A/B Testing File Size Per-User Formula
Learn how to estimate compressed event data per participant, total experiment file size, and average file size per A/B test variant.
This calculation estimates the event-data volume created by an A/B test. It starts with each participant’s expected event activity, applies a compression reduction, then scales the result by participant count and divides it across variants for an even-split planning estimate.
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Compressed Data per User
Where:
Multiply the average number of events per participant by the average uncompressed event size, then keep the portion of data remaining after compression.
Variables Explained
| Variable | What It Means | Unit |
|---|---|---|
| averageEventsPerUser - Average events per user | The estimated number of tracked event records generated by one participant. | number |
| averageEventSizeKb - Average event size | The average uncompressed size of one event record, including its properties and metadata. | KB |
| compressionRate - Compression reduction | The estimated percentage by which compression reduces the raw event data. | percent |
| participants - Test participants | The number of unique users expected to participate in the experiment. | number |
| variants - Number of variants | The total number of experiment groups, including the control. | number |
Step-by-Step Calculation
Calculate the compression factor
Convert the compression reduction into the share of raw data that remains after compression.
compressionFactor = 1 - compressionRate / 100
Calculate raw data per user
Estimate one participant’s event-data volume before compression.
rawDataPerUserKb = averageEventsPerUser * averageEventSizeKb
Calculate compressed data per user
Apply the compression factor to the raw per-user amount.
compressedDataPerUserKb = rawDataPerUserKb * compressionFactor
Calculate the total experiment file size
Multiply compressed per-user data by all participants and convert KB to MB.
totalFileSizeMb = compressedDataPerUserKb * participants / 1024
Calculate the average size per variant
Divide total estimated data by the number of variants, assuming participants are distributed evenly.
averageFileSizePerVariantMb = totalFileSizeMb / variants
Example: 10,000 participants in a two-variant experiment
Compression factor
1 - 40 / 100
0.60
Raw data per user
12 * 1.5 KB
18.00 KB
Compressed data per user
18.00 KB * 0.60
10.80 KB
Total experiment file size
10.80 KB * 10,000 / 1024
105.47 MB
Average file size per variant
105.47 MB / 2
52.73 MB
Final Result
Estimated compressed event data is 10.80 KB per participant, 105.47 MB in total, and 52.73 MB per variant with even allocation.
Assumptions
- ✓Each participant generates the same average number of tracked events.
- ✓Average event size includes the stored properties and metadata relevant to each event.
- ✓The selected compression reduction applies consistently across event data.
- ✓Participants are distributed evenly across variants for the per-variant average.
- ✓KB-to-MB conversion uses 1 MB = 1024 KB.
Limitations
- !Actual event counts can differ substantially between participant segments and variants.
- !Storage, export, and download sizes can include headers, indexes, partitions, and other overhead not modeled here.
- !Compression effectiveness depends on the file format, repeated values, field names, and event payload structure.
- !Uneven traffic allocation means individual variant files may differ from the average.
Common Mistakes to Avoid
Entering only treatment groups and excluding the control from the number of variants.
Using a compressed event size while also entering a compression reduction.
Estimating event size without including identifiers, timestamps, properties, and metadata.
Confusing the number of events per user with the total number of events in the test.
Treating the average per-variant result as the expected size of every individual variant when allocation is uneven.
Related Formulas
Frequently Asked Questions
How do you calculate A/B test file size per user?
Multiply average events per user by average event size, then multiply by one minus the compression reduction expressed as a decimal.
How is total experiment file size calculated?
Multiply compressed data per user by the participant count, then divide by 1024 to convert the result from KB to MB.
Does the number of variants affect total file size?
Not in this model. It affects only the average file size per variant because the same total participant data is split among more groups.
Should the control count as a variant?
Yes. A standard A/B test has two variants: one control and one treatment.
What does a 40% compression reduction mean?
It means the estimate assumes compression removes 40% of raw data, leaving 60% of the original volume.
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