A/B Testing API Cost Formula
Learn how annual A/B testing API cost is estimated from experiment traffic, API usage, included calls, overage pricing, and platform fees.
The calculator estimates annual API spending by first calculating monthly calls from experimental traffic. It then subtracts the included monthly allowance, prices any overage, adds the recurring platform fee, and projects the result across 12 months.
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Estimated Annual API Cost
Where:
Estimate monthly API calls, charge only for calls above the monthly allowance, add the fixed monthly fee, then multiply the monthly total by 12.
Variables Explained
| Variable | What It Means | Unit |
|---|---|---|
| monthlyVisitors - Monthly visitors evaluated | Average monthly visitors, users, or sessions evaluated by the testing API. | visitors |
| apiCallsPerVisitor - API calls per evaluated visitor | Average number of billable API or decision calls for each evaluated visitor. | calls |
| experimentTrafficPercent - Traffic included in experiments | Share of evaluated traffic actively participating in experiments. | percent |
| activeExperiments - Average active experiments | Average number of experiments assumed to create separate evaluations for an eligible visitor. | number |
| includedCallsMillions - Included API calls per month | Monthly plan allowance measured in millions of calls. | million calls |
| overageCostPerMillion - Overage cost per million calls | Entered price for one million calls above the included monthly allowance. | currency |
| monthlyPlatformFee - Monthly platform fee | Fixed recurring subscription or platform charge before overages. | currency |
Step-by-Step Calculation
Convert experiment traffic to a decimal
A percentage such as 60% becomes 0.60 for use in the usage calculation.
experimentTrafficRate = experimentTrafficPercent / 100
Estimate monthly API calls
This applies API calls per visitor only to experiment traffic and accounts for separate experiment evaluations.
monthlyApiCalls = monthlyVisitors * apiCallsPerVisitor * experimentTrafficRate * activeExperiments
Convert included usage to calls
The plan allowance is entered in millions, so it is converted to individual calls.
includedCalls = includedCallsMillions * 1000000
Find calls above the allowance
There is no negative overage when usage remains below the plan allowance.
monthlyOverageCalls = max(0, monthlyApiCalls - includedCalls)
Price the monthly overage
Excess usage is converted back to millions and multiplied by the entered overage rate.
monthlyOverageCost = (monthlyOverageCalls / 1000000) * overageCostPerMillion
Calculate annual cost
The fixed monthly fee and estimated monthly overage are projected over 12 months.
annualApiCost = (monthlyPlatformFee + monthlyOverageCost) * 12
Worked example: growing experiment program
Experiment traffic rate
60 / 100
0.60
Monthly API calls
500,000 * 2 * 0.60 * 3
1,800,000 calls
Monthly overage calls
max(0, 1,800,000 - 1,000,000)
800,000 calls
Monthly overage cost
0.8 * $25
$20
Monthly API cost
$99 + $20
$119
Annual API cost
$119 * 12
$1,428
Final Result
Estimated annual A/B testing API cost: $1,428, based on 21.6 million API calls per year.
Assumptions
- ✓Monthly traffic, experiment participation, calls per visitor, and active experiment counts stay consistent for 12 months.
- ✓Each active experiment creates a separate API evaluation for eligible traffic.
- ✓Included calls are a monthly allowance and do not roll over.
- ✓The fixed platform fee and overage rate remain unchanged throughout the year.
- ✓Only the entered subscription and overage charges are included.
Limitations
- !Providers may bill decisions, events, users, sessions, requests, or SDK evaluations differently.
- !Some platforms combine multiple experiment decisions into one API call, which can make the active-experiment multiplier unsuitable.
- !Usage can vary by season, campaign, rollout, and experiment schedule.
- !Taxes, annual commitments, discounts, support charges, implementation work, and minimum commitments are not included.
Common Mistakes to Avoid
Using total site visitors when only a smaller segment is evaluated by the API.
Entering 60 instead of 0.60 where a manual calculation requires a decimal experiment traffic rate.
Multiplying by active experiments when the platform batches all decisions into one call.
Treating included calls as an annual pool when the plan resets its allowance every month.
Forgetting SDK calls, server-side evaluations, or repeat decisions in the calls-per-visitor estimate.
Related Formulas
Frequently Asked Questions
How do you calculate annual A/B testing API cost?
Estimate monthly API calls, subtract the monthly included allowance, apply the overage price to excess calls, add the monthly platform fee, and multiply by 12.
Why are active experiments included in the API cost formula?
They account for cases where one eligible visitor receives a separate billable evaluation for each concurrent experiment. Use 1 if the provider combines those decisions.
What happens if API usage is below the included allowance?
Monthly overage is zero, so the estimate is the monthly platform fee multiplied by 12.
Does the formula include taxes or discounts?
No. It uses only the platform fee and overage cost entered in the calculator.
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