The Dale–Chall readability formula is useful for getting a quick measure of Readability.

Usage

The adjusted score roughly means:

• 0.0–4.9 easily understood by an average 4th-grade student or lower
• 5.0–5.9 easily understood by an average 5th or 6th-grade student
• 6.0–6.9 easily understood by an average 7th or 8th-grade student
• 7.0–7.9 easily understood by an average 9th or 10th-grade student
• 8.0–8.9 easily understood by an average 11th or 12th-grade student
• 9.0–9.9 easily understood by an average 13th to 15th-grade (college) student

The grades refer to American students, where 4th grade is a 9-year-old.

The main use I’ve found for the tool is highlighting words that aren’t common, prompting me to think about whether they’re needed.

I aim for my writing to have a score below 8. This isn’t easy, and in some cases isn’t worth it.

You might think that number is too low. Surely my target audience is at least high-school graduates?

Firstly, it’s only a target, and I often miss it without stressing about it.

Secondly, everything I write starts with plenty of words that can be cut or changed with no cost to clarity. Those can go right away. Why wouldn’t I make my writing more readable, if it comes at no cost? (Other than the time I spend editing, of course.)

Most importantly, readability isn’t just a binary Yes/No “Can the reader read this?” A 12th grade student will find it easier to read something with a score of 7 than a score of 8, even if they are capable of reading both. The less effort my reader spends reading, the more effort they can spend thinking about what the writing says.

About

The formula was first published in 1948, long before computers. Doing the calculation by hand takes a while, so testers would only test short samples of the text instead of doing the whole thing, and even then it was tedious. But it did have the upside that the human could handle weird input. Computers are not so good at that.

This calculator uses these rules to clean up the input:

• All non-letters except apostrophes, hyphens, and periods are turned into spaces
• A word is defined as a sequence of non-space characters
• For a word before a period, if it’s one letter that isn’t “I” or it contains no vowels, it’s considered an abbreviation. In this case, the period is apart of the word, rather than the end of a sentence.
• A sequence of 2 to 5 periods is considered an ellipsis and doesn’t end a sentence.

In Using a Computer to Calculate the Dale-Chall Formula, it says:

In addition, 5) names of persons and places are considered familiar even though they do not appear on the list; 6) names of organizations, laws, documents, titles of books or movies, when used several times in a sample of 100 words, are counted only twice.

For computers, this is not easy. This calculator doesn’t attempt to decide if a word is a proper noun, and it definitely doesn’t try to decide what type of proper noun it is.