A/B Testing Token Usage Formula
Learn how to estimate total token usage, API cost, and the incremental token impact of an A/B test variant.
This calculation estimates token consumption across control and variant traffic, then compares the observed test total with a control-only baseline. It helps quantify whether the variant increases or reduces token-related API usage at the test volume.
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Total Test Token Usage
Where:
Calculate requests and tokens for each group separately, then add the two token totals together.
Variables Explained
| Variable | What It Means | Unit |
|---|---|---|
| controlUsers - Control group users | Number of users assigned to the control group. | users |
| variantUsers - Variant group users | Number of users assigned to the variant group. | users |
| requestsPerUser - Average requests per user | Average token-consuming requests made by each user during the test. | requests |
| controlTokensPerRequest - Control tokens per request | Average total tokens used by one control request. | tokens |
| variantTokensPerRequest - Variant tokens per request | Average total tokens used by one variant request. | tokens |
| costPerMillionTokens - Cost per 1 million tokens | Blended token price in the selected currency for one million tokens. | currency |
Step-by-Step Calculation
Estimate control requests
Multiply control users by their average requests to estimate control traffic.
controlRequests = controlUsers * requestsPerUser
Estimate variant requests
Multiply variant users by their average requests to estimate variant traffic.
variantRequests = variantUsers * requestsPerUser
Calculate each group's tokens
Apply the relevant average tokens per request to each group's requests.
controlTokenUsage = controlRequests * controlTokensPerRequest; variantTokenUsage = variantRequests * variantTokensPerRequest
Combine token usage
Add control and variant token usage to obtain total test consumption.
totalTokenUsage = controlTokenUsage + variantTokenUsage
Calculate the control-only baseline
This estimates usage if every observed request had used the control configuration.
controlOnlyBaselineTokens = (controlRequests + variantRequests) * controlTokensPerRequest
Find incremental variant impact
A positive result means the variant used more tokens than the control baseline; a negative result means it used fewer.
incrementalTokenUsage = totalTokenUsage - controlOnlyBaselineTokens; incrementalCost = incrementalTokenUsage / 1000000 * costPerMillionTokens
Equal split test with a higher-token variant
Control requests
5,000 × 3
15,000 requests
Variant requests
5,000 × 3
15,000 requests
Control token usage
15,000 × 1,200
18,000,000 tokens
Variant token usage
15,000 × 1,500
22,500,000 tokens
Total test usage and cost
(18,000,000 + 22,500,000) / 1,000,000 × 5
40,500,000 tokens; $202.50
Incremental variant usage and cost
40,500,000 - (30,000 × 1,200); 4,500,000 / 1,000,000 × 5
4,500,000 tokens; $22.50
Final Result
The test is estimated to consume 40,500,000 tokens at a token cost of $202.50. The variant adds an estimated 4,500,000 tokens and $22.50 versus the control-only baseline.
Assumptions
- ✓Each group has the entered average number of token-consuming requests per user.
- ✓The average token totals per request represent the traffic observed or expected during the test.
- ✓The entered price is a blended rate appropriate for the mix of input and output tokens.
- ✓Variant-group requests are compared with the assumption that they would otherwise have used the control configuration.
Limitations
- !Actual token counts can vary by prompt length, response length, retrieved context, model routing, and user behavior.
- !A single blended token price may not exactly match providers that bill input and output tokens at different rates.
- !The calculation excludes non-token charges, such as platform, storage, caching, tool calls, or other fees, unless included in the entered rate.
- !Token cost is only one experiment measure and does not show whether a variant performs better for users or the business.
Common Mistakes to Avoid
Entering prompt tokens only while excluding billed output tokens.
Using planned traffic with token averages taken from an unrepresentative small sample.
Using a price per thousand tokens in a field that expects a price per one million tokens.
Comparing total test cost with incremental variant cost as though they are the same measure.
Assuming equal group sizes when the observed control and variant populations differ.
Related Formulas
Frequently Asked Questions
How is total token usage calculated for an A/B test?
The calculator multiplies each group's users by requests per user and tokens per request, then adds the control and variant token totals.
What does incremental variant token usage mean?
It is actual test token use minus the estimated use if all observed requests had used the control token rate. A negative figure indicates a reduction.
How do I calculate A/B test token cost?
Divide total token usage by 1,000,000 and multiply by your blended cost per million tokens.
Should token usage include input and output tokens?
Yes. Use the average total billed tokens per request whenever possible. If rates differ by token type, use a representative blended price.
Can this formula handle unequal A/B test groups?
Yes. Control and variant user counts are calculated separately, so each group can have a different size.
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