A/B Testing Token Usage Formula
Learn how to estimate monthly input tokens, output tokens, total token volume, and token-based API cost for an A/B test.
This calculation estimates the AI token volume and token-based API spend generated by an A/B test over one month. It helps teams translate expected participant traffic, request frequency, prompt size, response size, and token prices into a practical experiment budget estimate.
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Estimated Monthly API Cost
Where:
Estimate monthly requests from participants and requests per participant. Calculate input and output tokens separately, apply the relevant price per million tokens to each, then add both costs.
Variables Explained
| Variable | What It Means | Unit |
|---|---|---|
| monthlyParticipants - Monthly test participants | The expected number of users or sessions exposed to the experiment during the month. | number |
| requestsPerParticipant - Requests per participant | The average number of AI requests made by each participant during the test. | number |
| inputTokensPerRequest - Average input tokens per request | Average tokens sent to the model, including prompts, instructions, context, and conversation history. | tokens |
| outputTokensPerRequest - Average output tokens per request | Average tokens generated by the model for each request. | tokens |
| inputPricePerMillion - Input price per 1 million tokens | The entered billing price for one million input tokens. | currency |
| outputPricePerMillion - Output price per 1 million tokens | The entered billing price for one million output tokens. | currency |
| numberOfVariants - Number of variants | The total number of versions in the test, including the control. | number |
Step-by-Step Calculation
Estimate monthly requests
Multiply expected monthly participants by their average request frequency.
monthlyRequests = monthlyParticipants * requestsPerParticipant
Calculate monthly input tokens
Multiply total requests by the average prompt and context tokens sent with each request.
monthlyInputTokens = monthlyRequests * inputTokensPerRequest
Calculate monthly output tokens
Multiply total requests by the average generated response tokens per request.
monthlyOutputTokens = monthlyRequests * outputTokensPerRequest
Calculate total token usage
Add input and output tokens to find the combined monthly token volume.
monthlyTotalTokens = monthlyInputTokens + monthlyOutputTokens
Calculate input token cost
Convert input tokens into millions, then multiply by the entered input-token price.
monthlyInputCost = (monthlyInputTokens / 1000000) * inputPricePerMillion
Calculate output token cost
Convert output tokens into millions, then multiply by the entered output-token price.
monthlyOutputCost = (monthlyOutputTokens / 1000000) * outputPricePerMillion
Calculate total cost and average tokens per variant
Add the two token charges. The per-variant figure divides total tokens evenly across variants for planning purposes.
monthlyEstimatedCost = monthlyInputCost + monthlyOutputCost; tokensPerVariant = monthlyTotalTokens / numberOfVariants
Example: Two-variant test with 10,000 monthly participants
Monthly requests
10,000 × 3
30,000 requests
Input tokens
30,000 × 800
24,000,000 tokens
Output tokens
30,000 × 400
12,000,000 tokens
Total token usage
24,000,000 + 12,000,000
36,000,000 tokens
Input cost
24,000,000 ÷ 1,000,000 × $3.00
$72.00
Output cost
12,000,000 ÷ 1,000,000 × $15.00
$180.00
Total cost and per-variant usage
$72.00 + $180.00; 36,000,000 ÷ 2
$252.00 and 18,000,000 tokens per variant
Final Result
The test is estimated to use 36,000,000 tokens per month and incur $252.00 in token-based API charges at the entered prices.
Assumptions
- ✓Each participant makes the stated average number of requests during the month.
- ✓Input and output token counts are representative averages across test traffic.
- ✓Each request is assigned to one variant rather than duplicated across all variants.
- ✓Variants receive an even traffic split when estimating tokens per variant.
- ✓The estimate includes only the entered input and output token prices.
Limitations
- !Actual requests can differ from forecasts because of traffic changes, abandonment, or repeat usage.
- !Token counts may vary materially with prompt length, conversation history, retrieval context, and response length.
- !Provider billing may use pricing tiers, cached-token rules, rounding, or model-specific accounting not represented here.
- !Retries, tool calls, storage, batch features, fixed fees, taxes, and other charges are not included.
Common Mistakes to Avoid
Multiplying total traffic by the number of variants even though each participant sees only one assigned variant.
Entering only the user prompt length and omitting system prompts, retrieved content, or conversation history.
Using a single blended token price when input and output prices are different.
Treating average tokens per variant as an actual measured allocation when traffic is uneven.
Ignoring retries and fallback requests when setting an operational budget.
Related Formulas
Frequently Asked Questions
How is monthly token usage for an A/B test calculated?
Monthly requests equal participants multiplied by requests per participant. Multiply that request total by average input and output tokens, then add the two results.
Do more A/B test variants automatically use more tokens?
No. With fixed overall traffic and one assigned variant per participant, variants redistribute traffic rather than multiply requests. Extra usage occurs if requests are duplicated for evaluation or other workflows.
How do I calculate input and output token costs separately?
Divide each monthly token total by 1,000,000 and multiply it by its respective price per million tokens. Add the two costs for the total estimate.
What counts as input tokens in this formula?
Input tokens can include system instructions, user messages, prior conversation turns, retrieved documents, tool context, and other text sent to the model.
Why can output tokens account for most of the estimated cost?
Output tokens may have a higher entered price per million than input tokens. Long generated responses can therefore have a large cost effect.
What does average tokens per variant mean?
It is total estimated monthly tokens divided by the number of variants. It is a planning estimate that assumes traffic is split evenly.
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