A/B Testing File Size (Monthly) Formula
Learn how to estimate the monthly data-transfer change caused by serving an A/B test variant with a different page file size.
This calculation estimates the monthly bandwidth difference between running an A/B test and serving the control page to every tested pageview. It helps quantify whether the variant reduces or increases transferred data while the experiment is live.
- 100% Free
- No Sign-Up Required
- Private & Secure
- Mobile Friendly
Monthly Bandwidth Difference
Where:
Estimate tested pageviews, find the share sent to the variant, then multiply it by the difference between variant and control file sizes. Divide by 1,000,000 to convert kilobytes to gigabytes.
Variables Explained
| Variable | What It Means | Unit |
|---|---|---|
| V - monthlyVisitors | Expected number of unique visitors who generate views of the tested page during the month. | visitors |
| P - pageviewsPerVisitor | Average number of tested-page views generated by each visitor. | pageviews |
| S - variantTrafficShare | Percentage of tested pageviews allocated to the variant. | percent |
| Fc - controlFileSize | Average transferred size of the control experience per tested pageview. | KB per pageview |
| Fv - variantFileSize | Average transferred size of the variant experience per tested pageview. | KB per pageview |
Step-by-Step Calculation
Estimate monthly tested pageviews
Multiply expected visitors by average tested-page views per visitor.
monthlyPageviews = monthlyVisitors * pageviewsPerVisitor
Allocate pageviews to the variant
Apply the variant traffic percentage to total tested pageviews.
variantPageviews = monthlyPageviews * (variantTrafficShare / 100)
Allocate pageviews to the control
The remaining tested pageviews are assigned to the control.
controlPageviews = monthlyPageviews - variantPageviews
Calculate control transfer
Multiply control pageviews by the control file size and convert KB to GB.
controlTransferGb = controlPageviews * controlFileSize / 1000000
Calculate variant transfer
Multiply variant pageviews by the variant file size and convert KB to GB.
variantTransferGb = variantPageviews * variantFileSize / 1000000
Compare the test with the all-control baseline
Subtract the transfer required by an all-control experience from total test-period transfer. A negative result represents estimated data savings.
monthlyBandwidthDifferenceGb = (controlTransferGb + variantTransferGb) - (monthlyPageviews * controlFileSize / 1000000)
Example: 50/50 test of a lighter page variant
Monthly tested pageviews
100,000 × 2
200,000 pageviews
Variant pageviews
200,000 × 50%
100,000 pageviews
Control transfer
100,000 × 1,500 ÷ 1,000,000
150 GB
Variant transfer
100,000 × 1,200 ÷ 1,000,000
120 GB
A/B test transfer
150 + 120
270 GB/month
Difference from all-control baseline
270 − (200,000 × 1,500 ÷ 1,000,000)
-30 GB/month
Final Result
The A/B test is estimated to transfer 270 GB per month, which is 30 GB less than the 300 GB all-control baseline. That is a 10% reduction.
Assumptions
- ✓File-size inputs represent the average number of kilobytes transferred for each tested pageview.
- ✓One GB is treated as 1,000,000 KB for this bandwidth estimate.
- ✓The selected variant traffic share remains stable over the month.
- ✓Each tested pageview is assumed to receive either the control or variant experience.
- ✓The comparison baseline is that all tested pageviews use the control file size.
Limitations
- !Browser caching and repeat visits can reduce actual asset transfers compared with a per-pageview estimate.
- !Device, connection, geography, compression, and personalization can cause file sizes to vary.
- !Lazy-loaded assets may not be transferred on every pageview.
- !The result estimates data volume only; it does not estimate CDN, hosting, or bandwidth charges.
- !Bot traffic and changes in monthly traffic can make actual totals differ from the estimate.
Common Mistakes to Avoid
Entering the total site pageviews instead of pageviews for the page included in the experiment.
Using an encoded or uncompressed asset size rather than the average transferred page weight.
Entering 0.5 for a 50% split when the field expects 50.
Excluding images, scripts, fonts, or other assets that are normally downloaded with the tested experience.
Interpreting a negative difference as an error; it indicates lower estimated transfer than the all-control baseline.
Related Formulas
Frequently Asked Questions
How is A/B test bandwidth difference calculated?
It is the variant pageview count multiplied by the difference between variant and control file size, converted from KB to GB. The result is negative when the variant is lighter.
Why does the control traffic cancel out in the main formula?
The test and the all-control baseline both include control-sized transfer for control traffic. The difference comes only from pageviews assigned to the variant.
What does a negative monthly bandwidth difference mean?
It means the A/B test is estimated to transfer less data during the month than serving the control to all tested pageviews.
How do I calculate the percentage bandwidth change?
Divide the monthly bandwidth difference by all-control transfer, then multiply by 100. A negative percentage indicates lower estimated transfer.
Does this formula include cached assets?
Not separately. Use average transferred file sizes that reflect the caching assumptions appropriate for your measurement method.
Ready to calculate your result?
Use the calculator to get instant results with your own inputs.