SMOG Readability Calculator

SMOG — the Simple Measure of Gobbledygook — was published in 1969 by G. Harry McLaughlin. It estimates the years of education a reader needs to fully understand a text, and it is the readability formula most often recommended for health information, patient leaflets, consent forms and public notices. Where some formulas predict partial understanding, SMOG aims at complete understanding, which is what matters when readers must follow instructions safely.

How SMOG works

SMOG counts polysyllables — words of three or more syllables — per sentence. The more long words packed into each sentence, the higher the grade. The calculator uses the regression formula 1.0430 × √(polysyllables × 30 ÷ sentences) + 3.1291 on your whole text. The “× 30 ÷ sentences” part scales the count to the 30-sentence sample the formula was designed for, so texts of any length can be scored.

McLaughlin’s hand method

The original method is designed to be done with a pencil: take 10 sentences from the beginning, 10 from the middle and 10 from the end, count the polysyllables, find the nearest perfect square, take its square root and add 3. When your text has at least 30 sentences, the tool does exactly this as well and shows the result next to the formula. The two usually agree within a grade.

What grade to aim for

Health literacy guidance in many countries recommends writing materials for the general public at around a grade 6 to 8 reading level, because a large share of adults read below high-school level, especially when they are anxious or unwell. A SMOG grade of 10 or more on a leaflet meant for everyone is a signal to simplify.

Seeing the long words

Every word of three or more syllables is highlighted. Many have everyday alternatives: “medication” → “medicine”, “approximately” → “about”, “experience” → “have”. Medical terms that patients must learn can stay, ideally with a short explanation the first time they appear.

Fry graph inputs

The panel also shows the number of syllables and sentences per 100 words. These are the two values used on Edward Fry’s readability graph, usually averaged over three 100-word samples. The tool gives the inputs so you can plot them on a printed Fry graph if your organisation uses one; it does not draw the graph itself.

A worked example

The sample medicine instructions contain several long words — “recommended”, “experience”, “medication”, “immediately”, “temperature”, “prescription” — across six sentences, giving a SMOG grade of about 11 — high-school level, well above what most health organisations recommend. Rewriting “If you experience dizziness, nausea or an allergic reaction, stop taking the medication immediately” as “If you feel dizzy or sick, or get a rash, stop taking the medicine and call your doctor” removes most of the long words in that sentence.

Testing with real readers

A formula is a first check. For important health materials, also test them with a few people from the intended audience: ask them to explain the instructions back in their own words. Where they struggle, simplify the wording, add headings and lists, and re-score the revised text.

Related tools

Compare with Flesch–Kincaid in the Readability Checker, check vocabulary with the Dale–Chall Calculator, count syllables precisely with the Syllable Counter, or measure other languages with the LIX Calculator.