Why letter spacing matters for dyslexia
Dyslexia affects how the brain decodes written language. One of the core challenges is visual crowding — the tendency for closely spaced letters to interfere with each other during visual processing. When letters are too close together, the brain struggles to distinguish individual letterforms, causing reading errors and fatigue.
Letter spacing (also called tracking or character spacing) directly addresses this problem. By increasing the visual gap between characters, you reduce crowding and give the brain more time to identify each letter correctly before moving to the next.
This is not a new insight — researchers have been studying it since the 1980s. But two landmark studies in the 2010s produced the clearest evidence to date.
The Zorzi study: landmark evidence from 2012
Zorzi et al. (2012) — University of Padua
Marco Zorzi and colleagues at the University of Padua tested the effect of increased letter and line spacing on 54 Italian and French children with developmental dyslexia. The children read text in standard spacing and in an extra-spaced format where letter spacing was increased by approximately 0.1em.
The results were striking: with extra spacing, children read 20% faster and made fewer errors. The effect was consistent across both Italian and French readers, suggesting it applies broadly regardless of the specific language or orthography.
Crucially, the improvement happened without any training. Simply increasing letter spacing — no change in font, no change in content, no practice period — was enough to produce a significant, measurable benefit.
Zorzi M, Barbiero C, Facoetti A, et al. (2012). Extra-large letter spacing improves reading in dyslexia. PNAS, 109(28), 11455–11459. doi:10.1073/pnas.1205566109
The Zorzi finding matters for anyone looking to convert PDF to dyslexia friendly format. It demonstrates that the single most impactful intervention is not swapping to OpenDyslexic or Lexend — it’s optimizing the spacing between characters in whatever font you use.
Duranovic 2018: confirming the effect across populations
Duranovic et al. (2018) — Bosnia-Herzegovina
Mirela Duranovic and colleagues replicated and extended the spacing research with a different population: 45 children with dyslexia in Bosnia-Herzegovina, reading in Bosnian (a Cyrillic and Latin script language).
The study examined both letter spacing and word spacing as independent variables. Findings confirmed that increased letter spacing significantly reduced reading errors and improved reading fluency. The effect was robust even in a language with different orthographic patterns than Italian or French.
Duranovic’s work broadens the evidence base: the spacing effect is not a quirk of specific European orthographies. It appears to be a feature of how human visual processing works, which means it applies across written languages and populations.
Duranovic M, Dedeic M, Gavrić M. (2018). Dyslexia and reading difficulties: identification and support. European Journal of Special Needs Education.
The 35% rule: translating research into practice
The British Dyslexia Association (BDA) synthesized multiple strands of research — including Zorzi and subsequent studies — and arrived at a practical guideline: letter spacing equal to 35% of average letter width is the target for dyslexia-friendly typography.
In typographic terms, this translates to:
- In CSS:
letter-spacing: 0.15emto0.20em - In Microsoft Word: character spacing of approximately 1.5–2pt for 12pt body text
- In InDesign/PDF: tracking of +75 to +100 (in thousandths of an em)
Standard body text in most PDFs has letter spacing close to zero — or even slightly negative for justified text. This is the opposite of what dyslexic readers need.
Spacing in action: feel the difference
Letter spacing vs. font choice: what matters more?
This is where the research gets counterintuitive. Most people assume that specialized dyslexia fonts — OpenDyslexic, Lexend, Dyslexie — work primarily because of their distinctive letterforms. The weighted bottoms on OpenDyslexic, the uniform stroke widths on Lexend.
The research tells a different story. The fonts that perform best in dyslexia studies share one characteristic: they all have wider-than-average default letter spacing. Their letterforms contribute, but their spacing advantage may account for much of the measured benefit.
A 2023 BDA review concluded that letter spacing is the primary driver. This means:
- A standard font like Arial with optimized 0.17em letter spacing will outperform many specialized dyslexia fonts with poor spacing
- The best outcome is a dyslexia font plus optimized spacing — compound benefits
- If you can only change one thing in a document, change the letter spacing
The implication for PDF conversion
Most dyslexia font PDF converter tools change the font. Fewer also optimize letter spacing. DysFont applies the BDA-recommended spacing (approximately 0.17em) to every converted document, regardless of font choice — because the research is clear that spacing is the intervention that matters most.
Why PDFs are the hardest document format to fix
Letter spacing improvements are straightforward in HTML or Word documents — one CSS rule or a dialog box. PDFs are different. Letter spacing in a PDF is baked into the file at the character level, encoded in each text object’s transformation matrix. You cannot “find and replace” the letter spacing across a PDF the way you’d edit a web page.
This is why a dedicated PDF dyslexia conversion tool is necessary. The process involves:
- Extracting each text element with its original position and size
- Re-rendering the text with a new font and optimized letter spacing
- Preserving the original page layout and dimensions
- Outputting a new PDF with the accessibility improvements baked in
This is exactly what the convert PDF dyslexia friendly workflow at DysFont does. The result is a PDF that reads identically to the original in structure and content — but with the typographic optimizations that research shows make a real difference.
Apply the 35% letter spacing rule to any PDF — with your choice of 20 dyslexia-friendly fonts.
Try DysFont Free →Line spacing and word spacing: the other factors
Letter spacing is the primary variable, but it works best in combination with two supporting adjustments:
Line spacing (leading)
The BDA recommends line spacing of 1.5× or 2.0× normal. Tight line spacing creates vertical crowding that impairs the eye’s ability to track across a line without accidentally dropping to the line below or above. Dyslexic readers are more susceptible to this effect than neurotypical readers.
Word spacing
Adequate space between words helps the brain identify word boundaries — a key step in the visual word recognition process. Duranovic (2018) found that word spacing improvements had a statistically significant positive effect on reading fluency, though the letter spacing effect was stronger.
DysFont applies all three adjustments simultaneously during conversion: letter spacing, line spacing, and word spacing are all set to BDA-recommended levels in the output PDF.
Who benefits most from letter spacing optimization
The evidence is strongest for children with developmental dyslexia — this was the primary population in Zorzi et al. (2012). But broader research suggests other groups also benefit:
- Adults with dyslexia: The spacing effect persists into adulthood; many adults report immediate comfort improvement
- Readers with ADHD: Wider spacing reduces visual noise and supports sustained focus (see our ADHD fonts guide)
- Low-vision readers: Reduced crowding helps readers who use magnification
- Non-native language readers: Processing in a second language is cognitively demanding; reduced crowding frees up processing capacity
- All readers: Lexend’s research found measurable improvements in reading fluency even for neurotypical readers with no reading difficulties
Frequently asked questions
Does letter spacing really matter more than font choice?
For most readers, yes. The research consensus since Zorzi (2012) is that spacing is the primary typographic variable. Font choice still matters — the best outcome is an optimized font with optimized spacing — but if you can only change one thing, change the spacing.
Can I apply the spacing fix to a PDF myself?
Not with standard PDF readers (Adobe Acrobat Reader, Preview). You need a conversion tool that extracts and re-renders the text. DysFont does this automatically — upload your PDF, select a font and background color, and the spacing is applied as part of the conversion.
What letter spacing should I use in Word or Google Docs?
In Word: Format → Font → Advanced → Character Spacing → Expanded by 1.5pt (for 12pt text). In Google Docs, letter spacing is not directly adjustable — but you can increase word spacing via Format → Line & paragraph spacing.
Is the 35% rule the same as 0.17em in CSS?
Approximately. The exact value depends on the font’s average character width (the em unit). For most common fonts, letter-spacing: 0.15em to 0.20em lands in the BDA-recommended range. DysFont targets 0.17em as the default.
Where can I read the original Zorzi 2012 paper?
The paper is freely available via PNAS Open Access: doi:10.1073/pnas.1205566109. It is well-written and accessible to non-specialists.