The Dale Chall formula stands out among readability formulas because it checks vocabulary, not just sentence length. It compares your words against a list of words familiar to most fourth-graders, and words outside that list raise the score.
How it works
The original 1948 formula is: score = 0.1579 x (percentage of difficult words) + 0.0496 x (average sentence length) + 3.6365. A difficult word is any word not on the Dale Chall familiar-word list, excluding proper nouns, numbers, and some inflected forms. The 1995 new Dale Chall formula uses a different word list and adjusted constants, so state which version you use. Scores map roughly to grades: below 5 is fourth grade and below, 8 to 8.9 is eleventh to twelfth grade, and above 10 is college level.
Step by step
- Pick a sample of at least 100 words, ending at a sentence boundary.
- Count words and sentences, then compute average sentence length.
- Mark each word as familiar or difficult using the Dale Chall list, excluding proper nouns.
- Compute the percentage of difficult words.
- Apply the formula above for the original version.
- Map the score to a grade band and compare with your audience.
Worked example
A sample has 120 words, 8 sentences, and 24 difficult words. Average sentence length = 120 / 8 = 15. Difficult percentage = 24 / 120 x 100 = 20. Score = 0.1579 x 20 + 0.0496 x 15 + 3.6365 = 3.158 + 0.744 + 3.6365 = 7.54, about seventh to eighth grade. If difficult words rose to 40 percent, the score would be 0.1579 x 40 + 0.744 + 3.6365 = 10.70, college level.
Common mistakes
- Counting proper nouns as difficult words. The formula excludes them.
- Using the new-formula constants with the old word list, or the reverse.
- Applying the formula to a tiny sample, where a few words swing the percentage.
- Treating the score as a measure of quality. A low score is not better writing, only easier reading.
Try it with the free tool
You can follow every step above by hand, but the free tool below does the same calculation instantly. Try your own numbers in it, then check one result by hand so you trust the method as well as the answer. Open the full tool here: open the tool.
Checklist before you finish
- Write down every input number first, with its unit, before you calculate anything.
- Work through the steps in order, and do not skip the unit conversions.
- Check the answer against the worked example above: is it the right size, and does the direction make sense?
- Round only at the end, and keep the unrounded figure for any follow-up calculation.
Practice with your own numbers
Pick three cases of your own and work each one by hand. First, try a simple round-number case you can check mentally. Second, try a realistic case with awkward numbers, the kind you will actually meet. Third, try an edge case, such as a very small or very large input, and confirm the answer still behaves sensibly. If any case gives a surprising result, re-check the units and the formula step by step before you trust it.
If your answer looks wrong
Most errors come from a small set of causes. Check that you used the same units throughout, that you divided where the formula divides and multiplied where it multiplies, and that you applied any percentage as a decimal. Check the order of operations: powers and brackets come before multiplication and division, which come before addition and subtraction. Finally, test the method on the worked example in this guide. If your method reproduces the example answer, the method is right and the inputs deserve a second look.
Quick tips
If your score is too high for your audience, replace difficult words with familiar ones before shortening sentences, because vocabulary drives this formula more than sentence length does.