MLU stands for Mean Length of Utterance — the average number of morphemes per utterance in a child's language sample. It is a standard, research-backed index of expressive syntactic development (Brown, 1973).

How do you calculate MLU?

Count the total number of morphemes across all included utterances, then divide by the number of included utterances. Use Brown's morpheme rules: each word is 1 morpheme, with +1 for each bound morpheme (plural -s, possessive 's, past -ed, progressive -ing, third-person -s). Contractions count as 2.

How many utterances do I need for MLU?

50+ complete, intelligible, spontaneous utterances are generally recommended for a stable MLU estimate (Miller & Chapman). Between 15 and 49 utterances, report MLU with caution. Below 15, MLU is not considered reportable.

What are Brown's Stages?

Brown's Stages map MLU to approximate expressive syntactic stages: Stage I (MLU 1.0–1.99), Stage II (2.0–2.49), Stage III (2.5–2.99), Stage IV (3.0–3.74), Stage V (3.75–4.49), and Stage V+ (4.5+).

Is this MLU calculator free?

Yes — completely free, no signup, no email required. It runs entirely in your browser and never sends your language sample to a server.

Can this calculator handle Spanish or bilingual samples?

Yes, with manual review. Spanish has rich verb inflection that cannot be reliably auto-counted, so each Spanish word defaults to 1 morpheme and every row is flagged for clinician review before the MLU is reported.

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Free MLU Calculator

Paste a language sample, get morpheme counts, MLU, and Brown's Stage — in seconds. Rule-based and transparent: every count is shown and editable. Built by a school SLP. Yours to use, free, forever.

Step One

1. Paste the sample

One utterance per line. Mark unintelligible with xxx.

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Step Two

2. Results

MLU updates live as you edit utterances below.

Mean Length of Utterance

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Total morphemes ÷ included utterances.

Included

Utterances

Morphemes

Total count

How to calculate MLU (Mean Length of Utterance)

MLU = total morphemes ÷ included utterances. The calculator above uses Brown's morpheme rules — the same rules used in graduate SLP coursework and in published language sample analysis tools.

Each word = 1 morpheme by default.
Bound morphemes add +1 each: possessive 's, plural -s/-es, past -ed, progressive -ing, third-person -s.
Contractions (don't, it's, I'll) split into 2 morphemes.
Irregular plurals (children, feet) and irregular past (went, ate) stay 1.
Fillers (um, uh, hmm) and unintelligible tokens (xxx) count 0.
50+ utterances are recommended for a stable estimate (Miller & Chapman, 1981). Below 15, MLU is not reportable.

Brown's Stages of Language Development

Roger Brown (1973) mapped MLU values to five developmental stages. The calculator automatically flags the stage that matches each sample.

Stage	MLU range	Approx. age	Typical features
I	1.0–1.99	12–26 mo	Single words; emerging two-word combinations.
II	2.0–2.49	27–30 mo	Early grammatical morphemes (-ing, in/on, plural -s).
III	2.5–2.99	31–34 mo	Simple sentences, yes/no questions, basic negation.
IV	3.0–3.74	35–40 mo	Complex sentences, emerging embedded clauses.
V	3.75–4.49	41–46 mo	Conjoined clauses (and, because), elaborated structures.
V+	4.5+	47+ mo	Adult-like grammar, advanced clause structures.

Age bands are approximate; clinical judgement always overrides stage tables.

Worked example

Five utterances from a 3-year-old:

#	Utterance	Morphemes	Why
1	I want cookie.	3	3 words, no bound morphemes.
2	He's running fast.	5	he + is (contraction) + run + -ing + fast.
3	Two doggies sleeping.	5	two + doggy + -s + sleep + -ing.
4	Mommy's car.	3	mommy + 's (possessive) + car.
5	We went home.	3	we + went (irregular past, counts 1) + home.

Total morphemes: 19. Included utterances: 5. MLU = 19 ÷ 5 = 3.80 → Brown's Stage V.

(This is a demonstration only — a real clinical MLU needs 50+ utterances.)

Frequently asked questions

What does MLU mean?

MLU stands for Mean Length of Utterance — the average number of morphemes per utterance in a child's language sample. It's a research-backed index of expressive syntactic development.

How do I calculate MLU by hand?

Transcribe 50+ spontaneous, intelligible utterances (one per line). Count morphemes for each: 1 per word, +1 for each bound morpheme (plural -s, possessive 's, past -ed, -ing, 3rd-sg -s). Contractions count as 2. Total morphemes ÷ utterances = MLU. This calculator does the arithmetic and lets you override every count.

How many utterances do I need?

50+ for a stable, reportable MLU (Miller & Chapman, 1981). 15–49 utterances can be reported with caution. Fewer than 15 should not be reported as MLU.

Does this work for Spanish or bilingual samples?

Yes, with manual review. Spanish verbs encode person, number, tense, and mood in their endings — reliable counting requires a lexicon, so each Spanish word defaults to 1 morpheme and every row is flagged. Edit each count before relying on the final MLU.

Is the calculator free? Do you store my data?

100% free, no signup. The calculator runs entirely in your browser — your language sample is never sent to a server or saved.

What if I want the whole IEP report drafted?

That's what SLP Draft does. The MLU calculator is the same engine that lives inside the paid app, where it's part of a full evaluation report workflow (present levels, goals, narrative, the works). 14-day free trial, no credit card.

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