What this covers
- Fit to page typically shrinks a document by 4 to 6 per cent
- That changes every measured length, including graph axes
- The printout looks entirely normal, which is what makes it dangerous
- Print one page and measure a known length before printing a set
Somewhere in every print dialogue is a scaling option, and on most systems it defaults to fit to page or shrink oversized pages. For a page of prose, it is harmless. For anything you will put a ruler on, it silently invalidates the exercise.
The insidious part is that the printout looks completely correct. There is no visual cue at all. You only find out when your answer is consistently a few per cent off and you cannot work out why.
What the setting actually does
A printer cannot print to the edge of a sheet. There is an unprintable margin - typically three to five millimetres on a consumer inkjet - where the paper-handling mechanism grips.
Fit to page handles this by scaling the whole page down until its content fits inside the printable area. On our measurements, an A4 document printed to A4 with fit to page enabled came out at 95.5 per cent on one printer and 94.2 per cent on another. A reference line drawn at exactly 100 millimetres measured 95.5 and 94.2 millimetres respectively.
What that does to a gradient
A gradient is a ratio of two lengths, so uniform scaling cancels out and the gradient is unchanged - which is why people conclude this does not matter. It does, because most graph questions are not pure gradients. They ask for a value read from an axis, an area under a curve, or a length measured against a printed scale, and every one of those is wrong by the scaling factor.
What scaling breaks
Measured on a page printed at 95 per cent.
| Task | Affected? | Error |
|---|---|---|
| Reading a value from a printed axis | Yes | About 5 per cent |
| Measuring a length on a scale drawing | Yes | About 5 per cent |
| Estimating an area under a curve | Yes | About 10 per cent - the error compounds |
| Geometry construction with compasses | Yes | Constructions no longer match the printed figure |
| A gradient from two points on the same graph | No | The ratio cancels |
| Reading text | No | Just very slightly smaller |
Setting it correctly, once
Two minutes, and the setting usually persists.
- Find the scaling control
In the print dialogue it is called Scale, Page Sizing, Page Scaling or Print Size, depending on the application.
- Set it to 100 per cent or Actual Size
Not 'fit', not 'shrink oversized pages', not 'auto'.
- Print one page
A page with a graph or a ruled scale on it, ideally.
- Measure a known length
Compare a printed axis against the same axis measured on screen at 100 per cent zoom, or against a scale printed on the page if there is one.
- Only then print the set
One test page costs a sheet. Six weeks of wrong answers costs considerably more.
What happens at 100 per cent instead
The page prints at its true size, and anything falling inside the printer's unprintable margin is clipped. On a document with a normal 20-millimetre margin, nothing is near the edge and nothing is lost.
On a document with very tight margins - or one that has already been cropped - clipping is a real possibility, and the answer is to resize the document to the paper with a small margin rather than to let the print driver scale it. Resizing is a deliberate, uniform, known change; driver scaling is an unknown amount decided by the printer.
Preparing a document to print at full size
The A4 to US Letter case
Printing an A4 document on US Letter is where this most often bites, because the two are genuinely different sizes and something has to give.
Letter is six millimetres wider and eighteen shorter than A4. A driver asked to fit A4 onto Letter scales by about 94 per cent. Resizing the document to Letter first, with a five-millimetre margin, and then printing at 100 per cent gives you a known result instead of a driver's judgement.
Two ways to get A4 content onto Letter paper
Both produce a printout. Only one is predictable.
Let the driver scale it
- One setting, no preparation
- Scaling amount varies between printers
- Every measured length is wrong by an unknown factor
- Fine for prose, wrong for anything measured
Resize, then print at 100 per cent
- One extra step before printing
- Scaling is uniform, deliberate and the same everywhere
- Measured lengths are wrong by a factor you know
- Still not ideal for measured work - use A4 if you can
Measured results
- A4 on A4, scale 100%
- 100.0 mm reference line measured 100 mm
- A4 on A4, fit to page, printer A
- Measured 95.5 mm
- A4 on A4, fit to page, printer B
- Measured 94.2 mm
- A4 on US Letter, fit to page
- Measured 93.9 mm
- Typical unprintable margin
- 3 to 5 mm per edge
Before printing anything you will measure
- Scale set to 100 per cent or Actual Size
- Paper size in the driver matches the document
- One test page printed and measured
- Nothing important clipped at the edges
- For measured work, print on the paper size the document was made for
Questions and answers
Does fit to page affect gradients?
A gradient calculated from two points on the same scaled graph is unaffected, because the ratio cancels. Almost everything else - reading a value off an axis, measuring against a printed scale, estimating an area - is wrong by the scaling factor.
How do I know whether my printout was scaled?
Measure a known length. If the document has a printed scale, use that. Otherwise compare an axis against the same axis on screen at 100 per cent zoom, or print a page with a 100 mm line drawn on it as a test.
Why is fit to page the default?
Because it guarantees nothing is clipped, which is the safer failure for general documents. It is the wrong default for measured work and the print dialogue has no way to know which you have.
What if my content gets clipped at 100 per cent?
Resize the document to the paper size with a small margin, then print at 100 per cent. That makes the scaling deliberate and uniform rather than leaving it to the driver.
