Mean Length of Utterance (MLU) Calculator

An essential tool for speech-language pathologists, researchers, and parents to measure linguistic development.

Brown’s Stages of Syntactic and Morphological Development

Stage	MLU Range	Typical Age Range	Characteristics
I	1.0 – 2.0	12-26 months	Single-word and two-word utterances (e.g., “more juice”).
II	2.0 – 2.5	27-30 months	Present progressive (-ing), in, on, plural -s.
III	2.5 – 3.0	31-34 months	Irregular past tense, possessive ‘s, uncontractible copula.
IV	3.0 – 3.75	35-40 months	Articles (a, the), regular past tense (-ed), third person regular.
V	3.75 – 4.5	41-46 months	Third person irregular, uncontractible auxiliary, contractible copula & auxiliary.
Post-V	4.5+	47+ months	Complex sentence structures and multi-clause utterances.

What is Mean Length of Utterance (MLU)?

Mean Length of Utterance (MLU) is a widely used measure in linguistics and speech-language pathology to gauge a child’s linguistic development. Proposed by Roger Brown in 1973, it provides a more sensitive and accurate index of language ability than chronological age alone. MLU represents the average number of morphemes a child uses per utterance, offering a snapshot of their grammatical complexity. It is a crucial tool for identifying potential language delays and for tracking progress during language intervention. Anyone involved in a child’s development, including parents, educators, and speech-language pathologists, can use this metric to understand and support language acquisition.

A common misunderstanding is that MLU is simply the average number of words in a sentence. However, the “morpheme” is the key unit. A morpheme is the smallest unit of meaning in a language. For example, the word “dogs” contains two morphemes: “dog” (the animal) and “-s” (the plural marker). Calculating MLU in morphemes provides a deeper insight into a child’s grasp of grammatical rules, which is why it is the standard for how to calculate mean length of utterance.

Mean Length of Utterance Formula and Explanation

The formula for how to calculate mean length of utterance is straightforward and powerful. It provides a clear numerical value representing a child’s average utterance complexity.

MLU = (Total Number of Morphemes) / (Total Number of Utterances)

To use this formula, you first need to collect and transcribe a language sample, typically consisting of 50-100 spontaneous utterances from the child. Then, you meticulously count the morphemes in each utterance and sum them up. Finally, you divide by the total number of utterances to get the MLU.

Variable Explanations

Variable	Meaning	Unit (Auto-Inferred)	Typical Range
Total Number of Morphemes	The sum of all meaningful language units (base words and affixes) in the sample.	Morphemes	50 – 500+ (highly dependent on sample size and age)
Total Number of Utterances	The number of distinct, complete expressions (sentences or phrases) in the sample.	Utterances	50 – 100 (for a standard sample)
Mean Length of Utterance (MLU)	The resulting ratio, indicating the average grammatical complexity per utterance.	Morphemes per Utterance	1.0 – 5.0+ (increases with age)

Practical Examples of How to Calculate Mean Length of Utterance

Example 1: Early Language Stage

A speech-language pathologist collects a sample from a 28-month-old child. They transcribe 5 utterances:

1. “Mommy go.” (2 morphemes)
2. “Want juice.” (2 morphemes)
3. “Big doggy.” (2 morphemes)
4. “My toys.” (My + toy + s = 3 morphemes)
5. “Go bye-bye.” (2 morphemes)

• Inputs: Total Morphemes = 2 + 2 + 2 + 3 + 2 = 11
• Units: Total Utterances = 5
• Results: MLU = 11 / 5 = 2.2

An MLU of 2.2 places the child squarely in Brown’s Stage II, which is age-appropriate.

Example 2: Developing Language Stage

A parent analyzes a language sample from their 42-month-old child. They transcribe 3 more complex utterances:

1. “The boys are running.” (The + boy + s + are + run + ing = 6 morphemes)
2. “He goed outside.” (He + go + ed (irregular past) = 3 morphemes. Note: overregularization is counted.)
3. “I’m eating apples.” (I + am + eat + ing + apple + s = 6 morphemes)

• Inputs: Total Morphemes = 6 + 3 + 6 = 15
• Units: Total Utterances = 3
• Results: MLU = 15 / 3 = 5.0

This MLU of 5.0 indicates the child is operating at or beyond Stage V, demonstrating a strong command of grammatical structures. For more details on language stages, see our guide on Child Development Milestones.

How to Use This Mean Length of Utterance Calculator

This calculator simplifies the process of determining MLU. Follow these steps for an accurate result:

1. Collect a Language Sample: Record 50-100 spontaneous utterances from the child in a natural play-based setting. Avoid asking too many questions that elicit yes/no answers.
2. Transcribe and Count Morphemes: Write down each utterance. Carefully count the morphemes for each one, paying attention to plurals (-s), possessives (‘s), past tense (-ed), and present progressives (-ing).
3. Enter Total Morphemes: Sum all the morphemes you counted and enter the total into the “Total Number of Morphemes” field.
4. Enter Total Utterances: Enter the number of utterances you collected (e.g., 50 or 100) into the “Total Number of Utterances” field.
5. Interpret the Results: The calculator will instantly display the MLU. You can compare this value to Brown’s Stages of Development in the table above to assess the child’s current language level. The corresponding stage will be highlighted for your convenience.

Key Factors That Affect Mean Length of Utterance

Several factors can influence a child’s MLU. Understanding these provides context for the calculated value.

• Age: The most significant factor. MLU is expected to increase predictably with age through early childhood.
• Vocabulary Size: A child needs a critical mass of words (at least 50) before they can begin combining them into multi-word utterances. A diverse vocabulary is crucial for creating longer sentences.
• Language Environment: Children who are exposed to rich, complex language from caregivers tend to develop higher MLUs more quickly.
• Cognitive Development: General cognitive abilities and nonverbal intelligence are correlated with language development, including MLU.
• Language Impairments: Children with Specific Language Impairment (SLI) or other developmental disorders often have a significantly lower MLU than their typically developing peers.
• Sampling Context: The type of interaction can affect MLU. For instance, a narrative or storytelling task may elicit more complex language than a simple question-answer session. For more information, you might be interested in our Language Assessment Tools overview.

Frequently Asked Questions (FAQ)

1. What is a “morpheme”?

A morpheme is the smallest unit of language that carries meaning. It can be a full word (like “cat,” “run”) or a prefix/suffix (like plural “-s,” past-tense “-ed,” or progressive “-ing”). For example, “running” has two morphemes: “run” and “-ing”.

2. How many utterances do I need for a reliable MLU calculation?

For a reliable and stable measure, a sample of 50 to 100 utterances is the standard recommendation in research and clinical practice. Some studies suggest sample sizes up to 175-200 for maximum reliability.

3. Should I count words or morphemes?

While counting words is simpler, counting morphemes provides a much more accurate picture of a child’s grammatical development. Professionals almost exclusively use MLU-m (in morphemes) for assessment. This calculator is designed for the standard morpheme-based approach to how to calculate mean length of utterance.

4. What if my child’s MLU is low for their age?

A low MLU can be an indicator of a potential language delay. If you have concerns, it is best to consult with a certified speech-language pathologist for a comprehensive evaluation. Explore our Early Intervention Guide for more resources.

5. How do I count morphemes for irregular words?

Irregular forms are counted as one morpheme until the regular rule is also acquired. For example, when a child says “went,” it is one morpheme. If they say “goed,” it is two morphemes (go + -ed) because they are applying a rule, even if incorrectly.

6. Do compound words count as one or two morphemes?

Compound words like “butterfly” or “pancake” are counted as a single morpheme because the individual parts (“butter” and “fly”) do not retain their original meanings to form the new word’s meaning.

7. What doesn’t get counted in a morpheme count?

You typically exclude fillers (um, uh), false starts, and repetitions unless the repetition is for emphasis. You also don’t count dysfluencies (stuttering).

8. Is a higher MLU always better?

Generally, a higher MLU indicates more advanced linguistic skills. However, context is everything. Extremely long, rambling utterances that are grammatically incorrect may not reflect true competence. MLU is one of several tools used in a complete language assessment. See our article on Holistic Language Evaluation.

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