MLU Calculator: Calculate Mean Length of Utterance (Brown’s Method)

An expert tool for speech-language pathologists, researchers, and parents to accurately measure language development based on Roger Brown’s foundational rules.

MLU Calculator (Brown’s Rules)

What is Mean Length of Utterance (MLU)?

Mean Length of Utterance (MLU) is a measure of linguistic productivity in children. It was developed by psychologist Roger Brown and is widely used by speech-language pathologists and researchers to track the development of a child’s grammatical skills. Instead of just counting words, MLU is calculated in morphemes—the smallest units of meaning in a language. This makes it a more sensitive index of grammatical development than age alone, as it reflects the growing complexity of a child’s speech. For example, the word “dogs” contains two morphemes: the root word “dog” and the plural marker “-s”. A higher MLU generally indicates a higher level of language proficiency.

The MLU Formula and Explanation

The calculation for MLU is straightforward once the necessary data has been collected from a language sample.

MLU = Total Number of Morphemes / Total Number of Utterances

The most critical part of this formula is accurately counting the morphemes and utterances according to a specific set of rules, such as those laid out by Brown.

Table explaining the variables used in the MLU calculation.

Variable	Meaning	Unit	Typical Range
Total Morphemes	The sum of all morphemes counted in the language sample.	Count (unitless)	50 – 500+
Total Utterances	The total number of analyzable sentences or phrases spoken by the child.	Count (unitless)	50 – 100
MLU	The resulting average length of utterance in morphemes.	Ratio (morphemes/utterance)	1.0 – 5.0+

Practical Examples

Example 1: Early Language User

A speech-language pathologist collects a 50-utterance sample from a 28-month-old child. After transcribing and analyzing the sample, the total morpheme count is 115.

• Inputs: 115 Total Morphemes, 50 Total Utterances
• Units: These are unitless counts.
• Result: 115 / 50 = 2.3 MLU. This places the child in Brown’s Stage II.

Example 2: More Advanced Speaker

A 42-month-old child provides a 100-utterance language sample. The analysis reveals a total of 380 morphemes.

• Inputs: 380 Total Morphemes, 100 Total Utterances
• Units: These are unitless counts.
• Result: 380 / 100 = 3.8 MLU. This places the child in Brown’s Stage V.

How to Use This MLU Calculator

1. Collect a Language Sample: Record at least 50-100 consecutive, intelligible utterances from the child in a natural play setting.
2. Count Morphemes: Transcribe the sample and count the morphemes for each utterance according to Brown’s rules. This requires careful attention to grammatical markers like plurals, possessives, and verb tennses.
3. Enter Totals: Input your ‘Total Number of Morphemes’ and ‘Total Number of Utterances’ into the fields above.
4. Calculate and Interpret: Click the “Calculate MLU” button. The calculator will provide the MLU score, the formula used, and the corresponding developmental stage according to Brown. The chart helps visualize the result in context of typical development.

Key Factors That Affect MLU Calculation

The accuracy of an MLU calculation is highly dependent on the consistent application of counting rules. Here are six critical factors to consider, primarily based on Roger Brown’s method:

• 1. Defining an Utterance: An utterance is a complete spoken unit, often marked by pauses and intonation changes. It’s not always a perfect sentence. Deciding where one utterance ends and another begins is a fundamental first step.
• 2. Accurate Morpheme Counting: This is the most complex factor. You must apply Brown’s 14 morpheme acquisition order and rules consistently. For instance, regular past tense “-ed” is one morpheme, but an irregular past tense verb like “went” is also counted as one. Possessives (“mommy’s”) and plurals (“dogs”) count as two.
• 3. Handling Dysfluencies and Fillers: Stutters (e.g., “b-b-ball”) and fillers (e.g., “um,” “uh”) should not be counted. Only the complete, intended word (“ball”) is included in the morpheme count.
• 4. Excluding Imitations: If the child immediately repeats something an adult just said, this utterance is typically excluded from the sample because it may not reflect the child’s independent language ability.
• 5. Compound Words and Proper Nouns: Compound words (e.g., “birthday,” “railroad”) and proper names (e.g., “Big Bird”) are generally counted as a single morpheme until the child demonstrates they understand the individual parts.
• 6. Sample Size and Context: A sample should be large enough (at least 50 utterances) to be representative. The context should also be naturalistic (e.g., play) to elicit the child’s typical language use. A sample taken during a structured test may not be as accurate.

Frequently Asked Questions (FAQ)

1. What is a morpheme?

A morpheme is the smallest unit of language that carries meaning. It can be a whole word (like “cat”) or a part of a word (like the plural “-s” in “cats”).

2. Why is MLU a better measure than age?

MLU is a better index of grammatical complexity because children develop at different rates. Two children of the same age can have very different language abilities, and MLU captures this difference more effectively than age alone.

3. What are Brown’s 14 morphemes?

They are a set of 14 grammatical morphemes that English-speaking children acquire in a predictable order. The list includes the present progressive “-ing”, prepositions “in” and “on”, plural “-s”, possessive “-‘s”, and various verb forms.

4. What is a “good” MLU for a 3-year-old?

A typical 3-year-old (around 36 months) would have an MLU between 3.0 and 3.75, corresponding to Brown’s Stage IV.

5. What does it mean if my child’s MLU is low?

A low MLU for a child’s age can be an indicator of a language delay or impairment. However, MLU is just one data point and should not be used as the sole diagnostic tool. A full assessment by a speech-language pathologist is necessary.

6. How are contractions like “he’s” counted?

According to Brown’s rules, contractions of the verb “to be” like “he’s” or “it’s” are counted as two morphemes (e.g., “he” + “‘s”), as they represent two distinct units of meaning (the pronoun and the verb).

7. When does MLU stop being a useful measure?

MLU is most useful for children with an MLU up to about 4.5. Beyond this point, language growth becomes more about sentence complexity (e.g., using clauses) rather than just length, so other measures become more informative.

8. Can I use this for languages other than English?

While the concept of MLU is used in other languages, the specific rules for counting morphemes (especially grammatical ones) are language-specific. This calculator is designed based on Brown’s rules for English.

Related Tools and Internal Resources

Explore more of our tools and resources for parents and professionals:

• Child Development Milestone Tracker: See how language fits into overall development.
• Language Sample Analysis Guide: A deeper dive into collecting and analyzing language samples.
• Parent’s Guide to Speech Therapy: Understand the process and how to support your child.
• Phonological Processes Calculator: Analyze common error patterns in child speech.
• Articulation Development Chart: Learn which sounds to expect at which ages.
• Fun Activities for Language Development: Engaging ideas to encourage speech and language growth at home.