A/B Testing Video Bitrate Monthly Formula
Learn how monthly video data transfer and CDN delivery cost are estimated for two bitrate variants in an A/B test.
This calculation estimates the video data delivered each month when traffic is split between two bitrate variants. It helps quantify the bandwidth and CDN cost effect of testing a higher- or lower-bitrate stream against a Variant A baseline.
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Total Monthly Test Data Transfer
Where:
Split monthly plays between the two variants, multiply each group by watch time and bitrate, convert megabits to decimal gigabytes, and add both delivery totals.
Variables Explained
| Variable | What It Means | Unit |
|---|---|---|
| P - monthlyPlays | Expected number of video plays during the month. | number |
| W - averageWatchTime | Average minutes watched for each video play. | minutes |
| A - bitrateA | Average delivered bitrate for Variant A. | Mbps |
| B - bitrateB | Average delivered bitrate for Variant B. | Mbps |
| S - variantBShare | Percentage of monthly plays assigned to Variant B. | percent |
| C - cdnCostPerGb | Estimated CDN delivery charge per decimal gigabyte. | currency |
Step-by-Step Calculation
Convert watch time to seconds
Bitrate is measured per second, so the average viewing duration must be converted from minutes to seconds.
averageWatchSeconds = averageWatchTime * 60
Allocate plays to Variant A
Variant A receives the portion of traffic not assigned to Variant B.
variantAPlays = monthlyPlays * (1 - variantBShare / 100)
Allocate plays to Variant B
Variant B receives the selected test share of monthly plays.
variantBPlays = monthlyPlays * (variantBShare / 100)
Calculate Variant A transfer
Multiplying plays, seconds watched, and Mbps gives megabits; dividing by 8 converts to megabytes and dividing by 1000 expresses the estimate in decimal GB.
variantADataTransfer = variantAPlays * averageWatchSeconds * bitrateA / 8 / 1000
Calculate Variant B transfer
The same conversion is applied using Variant B's assigned plays and bitrate.
variantBDataTransfer = variantBPlays * averageWatchSeconds * bitrateB / 8 / 1000
Add test transfer and calculate cost
The two transfer totals are combined and multiplied by the entered CDN price per GB.
monthlyDeliveryCost = (variantADataTransfer + variantBDataTransfer) * cdnCostPerGb
Compare with an all-Variant-A baseline
This shows the estimated cost change relative to serving Variant A for every play.
additionalMonthlyCost = monthlyDeliveryCost - (monthlyPlays * averageWatchSeconds * bitrateA / 8 / 1000 * cdnCostPerGb)
Example: Equal split between 3 Mbps and 5 Mbps video
Convert watch time
5 * 60
300 seconds
Split plays
100,000 * 50%
50,000 plays per variant
Variant A transfer
50,000 * 300 * 3 / 8 / 1000
5,625 GB
Variant B transfer
50,000 * 300 * 5 / 8 / 1000
9,375 GB
Total test transfer
5,625 + 9,375
15,000 GB
Monthly delivery cost
15,000 * 0.08
$1,200
Cost change versus all Variant A
1,200 - (11,250 * 0.08)
$300 more per month
Final Result
The 50/50 bitrate test is estimated to transfer 15,000 GB and cost $1,200 per month, or approximately $300 more than serving the 3 Mbps Variant A to every viewer.
Assumptions
- ✓Each variant's entered bitrate represents its average delivered bitrate across watched seconds.
- ✓Average watch time is the same for Variant A and Variant B.
- ✓One decimal GB is treated as 1,000 MB.
- ✓The entered CDN price applies uniformly to all delivered video data.
- ✓The calculation measures viewer delivery transfer only.
Limitations
- !Adaptive bitrate streaming can cause delivered bitrate to differ by viewer, device, connection, and content.
- !Viewer behavior may differ between variants, making a shared average watch time inaccurate.
- !CDN prices may vary by region, delivery tier, contract, or volume.
- !The estimate excludes storage, transcoding, requests, origin transfer, platform fees, and minimum commitments.
Common Mistakes to Avoid
Entering full video duration instead of the average time viewers actually watch.
Using an encoded rendition bitrate when the delivered average bitrate is materially different.
Treating a 50% Variant B share as 50% for each variant rather than 50% for B and 50% for A.
Forgetting that Mbps is megabits per second, so the conversion requires division by 8.
Using binary storage units when the calculator and CDN rate use decimal GB.
Comparing the test cost with a baseline that does not use the same monthly plays and watch time.
Related Formulas
Frequently Asked Questions
How do you calculate video bandwidth from bitrate and watch time?
Multiply plays by average watched seconds and delivered Mbps, then divide by 8 and by 1,000 to estimate decimal GB.
Why is the formula divided by 8?
Bitrate is in megabits per second, while delivery volume is expressed in bytes. Eight bits equal one byte.
How does the traffic split affect monthly video transfer?
It determines the number of plays receiving each bitrate. Increasing the share of a higher-bitrate variant generally raises total transfer.
What does cost change versus all Variant A mean?
It is the estimated test delivery cost minus the estimated cost if every monthly play used Variant A.
Can the additional monthly cost be negative?
Yes. It can be negative when Variant B has a lower bitrate than Variant A and receives a positive share of traffic.
Should I use Mbps or MBps for bitrate?
Use Mbps, meaning megabits per second. MBps means megabytes per second and would need conversion before use.
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