Aaron Graves, PhDude (Replica)

Empowering people to reach their full potential

How to Calculate Mean Length of Utterance (MLU)

Posted on February 16, 2026 by Admin

Mean Length of Utterance (MLU) is a crucial metric in language development, offering insights into a child's linguistic complexity. This guide will walk you through what MLU is, how to count morphemes, and how to calculate it effectively, including a handy calculator to assist you.

MLU Calculator

What is Mean Length of Utterance (MLU)?

Mean Length of Utterance (MLU) is a widely used measure in linguistics to assess the linguistic development of children, particularly in the fields of speech-language pathology and developmental psychology. It refers to the average number of morphemes a child produces in an utterance. An utterance is a natural unit of speech bounded by pauses, changes in intonation, or a new topic.

MLU is considered a reliable indicator of a child's grammatical complexity because as children grow, their utterances tend to become longer and more complex, incorporating more morphemes (e.g., plural markers, verb tenses, prepositions).

Understanding Morphemes: The Building Blocks of MLU

Before you can calculate MLU, you must understand what a morpheme is. A morpheme is the smallest unit of meaning in a language. It cannot be broken down into smaller meaningful parts. Morphemes can be:

  • Free Morphemes: These can stand alone as words (e.g., "cat", "run", "happy").
  • Bound Morphemes: These must be attached to a free morpheme (e.g., prefixes like "un-" in "unhappy", suffixes like "-s" in "cats", "-ed" in "walked").

Key Rules for Counting Morphemes (Brown's Stages)

When calculating MLU, consistent application of morpheme counting rules is essential. The most commonly followed rules are based on Roger Brown's work. Here's a simplified guide:

  1. Count each word as one morpheme: Unless specified otherwise below.
    • Example: "The dog ran" = 3 morphemes.
  2. Count grammatical inflections as separate morphemes:
    • Plural -s (e.g., "dogs" = 2 morphemes: dog + s)
    • Possessive -'s (e.g., "Mommy's" = 2 morphemes: Mommy + 's)
    • Third-person singular present -s (e.g., "runs" = 2 morphemes: run + s)
    • Past tense -ed (e.g., "walked" = 2 morphemes: walk + ed)
    • Present progressive -ing (e.g., "running" = 2 morphemes: run + ing)
    • Comparatives -er and superlatives -est (e.g., "bigger" = 2 morphemes: big + er)
  3. Count compound words as one morpheme: Words like "baseball", "toothbrush", "birthday" are typically counted as one morpheme if they function as a single lexical item for the child.
  4. Count proper nouns as one morpheme: "Mickey Mouse", "New York" = 1 morpheme each.
  5. Count irregular past tense verbs and irregular plurals as one morpheme:
    • Example: "went" (go + past) = 1 morpheme, "men" (man + plural) = 1 morpheme.
    • This rule applies because the child learns them as single units, not by adding a bound morpheme.
  6. Count auxiliary verbs and catenatives as one morpheme: "is", "are", "have", "gonna", "wanna", "hafta" (as single units).
    • Example: "I'm going" ("I" + "am" + "go" + "ing") = 4 morphemes.
    • "I wanna go" ("I" + "wanna" + "go") = 3 morphemes.
  7. Count negative contractions as two morphemes: "can't" (can + not), "don't" (do + not), "isn't" (is + not).
    • Example: "He can't come" = 4 morphemes (He + can + not + come).
  8. Count other contractions as one morpheme: "I'm" (I + am), "he's" (he + is/has), "we'll" (we + will).
  9. Do not count disfluencies or fillers: "um", "uh", "like", "you know" (unless clearly intentional and meaningful).
  10. Do not count false starts or repetitions for emphasis: Only count the final, complete, and meaningful part of an utterance.
    • Example: "I want... I want the ball" = 4 morphemes (I + want + the + ball).
    • Example: "No, no, no!" = 3 morphemes (No + No + No).

Step-by-Step Guide to Calculating MLU

To get an accurate MLU, you'll need a language sample from the individual. Here's the process:

  1. Collect a Language Sample: Record spontaneous speech. For children, aim for 50-100 intelligible utterances. Ensure the sample is representative of their typical communication.
  2. Transcribe the Sample: Write down everything said, including pauses, repetitions, and non-verbal cues if relevant for utterance segmentation.
  3. Segment into Utterances: Divide the transcribed text into individual utterances. An utterance is typically defined by a pause, a change in intonation, or a complete thought.
    • Example: "I want milk. // My mommy said no." would be two utterances.
  4. Count Morphemes for Each Utterance: Apply the morpheme counting rules consistently to every utterance in your sample. This is the most labor-intensive part.
  5. Sum Total Morphemes: Add up the total number of morphemes from all utterances in your sample.
  6. Sum Total Utterances: Count the total number of utterances you segmented.
  7. Calculate MLU: Divide the total number of morphemes by the total number of utterances. 
    MLU = (Total Number of Morphemes) / (Total Number of Utterances)

Using the MLU Calculator

The MLU calculator above is designed to streamline the final step of your calculation. Once you have meticulously collected your language sample, transcribed it, segmented it into utterances, and counted the morphemes for each utterance according to the rules:

  1. Enter the Total Number of Morphemes you counted across your entire sample into the first field.
  2. Enter the Total Number of Utterances you identified in your sample into the second field.
  3. Click the "Calculate MLU" button.

The calculator will then display the Mean Length of Utterance for your sample.

Interpreting MLU Scores

MLU typically increases with age in young children. Roger Brown identified five stages of language development based on MLU ranges:

  • Stage I (MLU 1.0-2.0): Ages 12-26 months. Basic word combinations, semantic roles.
  • Stage II (MLU 2.0-2.5): Ages 27-30 months. Emergence of grammatical morphemes (e.g., -ing, plural -s).
  • Stage III (MLU 2.5-3.0): Ages 31-34 months. Simple sentence forms, "in", "on", possessive -'s.
  • Stage IV (MLU 3.0-3.75): Ages 35-40 months. Complex sentence structures, articles, regular past tense -ed.
  • Stage V (MLU 3.75-4.5+): Ages 41-46+ months. More complex sentences, irregular past tense, contractible auxiliary.

Remember that these are averages, and individual development can vary. MLU is best used as one of several measures in a comprehensive language assessment.

Limitations of MLU

While MLU is a valuable tool, it's important to be aware of its limitations:

  • Not a complete picture: MLU primarily measures grammatical complexity but doesn't capture semantic richness, pragmatic skills, or phonological development.
  • Age-dependent: Its utility diminishes after an MLU of around 4.0-4.5, as utterances become longer and more complex in ways not easily captured by simple morpheme counts (e.g., clause embedding).
  • Requires careful transcription: Accuracy heavily relies on the quality and consistency of transcription and morpheme counting.

Conclusion

Calculating MLU is a fundamental skill for anyone involved in assessing language development. By understanding morphemes, applying consistent counting rules, and following a systematic approach to language sample analysis, you can gain valuable insights into an individual's linguistic abilities. Use the calculator provided to simplify the final calculation step, but always remember that the accuracy begins with your meticulous counting of morphemes and utterances.