The test square is a box of known size printed on a pattern, such as 5 cm or 2 in, that you measure before printing or cutting anything else. If it measures exactly right, the whole pattern is at true scale; if it does not, the printer has resized the pages. Even a small error matters: printed at 97%, a pattern for a 100 cm (39 1/2 in) hip comes out 3 cm (1 1/4 in) smaller all round, which is close to a full size.
What is a test square?
A test square is a calibration mark. Because a PDF pattern can be printed on many different printers, papers and apps, the designer adds a square of exact size so you can check the printout with a ruler. Many patterns use a 5 cm or 2 in square; some use 1 in or 10 cm, or a scale line instead.
On Sewly patterns, the test square has two parts, a solid 5 cm square and a dashed 2 in square, so you can measure in whichever units your ruler shows. It appears on the cover page next to the printing instructions, again on the first pattern page where a corner is free, and on every copy shop and projector sheet.
How much does a small scaling error change the garment?
Scaling changes every line of the pattern by the same percentage, so the error grows with the size of what you measure: a fraction of a millimeter on the test square becomes several centimeters around the body.
| Printed at | 5 cm square measures | 2 in square measures | 100 cm hip becomes | 40 in hip becomes |
|---|---|---|---|---|
| 100% | 5.0 cm | 2 in | 100 cm | 40 in |
| 98% | 4.9 cm | 1.96 in | 98 cm | 39 1/4 in |
| 97% | 4.85 cm | 1.94 in | 97 cm | 38 3/4 in |
| 94% (A4 page fitted to Letter) | 4.7 cm | 1.88 in | 94 cm | 37 5/8 in |
| 103% | 5.15 cm | 2.06 in | 103 cm | 41 1/4 in |
Lengths change too: a dress 100 cm (39 1/2 in) long printed at 97% ends up 3 cm shorter, and sleeves, rise and armholes shrink in the same proportion. Size charts commonly step 4–6 cm (1 1/2–2 1/2 in) between sizes at the hip, so a 3% error is close to a whole size. The 94% row is what happens when an A4 page is shrunk onto US Letter; the post on A4 vs US Letter explains why.
Why do printers scale patterns without asking?
Most print dialogs are set up for documents, not drawings. Their defaults try to make sure nothing is cut off, so they shrink the page to fit the paper's printable area. The usual culprits:
- "Fit to page", "Fit", "Shrink oversized pages" or "Fit to printable area" in the print dialog.
- The wrong paper size, such as an A4 file printed on Letter, which pushes the dialog to scale.
- Borderless printing, which enlarges the page slightly.
- Printing from a phone or a browser with limited scaling options.
- Printer driver settings that override the app, such as a reduce, enlarge or poster mode left on from an earlier job.
How do you print a pattern at 100%?
- Open the PDF in a PDF reader rather than a web browser when you can.
- In the print dialog, choose the paper that is loaded in the printer.
- Set the scale to 100%, "Actual size" or "None", and switch off every fit or shrink option.
- Print only the page with the test square first.
- Measure it. If it is right, print the remaining pages with exactly the same settings.
In Adobe Acrobat Reader, choose "Actual size" under page sizing. In macOS Preview, set the scale to 100%. In a browser's print dialog, open the extra settings and look for a scale or actual-size option; if it only offers to fit the page, save the file and use a PDF reader instead. The guide to printing PDF sewing patterns at the right scale covers more apps and printers.
How do you measure the test square correctly?
- Use a ruler with millimeters, not a tape measure, which can bend and stretch.
- Measure both directions. Printers can scale width and height differently, especially when the paper slips.
- Measure consistently, from the outside of the lines, or from the same side of each line.
- Aim for exact. Half a millimeter on a 5 cm square is a 1% error and about the limit of what you can read on a ruler; anything more is worth tracking down.
What if the test square is still wrong?
Check the print dialog again, then the printer's own settings (printer preferences on Windows, the printer options menu in the print dialog on a Mac). Try another PDF reader, update the printer driver, and make sure the paper size selected matches the tray. If one direction is consistently off by a small amount, the printer itself may be slightly inaccurate; printing the same file at a copy shop is a good test.
Never correct an error by scaling the other way by eye, such as printing at 103% to compensate. Find the setting that causes it, or the next print will be wrong in a different way.
Does the test square matter for copy shops and projectors?
Yes, as much as at home. Copy shops print large files on plotters that can be set to fit the roll width, so ask for 100% and measure the square when you collect the print. For projector sewing, the square checks your calibration: project it onto the cutting mat and measure it at the center and near the corners of the image.
Every Sewly PDF, on any paper, carries the test square and a reminder to print at 100%, and the page map on the cover shows which pages are printed and how they fit together. More about printing Sewly patterns.