A/B Testing API Cost Formula
Learn how to estimate an A/B test's monthly variable API cost from traffic, allocation, calls per visitor, and price per million calls.
This calculation estimates the API usage cost attributable to one A/B test during a billing month. It turns test traffic into visitor volume, estimates the resulting API calls, and applies your provider's effective usage price.
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Estimated Monthly API Cost
Where:
First calculate how many visitors enter the test each day. Multiply by the billing days and calls per visitor, then multiply the total millions of calls by the provider's price.
Variables Explained
| Variable | What It Means | Unit |
|---|---|---|
| dailyVisitors - Average daily visitors | Average number of visitors or users evaluated each day before applying the test allocation. | visitors |
| testTrafficPercent - Traffic included in test | Percentage of total daily visitors exposed to the experiment. | percent |
| daysPerMonth - Days in billing month | Number of days included in the selected billing period. | days |
| apiCallsPerVisitor - API calls per test visitor | Average number of billable experiment-related API calls generated by each visitor in the test. | number |
| costPerMillionCalls - API cost per million calls | Effective provider price for one million API calls in the chosen currency. | currency |
Step-by-Step Calculation
Calculate daily test visitors
Apply the experiment's traffic allocation to average daily visitor volume.
dailyTestVisitors = dailyVisitors * (testTrafficPercent / 100)
Estimate monthly test visitors
Multiply daily test visitors by the number of billing days.
monthlyTestVisitors = dailyTestVisitors * daysPerMonth
Estimate monthly API calls
Apply the average calls generated by each test visitor.
monthlyApiCalls = monthlyTestVisitors * apiCallsPerVisitor
Convert calls to millions
Provider usage pricing is entered per one million calls, so total calls are converted to millions.
monthlyCallMillions = monthlyApiCalls / 1000000
Calculate monthly API cost
Multiply estimated call volume in millions by the stated effective price.
monthlyApiCost = monthlyCallMillions * costPerMillionCalls
Calculate the daily cost average
Divide the monthly estimate by billing days for an average daily cost.
dailyApiCost = monthlyApiCost / daysPerMonth
Example: Full-traffic test with 10,000 daily visitors
Daily test visitors
10,000 × (100 ÷ 100)
10,000 visitors per day
Monthly test visitors
10,000 × 30
300,000 visitors
Monthly API calls
300,000 × 2
600,000 calls
Calls in millions
600,000 ÷ 1,000,000
0.6 million calls
Monthly API cost
0.6 × $5.00
$3.00
Average daily API cost
$3.00 ÷ 30
$0.10 per day
Final Result
Estimated monthly API cost: $3.00 for 600,000 API calls.
Assumptions
- ✓Traffic is spread evenly across the selected billing days.
- ✓Each visitor included in the test generates the entered average number of API calls.
- ✓The cost per million calls remains constant throughout the month.
- ✓The calculation measures variable API usage charges only.
- ✓All calls used in the calculation are billable under the provider's plan.
Limitations
- !Actual traffic can vary significantly by day, campaign, season, or channel.
- !Provider billing may include free quotas, minimum fees, volume tiers, rounding, or overage rules not represented here.
- !Retries, bot traffic, cache misses, multiple sessions, and event tracking can increase call volume.
- !Fixed subscriptions, implementation costs, taxes, and other platform charges are excluded.
- !A visitor may generate a different number of calls depending on device, journey, or experiment design.
Common Mistakes to Avoid
Using total site traffic when only a smaller share is evaluated by the experiment.
Counting only assignment calls while omitting feature-flag, configuration, exposure, or event calls.
Entering a price per thousand calls instead of a price per million calls.
Forgetting to update the number of billing days for a 28-, 29-, or 31-day period.
Assuming free included calls or tiered provider rates are already reflected in a list price.
Treating the estimate as the full experimentation platform bill rather than a variable usage estimate.
Related Formulas
Frequently Asked Questions
How do you calculate monthly A/B testing API cost?
Multiply daily visitors by the test traffic percentage, billing days, and API calls per test visitor. Divide by one million, then multiply by the price per million calls.
Should test traffic be 100% for an A/B test?
Use 100% only if every visitor is evaluated by the test. Use the actual allocated share when the experiment runs on a subset of traffic.
What API calls should be included per test visitor?
Include billable calls such as assignment, flag evaluation, configuration retrieval, exposure logging, and conversion-event calls when they are charged by your provider.
Does this formula include an API provider's free allowance?
No. The formula estimates raw variable usage cost from the price you enter. Adjust the calls or effective price separately if included usage applies.
Why is the estimated daily cost useful?
It provides a normalized daily view that can help compare experiments with different billing periods or track cost as traffic changes.
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