how much the area grows and splits big jumps into equal passes for a copier with a maximum. Runs in your browser.
Original and desired size
Scale percentage
Fill the form and press Calculate.
Answer
To make a pattern line that measures 8 become 12, set the copier or printer to 150%. The percentage is desired size divided by original size, times 100.
Worked example
12 / 8 = 1.5, so 150%. Every length scales by 1.5, so a 5 measurement becomes 7.5. Areas scale by the square of that, 1.5 x 1.5 = 2.25, so the pattern piece covers 2.25 times as much fabric. If you need a 10x enlargement (2 to 20, 1,000%) and your copier tops out at 200% per pass, it needs 4 passes of 177.8% each, because 177.8% multiplied together 4 times gives 10x. Using 4 equal passes keeps each one within the limit.
How it works
Scale percent = desired / original x 100. Because the same factor applies to width and height, shapes keep their proportions. Area changes by the square of the linear factor. For a copier limit L (a percentage above 100 to enlarge, or below 100 to reduce), the passes are the smallest whole number n with (L / 100) to the power n at least (to enlarge) or at most (to reduce) the scale factor, and each pass uses the n-th root of the factor. Measure a test line on the output to check the scale. Measure along a line printed on the pattern rather than a whole piece, and remember seam allowances scale too, which usually isn't wanted.
FAQ
How do I calculate the percentage to enlarge a pattern?
Divide the size you want by the original size and multiply by 100. To turn 8 into 12 that is 12 / 8 x 100 = 150%.
Does doubling the percentage double the area?
No. Area grows with the square of the scale. 200% is twice as long and wide but four times the area.
What if my copier cannot enlarge enough in one pass?
Use several passes. Enter the copier maximum and the tool gives the number of equal passes, for example 4 passes of 177.8% for a 10x enlargement with a 200% limit.
Do seam allowances scale too?
Yes. Scaling the whole page scales the seam allowances as well, so a 5/8 in seam printed at 150% becomes 15/16 in. Check your pattern for seam allowances before resizing.
Which measurement should I use?
Choose a long, clearly marked line on the pattern, such as a printed scale bar or the length of a seam, and measure it carefully. Small errors in a short line are magnified.
Disclaimer
Informational arithmetic for crafts. Test with a small print and measure it before printing the full pattern.