Readability
Date: 2026-08-17
How easily text can be understood. The formulas measure sentence and word length only, which makes them a useful smell test and a poor target — optimising for the score produces choppy text that scores well and reads badly.
Readability is the ease with which written text is understood. Legibility is a separate thing — whether the characters can be distinguished, which is typography and contrast.
READABILITY can they understand it?
→ words, sentences, structure
LEGIBILITY can they see it?
→ type size, contrast,
line length
— Colour Contrast, Type Scale
See: Colour Contrast · Type Scale
What the formulas actually measure
FLESCH READING EASE
based on: average sentence length
average syllables per word
→ 0–100, higher is easier
FLESCH-KINCAID GRADE
the same inputs, expressed as a
US school grade
GUNNING FOG
sentence length + % of "complex"
(3+ syllable) words
Every mainstream formula uses only sentence length and word length. That’s the whole input.
THEY CANNOT DETECT
jargon made of short words
→ "we will action your P&P query"
ambiguity
bad structure
passive voice
whether it's TRUE
whether it answers the question
“Ambient” is two syllables and incomprehensible to a customer. The formula rates it easy.
Using them properly
As a smell test, not a target.
GOOD USE
flagging a paragraph that's much
harder than the rest
catching a 60-word sentence
comparing a rewrite to the original
BAD USE
a KPI
chopping sentences to hit a grade
→ produces staccato text that
scores well and reads badly
A common public-sector target is around a 9-year-old reading level for general content, which sounds patronising and isn’t — it means short sentences and common words, and expert readers prefer it too.
[CHECK: the specific reading-age target if you’re writing to a published content standard — they vary by organisation and by content type.]
What actually improves readability
SHORTER SENTENCES
one idea each. Aim ~15–20 words
average, with variety
COMMON WORDS
"use" not "utilise"
"start" not "commence"
"about" not "regarding"
ACTIVE VOICE
"we'll send your order" not "your
order will be dispatched"
SECOND PERSON
"you" — direct and shorter
CUT QUALIFIERS
"may potentially be able to" →
"can"
STRUCTURE
headings, lists, one idea per
paragraph
— Reading Behaviour Online
DEFINE JARGON on first use, or
remove it
Removing jargon does more than any sentence-length change, and no formula will tell you to.
Why it matters commercially
Not only accessibility:
non-native speakers
→ a large share of UK customers
low literacy
→ a significant proportion of UK
adults have literacy below the
level of much commercial writing
cognitive differences
— Cognitive Accessibility
EVERYONE, when
distracted, tired, stressed, on a
phone, in a hurry
[CHECK: current UK adult literacy statistics before quoting a figure — definitions and surveys differ.]
The situational group is the largest. Plain writing isn’t a concession to a minority; it’s the version that works for people shopping on a train.
Line length and legibility
Separate from the formulas, and it affects comprehension:
45–75 characters per line
← the usual comfortable range
→ about 65 is a good target
TOO LONG the eye loses the line on
return
TOO SHORT too many returns, rhythm breaks
p { max-width: 65ch; }One property, and it’s routinely missing on content pages that stretch to a 1600px viewport — Type Scale.
The honest summary
Write plainly, structure well, remove jargon, then check the score to catch what you missed. The formula is the last step, not the brief.