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). Below 50, the MLU is still calculated but always shown with a caution statement; under 15 it should be treated as a rough screening indicator only.

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+).

Does this MLU calculator store my data?

No. It runs entirely in your browser and never sends your language sample to a server.

Can this calculator handle Spanish or bilingual samples?

Yes. The calculator applies deterministic Spanish morpheme rules: plurals, diminutives (-ito/-cito), gerunds, infinitives, contractions (al/del), and common irregular verb forms each count their morphemes automatically. Only ambiguous inflected forms are flagged for clinician review, and every count stays editable.

SLP Draft Toolkit

MLU Calculator

Q: Can you calculate MLU for a gestalt language processor?

A standard MLU is misleading for gestalt language processors because echolalic scripts inflate the average. This calculator has a GLP mode: mark each utterance as a gestalt or self-generated, and MLU is computed from self-generated utterances only. Brown's Stage is not assigned; progress is tracked through the Natural Language Acquisition stages (Blanc, 2012) instead.

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 and available when you need it.

Step One

1. Paste the sample

One utterance per line. Mark unintelligible with xxx.

Language mode

Gestalt language learner (GLP)

For echolalic / gestalt speakers. You mark each utterance as a gestalt or self-generated; MLU runs on self-generated language only, and Brown’s Stage is not assigned.

Step Two

2. Results

MLU updates live as you edit utterances below.

Mean Length of Utterance

—

Total morphemes ÷ included utterances.

Included

Utterances

Morphemes

Total count

This MLU is based on only 0 included utterances — far below the 50-utterance minimum recommended for stable MLU estimates (Miller & Chapman). Treat this value as a rough screening indicator only, not a reportable MLU, and collect a longer sample before drawing conclusions.

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). Shorter samples still get an MLU, always flagged with a caution statement.
Gestalt language learners: turn on GLP mode and mark echolalic gestalts separately — they are excluded from the MLU, and no Brown’s Stage is assigned (NLA framework, Blanc 2012).

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). Below 50 the MLU is still calculated and reported with a caution statement; under 15 it should be treated as a rough screening indicator only.

Does this work for Spanish or bilingual samples?

Yes. Spanish samples are counted with deterministic Spanish morpheme rules — plurals, diminutives (-ito/-cito), gerunds, infinitives, contractions (al/del), and common irregular verb forms are counted automatically. Only ambiguous inflected forms get flagged for review, and every count stays editable before you rely on the final MLU.

Can you calculate MLU for a gestalt language processor?

A standard MLU alone is misleading for GLPs — echolalic scripts and gestalts inflate the average. Turn on GLP mode (the toggle under the sample box), mark each utterance as a gestalt or self-generated, and the calculator computes MLU from self-generated utterances only. It deliberately does not assign a Brown's Stage: for gestalt language processors, progress is tracked through the Natural Language Acquisition stages (Blanc, 2012) instead.

Do you store my data?

No account is required. 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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