How do you calculate MLU? Count the morphemes in each of 50 to 100 intelligible utterances, add them up, and divide by the number of utterances. MLU analysis software such as EASI (Evaluative Artificial Speech Intelligence) calculates MLU from an English language sample after the SLP reviews the draft transcript, corrects any morphemes, and attests each line.
What MLU measures
Mean length of utterance (MLU) is the average length of a child's utterances in a language sample. It is usually counted in morphemes, the smallest units of meaning, so "dogs" is two morphemes (dog + plural s). Roger Brown introduced the counting rules most SLPs still use (Brown, 1973). MLU in words is a simpler version that counts words instead of morphemes.
How to calculate MLU in morphemes
- Collect a sample. Most guidance says 50 to 100 utterances. See how long a language sample should be.
- Divide it into utterances. Use the same utterance rules every time.
- Leave out what you don't count. Skip partly unintelligible utterances, exact imitations of the adult, and fillers such as "um". Count only the final version of a false start or repeated word.
- Count morphemes in each utterance. Count each root word, plus the plural s, possessive s, ed, ing and third person s endings.
- Count these as one morpheme: irregular past tense (went), compound words and proper names (birthday, Mickey Mouse), diminutives (doggie), and catenatives (gonna, wanna).
- Add and divide. Total morphemes ÷ total utterances = MLU.
Brown's rules count contractions such as "don't" as one morpheme for young children, and many SLPs count "it's" or "I'm" as two once the child also uses both parts. Conventions vary. Pick one, write it down, and use the rules of whichever norms you plan to compare to.
A worked example
| Utterance | Morphemes | Count |
|---|---|---|
| doggie running | doggie + run + ing | 3 |
| he jumped | he + jump + ed | 3 |
| my mom's car | my + mom + 's + car | 4 |
| two cats | two + cat + s | 3 |
| I wanna go | I + wanna + go | 3 |
Total: 16 morphemes ÷ 5 utterances = MLU of 3.2. (A real sample needs far more than 5 utterances.)
Options for MLU analysis software
| Tool | What it is |
|---|---|
| Free online MLU calculators | Paste utterances and get a count; you still check morpheme rules yourself |
| CLAN (TalkBank) | Free research software with automatic morpheme tagging, using CHAT transcripts |
| SUGAR | Free, word processor based method with its own MLU rules and norms |
| MiMo | Free browser based tool that calculates MLU |
| SALT | Paid language sample analysis software with reference databases |
| EASI | Records, uploads or imports an English sample, drafts the transcript, and calculates MLU and other measures after clinician review and attestation, inside a caseload workspace |
How MLU works in EASI
- Draft: record live, upload audio, or paste or import a transcript. EASI drafts an English transcript.
- Review: check words, speaker labels and morphemes. Automatic transcription can miss small grammatical forms, such as a plural s or ed, so you correct them here.
- Attest: confirm each line of the transcript.
- Score: EASI calculates MLU from the reviewed, attested transcript, along with other measures the sample supports, such as IPSyn, number of different words, vocabulary diversity and PCC.
EASI does not calculate MLU silently from an unreviewed machine draft. You decide which norms apply and what the results mean. Learn more about language sample analysis in EASI, or see how EASI compares with SALT.
Frequently asked questions
Is MLU counted in words or morphemes? Both are used. MLU in morphemes follows Brown's (1973) counting rules and is the more common clinical measure; MLU in words counts whole words. Use whichever matches the norms you compare to.
How many utterances do I need for MLU? Most guidance says 50 to 100. One study of 3 year olds found MLU reached strong reliability at about 10 minutes of sampling (Guo & Eisenberg, 2015).
Does EASI calculate MLU from the AI transcript? No. EASI calculates MLU from the transcript after the SLP reviews it, corrects morphemes, and attests each line.
Can EASI calculate MLU for languages other than English? EASI transcription is English only. For other languages, CLAN and MiMo support many languages.
Is an MLU score a diagnosis? No. MLU is one measure. It does not replace clinical judgment or a complete evaluation.
Get EASI
Individual SLPs and groups of 10 or more get EASI through Northern Speech Services: Get EASI at Northern Speech.
References
Brown, R. (1973). A first language: The early stages. Harvard University Press. Guo, L. Y., & Eisenberg, S. (2015). Sample length affects the reliability of language sample measures in 3 year olds: Evidence from parent elicited conversational samples. Language, Speech, and Hearing Services in Schools, 46(2), 141 to 153. https://doi.org/10.1044/2015_LSHSS-14-0052 Pezold, M. J., Imgrund, C. M., & Storkel, H. L. (2020). Using computer programs for language sample analysis. Language, Speech, and Hearing Services in Schools, 51(1), 103 to 114. https://doi.org/10.1044/2019_LSHSS-18-0148