How to count tokens with Tiktoken

Dec 16, 2022
Open in Github

tiktoken is a fast open-source tokenizer by OpenAI.

Given a text string (e.g., "tiktoken is great!") and an encoding (e.g., "cl100k_base"), a tokenizer can split the text string into a list of tokens (e.g., ["t", "ik", "token", " is", " great", "!"]).

Splitting text strings into tokens is useful because GPT models see text in the form of tokens. Knowing how many tokens are in a text string can tell you (a) whether the string is too long for a text model to process and (b) how much an OpenAI API call costs (as usage is priced by token).

Encodings

Encodings specify how text is converted into tokens. Different models use different encodings.

tiktoken supports three encodings used by OpenAI models:

Encoding nameOpenAI models
cl100k_basegpt-4, gpt-3.5-turbo, text-embedding-ada-002, text-embedding-3-small, text-embedding-3-large
p50k_baseCodex models, text-davinci-002, text-davinci-003
r50k_base (or gpt2)GPT-3 models like davinci

You can retrieve the encoding for a model using tiktoken.encoding_for_model() as follows:

encoding = tiktoken.encoding_for_model('gpt-3.5-turbo')

Note that p50k_base overlaps substantially with r50k_base, and for non-code applications, they will usually give the same tokens.

Tokenizer libraries by language

For cl100k_base and p50k_base encodings:

For r50k_base (gpt2) encodings, tokenizers are available in many languages.

(OpenAI makes no endorsements or guarantees of third-party libraries.)

How strings are typically tokenized

In English, tokens commonly range in length from one character to one word (e.g., "t" or " great"), though in some languages tokens can be shorter than one character or longer than one word. Spaces are usually grouped with the starts of words (e.g., " is" instead of "is " or " "+"is"). You can quickly check how a string is tokenized at the OpenAI Tokenizer, or the third-party Tiktokenizer webapp.

%pip install --upgrade tiktoken
%pip install --upgrade openai
import tiktoken

2. Load an encoding

Use tiktoken.get_encoding() to load an encoding by name.

The first time this runs, it will require an internet connection to download. Later runs won't need an internet connection.

encoding = tiktoken.get_encoding("cl100k_base")

Use tiktoken.encoding_for_model() to automatically load the correct encoding for a given model name.

encoding = tiktoken.encoding_for_model("gpt-3.5-turbo")

The .encode() method converts a text string into a list of token integers.

encoding.encode("tiktoken is great!")

[83, 1609, 5963, 374, 2294, 0]

Count tokens by counting the length of the list returned by .encode().

def num_tokens_from_string(string: str, encoding_name: str) -> int:
    """Returns the number of tokens in a text string."""
    encoding = tiktoken.get_encoding(encoding_name)
    num_tokens = len(encoding.encode(string))
    return num_tokens

num_tokens_from_string("tiktoken is great!", "cl100k_base")

6

.decode() converts a list of token integers to a string.

encoding.decode([83, 1609, 5963, 374, 2294, 0])

'tiktoken is great!'

Warning: although .decode() can be applied to single tokens, beware that it can be lossy for tokens that aren't on utf-8 boundaries.

For single tokens, .decode_single_token_bytes() safely converts a single integer token to the bytes it represents.

[encoding.decode_single_token_bytes(token) for token in [83, 1609, 5963, 374, 2294, 0]]

[b't', b'ik', b'token', b' is', b' great', b'!']

(The b in front of the strings indicates that the strings are byte strings.)

5. Comparing encodings

Different encodings vary in how they split words, group spaces, and handle non-English characters. Using the methods above, we can compare different encodings on a few example strings.

def compare_encodings(example_string: str) -> None:
    """Prints a comparison of three string encodings."""
    # print the example string
    print(f'\nExample string: "{example_string}"')
    # for each encoding, print the # of tokens, the token integers, and the token bytes
    for encoding_name in ["r50k_base", "p50k_base", "cl100k_base"]:
        encoding = tiktoken.get_encoding(encoding_name)
        token_integers = encoding.encode(example_string)
        num_tokens = len(token_integers)
        token_bytes = [encoding.decode_single_token_bytes(token) for token in token_integers]
        print()
        print(f"{encoding_name}: {num_tokens} tokens")
        print(f"token integers: {token_integers}")
        print(f"token bytes: {token_bytes}")
        
compare_encodings("antidisestablishmentarianism")

Example string: "antidisestablishmentarianism"

r50k_base: 5 tokens
token integers: [415, 29207, 44390, 3699, 1042]
token bytes: [b'ant', b'idis', b'establishment', b'arian', b'ism']

p50k_base: 5 tokens
token integers: [415, 29207, 44390, 3699, 1042]
token bytes: [b'ant', b'idis', b'establishment', b'arian', b'ism']

cl100k_base: 6 tokens
token integers: [519, 85342, 34500, 479, 8997, 2191]
token bytes: [b'ant', b'idis', b'establish', b'ment', b'arian', b'ism']

compare_encodings("2 + 2 = 4")

Example string: "2 + 2 = 4"

r50k_base: 5 tokens
token integers: [17, 1343, 362, 796, 604]
token bytes: [b'2', b' +', b' 2', b' =', b' 4']

p50k_base: 5 tokens
token integers: [17, 1343, 362, 796, 604]
token bytes: [b'2', b' +', b' 2', b' =', b' 4']

cl100k_base: 7 tokens
token integers: [17, 489, 220, 17, 284, 220, 19]
token bytes: [b'2', b' +', b' ', b'2', b' =', b' ', b'4']

compare_encodings("お誕生日おめでとう")

Example string: "お誕生日おめでとう"

r50k_base: 14 tokens
token integers: [2515, 232, 45739, 243, 37955, 33768, 98, 2515, 232, 1792, 223, 30640, 30201, 29557]
token bytes: [b'\xe3\x81', b'\x8a', b'\xe8\xaa', b'\x95', b'\xe7\x94\x9f', b'\xe6\x97', b'\xa5', b'\xe3\x81', b'\x8a', b'\xe3\x82', b'\x81', b'\xe3\x81\xa7', b'\xe3\x81\xa8', b'\xe3\x81\x86']

p50k_base: 14 tokens
token integers: [2515, 232, 45739, 243, 37955, 33768, 98, 2515, 232, 1792, 223, 30640, 30201, 29557]
token bytes: [b'\xe3\x81', b'\x8a', b'\xe8\xaa', b'\x95', b'\xe7\x94\x9f', b'\xe6\x97', b'\xa5', b'\xe3\x81', b'\x8a', b'\xe3\x82', b'\x81', b'\xe3\x81\xa7', b'\xe3\x81\xa8', b'\xe3\x81\x86']

cl100k_base: 9 tokens
token integers: [33334, 45918, 243, 21990, 9080, 33334, 62004, 16556, 78699]
token bytes: [b'\xe3\x81\x8a', b'\xe8\xaa', b'\x95', b'\xe7\x94\x9f', b'\xe6\x97\xa5', b'\xe3\x81\x8a', b'\xe3\x82\x81', b'\xe3\x81\xa7', b'\xe3\x81\xa8\xe3\x81\x86']

6. Counting tokens for chat completions API calls

ChatGPT models like gpt-3.5-turbo and gpt-4 use tokens in the same way as older completions models, but because of their message-based formatting, it's more difficult to count how many tokens will be used by a conversation.

Below is an example function for counting tokens for messages passed to gpt-3.5-turbo or gpt-4.

Note that the exact way that tokens are counted from messages may change from model to model. Consider the counts from the function below an estimate, not a timeless guarantee.

In particular, requests that use the optional functions input will consume extra tokens on top of the estimates calculated below.

def num_tokens_from_messages(messages, model="gpt-3.5-turbo-0613"):
    """Return the number of tokens used by a list of messages."""
    try:
        encoding = tiktoken.encoding_for_model(model)
    except KeyError:
        print("Warning: model not found. Using cl100k_base encoding.")
        encoding = tiktoken.get_encoding("cl100k_base")
    if model in {
        "gpt-3.5-turbo-0613",
        "gpt-3.5-turbo-16k-0613",
        "gpt-4-0314",
        "gpt-4-32k-0314",
        "gpt-4-0613",
        "gpt-4-32k-0613",
        }:
        tokens_per_message = 3
        tokens_per_name = 1
    elif model == "gpt-3.5-turbo-0301":
        tokens_per_message = 4  # every message follows <|start|>{role/name}\n{content}<|end|>\n
        tokens_per_name = -1  # if there's a name, the role is omitted
    elif "gpt-3.5-turbo" in model:
        print("Warning: gpt-3.5-turbo may update over time. Returning num tokens assuming gpt-3.5-turbo-0613.")
        return num_tokens_from_messages(messages, model="gpt-3.5-turbo-0613")
    elif "gpt-4" in model:
        print("Warning: gpt-4 may update over time. Returning num tokens assuming gpt-4-0613.")
        return num_tokens_from_messages(messages, model="gpt-4-0613")
    else:
        raise NotImplementedError(
            f"""num_tokens_from_messages() is not implemented for model {model}. See https://github.com/openai/openai-python/blob/main/chatml.md for information on how messages are converted to tokens."""
        )
    num_tokens = 0
    for message in messages:
        num_tokens += tokens_per_message
        for key, value in message.items():
            num_tokens += len(encoding.encode(value))
            if key == "name":
                num_tokens += tokens_per_name
    num_tokens += 3  # every reply is primed with <|start|>assistant<|message|>
    return num_tokens

# let's verify the function above matches the OpenAI API response

from openai import OpenAI
import os

client = OpenAI(api_key=os.environ.get("OPENAI_API_KEY", "<your OpenAI API key if not set as env var>"))

example_messages = [
    {
        "role": "system",
        "content": "You are a helpful, pattern-following assistant that translates corporate jargon into plain English.",
    },
    {
        "role": "system",
        "name": "example_user",
        "content": "New synergies will help drive top-line growth.",
    },
    {
        "role": "system",
        "name": "example_assistant",
        "content": "Things working well together will increase revenue.",
    },
    {
        "role": "system",
        "name": "example_user",
        "content": "Let's circle back when we have more bandwidth to touch base on opportunities for increased leverage.",
    },
    {
        "role": "system",
        "name": "example_assistant",
        "content": "Let's talk later when we're less busy about how to do better.",
    },
    {
        "role": "user",
        "content": "This late pivot means we don't have time to boil the ocean for the client deliverable.",
    },
]

for model in [
    "gpt-3.5-turbo-0301",
    "gpt-3.5-turbo-0613",
    "gpt-3.5-turbo",
    "gpt-4-0314",
    "gpt-4-0613",
    "gpt-4",
    ]:
    print(model)
    # example token count from the function defined above
    print(f"{num_tokens_from_messages(example_messages, model)} prompt tokens counted by num_tokens_from_messages().")
    # example token count from the OpenAI API
    response = client.chat.completions.create(model=model,
    messages=example_messages,
    temperature=0,
    max_tokens=1)
    print(f'{response.usage.prompt_tokens} prompt tokens counted by the OpenAI API.')
    print()

gpt-3.5-turbo-0301
127 prompt tokens counted by num_tokens_from_messages().
127 prompt tokens counted by the OpenAI API.

gpt-3.5-turbo-0613
129 prompt tokens counted by num_tokens_from_messages().
129 prompt tokens counted by the OpenAI API.

gpt-3.5-turbo
Warning: gpt-3.5-turbo may update over time. Returning num tokens assuming gpt-3.5-turbo-0613.
129 prompt tokens counted by num_tokens_from_messages().
129 prompt tokens counted by the OpenAI API.

gpt-4-0314
129 prompt tokens counted by num_tokens_from_messages().
129 prompt tokens counted by the OpenAI API.

gpt-4-0613
129 prompt tokens counted by num_tokens_from_messages().
129 prompt tokens counted by the OpenAI API.

gpt-4
Warning: gpt-4 may update over time. Returning num tokens assuming gpt-4-0613.
129 prompt tokens counted by num_tokens_from_messages().
129 prompt tokens counted by the OpenAI API.