This study examines the effects of font size and line spacing on online readability, determining that 18-point font size provides optimal readability compared to smaller sizes, which impair comprehension. The research indicates that better comprehension and subjectively perceived readability occur with 1.0 or 1.4 line spacing, while smaller font sizes (10 and 12 points) are discouraged. The findings challenge common recommendations, suggesting settings like Chrome or Firefox should default to 24 pixels to achieve the recommended 18 points font size.

1. MAKE IT BIG
The Effect of Font Size and Line Spacing on Online Readability.
Luz Rello Martin Pielot Mari-Carmen Marcos
CHI 2016: ACM Conference on Human Factors in Computing Systems
San Jose McEnery Convention Center, May 7 – 12 at San Jose, CA, USA
Presenter: Martin Pielot, @martinpielot

2. Online Reading is a daily habit for many

3. WHICH FONT SIZE FOR BEST READABILITY?

4. readability refers to the ease with
which a reader can read and
understand a written text. It only
refers to the properties of the
presentation of the text, not the
content of the text itself

10. 16 pixels 12 points

12. PIXELS AND POINTS
• Points (pts)
– resolution-independent unit
– 72 pts = 1 inch = 2.54 cm
• Pixels (px)
– used to be resolution dependent
– nowadays ¾ of 1 point
16 px = 12pt (*)
* https://developers.google.com/speed/docs/insights/UseLegibleFontSizes

14. PREVIOUS SCIENTIFIC STUDIES
Authors Year Method Tested Sizes Best Size
Bernard et al. 2001,
2002,
2003
read-aloud 10-14 pts 14
Banerjee et al. 2011 read-aloud 10-14 pts 14
Bhatia et al. 2011 preference 10-14 pts -
Beymer & Russel 2005,
2007,
2008
eye-tracker 10-14 pts 14

15. • Confusing, conflicting, and evidence-
lacking advice in forums
• Scientific research found „bigger is
better“ but we did not find any test
with fonts larger than 14 points.

16. IN THIS PAPER
• control group (n=104) of study to determine
optimal font size for people with dyslexia
• much larger font sizes (up to 26 points)
results challenge existing insights and
recommendations

17. methodology

19. STUDY SETUP
• screen : 17 inch TFT, 1024x768
• eye tracker : tobii 1750
• website : wikipedia (animals)
• browser : firefox
• font : arial

20. INDEPENDENT MEASURES
• font size (pt) : 10, 12, 14, 18, 22, 26
within-subject factor
• line spacing : 0.8, 1.0, 1.4, 1.8
between-group factor

21. Dependent Measures

22. MEAN FIXATION DURATION
as objective measure for readability

23. COMPREHENSION QUESTIONS
to avoid that participants just „skim“ the text

24. SUBJECTIVE READABILITY & COMPREHENSION
to identify mismatch between perceived and objective performance
reading

25. PARTICIPANTS
• 104 volunteers (61 female, 43 male)
• 92 studying or had finished university
• aged 14* – 54 (M = 30.24, SD = 9.13)
• normal or corrected-to-normal vision
* Minors were accompanied by parents

26. results

27. Font Size

28. 10 12 14 18 22 26
0.100.200.300.40
Font Size
FixationDurationMean(ms)
0.8 1
0.100.200.300.40
Line Sp
FixationDurationMean(ms)
FixationDurationMean(s)
FixationDurationMean(s)
Font Size Line Sp
sig. effect of font size
F(5,445) = 66.825,p < .001
large effect size
(η2 = 0.159)
pairwise comparison
• 10pt > than all larger
• 12pt > than all larger
• 14pt > than all larger
• 18pt > than 22pt
MEAN FIXATION DURATION
objective readability best for 18/22/26 pts

29. sig. effect of font size
F(5,445) = 5.2489, p < .001
small effect size
(η2 = 0.025)
pairwise comparison
• 10pt <12pt
MEAN NUMBER OF FIXATIONS
limited effect on number of fixations
10 12 14 18 22 26
0100200300400500600
Font Size
FixN

30. sig. effect of font size
χ2 (5) = 135.85, p < .001
pairwise comparison
• 10pt < than all larger
• 12pt < than all larger
• 14pt < than 18pt
• 26pt < than 18pt
SUBJECTIVE READABILITY
best subjective readability for 18 pt font size
0%!
20%!
40%!
60%!
80%!
100%!
10! 12! 14! 18! 22! 26!
ReadabilityRating!
Font Size!
very good!
good!
neutral!
bad!
very bad!

31. sig. effect of font size
χ2 (5) = 27.29, p < .001
sig. less correct answers for
• 10 & 12 compared to 18 pt
• 12 pt compared to 26 pt
COMPREHENSION SCORES
small fonts may impair comprehension
0%!
20%!
40%!
60%!
80%!
100%!
10! 12! 14! 18! 22! 26!
CorrectResponses!
Font Size!
Wrong!
Correct!

32. sig. effect of font size
F (5, 363) = 18.614, p < .001
pairwise comparison
• 10pt < 14pt +
• 12pt < 14pt +
SUBJECTIVE COMPREHENSION
 subjective comprehension impaired for 10 & 12 pt
0%!
20%!
40%!
60%!
80%!
100%!
10! 12! 14! 18! 22! 26!
ComprehensionRating!
Font Size!
very good!
good!
neutral!
bad!
very bad!

33. Line Spacing

34. 0.8 1 1.4 1.8
0.100.200.300.40
Line Spacing
FixationDurationMean(ms)FixationDurationMean(s)
Line Spacing
no sig. effect of line spacing
F (3, 89) = 0.064, p = .978
MEAN FIXATION DURATION
no evidence re: line spacing

35. no sig. effect of font size
F (3, 365) = 2.074, p = .103
SUBJECTIVE READABILITY
no evidence re: line spacing
0%!
20%!
40%!
60%!
80%!
100%!
10! 12! 14! 18! 22! 26!
ReadabilityRating!
Font Size!
very good!
good!
neutral!
bad!
very bad!
0%!
20%!
40%!
60%!
80%!
100%!
0.8! 1! 1.4! 1.8!
ReadabilityRating!
Line Spacing!
very good!
good!
neutral!
bad!
very bad!

36. sig. interaction effect between
font size x line spacing
F (15, 445) = 4.098, p < .001)
small effect size
(η2 = 0.034)
no interact. effect for 10-18 pt
marginal effect for 10-22 pt
MEAN FIXATION DURATION
 larger fonts are affected by line spacing
0.000.100.200.30
Fontsize
FixationDuration(mean)
10 12 14 18 22 26
●
● ●
●
● ●
● ●
●
● ●
●
●
●
1.4
1.8
0.8
1

37. sig. effect of line spacing
χ2 (3) = 19.56, p < .001
sig. less correct answers for
• 0.8 compared to all other
line spacings
COMPREHENSION SCORES
 small line spacings may impair comprehension
0%!
20%!
40%!
60%!
80%!
100%!
0.8! 1! 1.4! 1.8!
CorrectResponses!
Line Spacing!
Wrong!
Correct!

38. sig. effect of line spacing
F (3, 365) = 3.249, p = .022
pairwise comparison
• 1.0 > 1.8
SUBJECTIVE COMPREHENSION
 subj. comprehension impaired for largest spacing
0%!
20%!
40%!
60%!
80%!
100%!
0.8! 1! 1.4! 1.8!
ComprehensionRating!
Line Spacing!
very good!
good!
neutral!
bad!
very bad!

39. conclusions

40. USE 18 POINT FONT SIZE
• 18 pts and larger had best objective readability
• subjective readability was highest for 18 pts
USE 1.0 or 1.4 LINE SPACING
• comprehension scores were lower for 0.8
• subjective comprehension was affected for 1.8
AVOID 10 AND 12 POINT FONT SIZE
• comprehension scores were lowest
• readability (subj + obj) was worst

41. STARK CONTRAST TO COMMON
RECOMMENDATIONS

42. Set Chrome or Firefox to 24 (pixels)
to have 18 points font size

43. MAKE IT BIG
The Effect of Font Size and Line Spacing on Online Readability.
CHI 2016: ACM Conference on Human Factors in Computing Systems
San Jose McEnery Convention Center, May 7 – 12 at San Jose, CA, USA
18 POINT FONT SIZE (24 px)
default OR slightly larger LINE SPACING
10 AND 12 PTS IMPAIRED UNDERSTANDING
Presenter: Martin Pielot, @martinpielot
Luz Rello Martin Pielot Mari-Carmen Marcos