Backup Storage Calculator
Size a grandfather-father-son backup footprint from change rate and retention, split across warm and cold storage.
Inputs
Re-writing the same hot blocks adds no new backup storage.
Monthly Backup Cost
$384.40
Total Backup Footprint
18,056GB
Warm Storage
5,096GB
Cold Storage
12,960GB
Footprint vs Primary Data
9.03x primary
Footprint in Bytes
18 TiB
Step by step
Values used
Primary data protected = 2,000 GB; Daily change rate = 3 %; Share of changed blocks that change again = 40 %; Daily recovery points kept = 30 points; Weekly recovery points kept = 8 points; Monthly recovery points kept = 12 points; Warm backup storage price = 0.0500 USD per GB-month; Cold backup storage price = 0.0100 USD per GB-month
Backup Storage
footprint = full copy + Σ(recovery points x days covered x daily delta x unique-block factor), where daily delta = primary GB x change rate.
Monthly Backup Cost
= 384.40
Total Backup Footprint
= 18,056 GB
Warm Storage
= 5,096 GB
Cold Storage
= 12,960 GB
Footprint vs Primary Data
= 9.03 x primary
Footprint in Bytes
= 19,387,482,374,144
How it works
Incremental-forever backup stores one full copy plus the changed blocks behind every retained recovery point, so the footprint is driven by change rate and retention depth rather than by the size of the source. Long-retention monthly points each anchor a month of change, which is why they usually dominate the total even though they are the fewest in number, and why they belong in cold storage. Both prices are user-supplied list prices to confirm on the AWS Backup pricing page for your region. It explains the recurring surprise that protecting 2 TB needs closer to 18 TB of backup storage, and shows that trimming monthly retention moves the bill far more than trimming daily retention.
Formula
Backup Storage
footprint = full copy + Σ(recovery points x days covered x daily delta x unique-block factor), where daily delta = primary GB x change rate.
- daily delta
- New bytes written each day that a backup must capture
- unique-block factor
- Share of changed blocks that are new rather than re-changed
Frequently Asked Questions
How is Backup Storage calculated?
footprint = full copy + Σ(recovery points x days covered x daily delta x unique-block factor), where daily delta = primary GB x change rate. Incremental-forever backup stores one full copy plus the changed blocks behind every retained recovery point, so the footprint is driven by change rate and retention depth rather than by the size of the source. Long-retention monthly points each anchor a month of change, which is why they usually dominate the total even though they are the fewest in number, and why they belong in cold storage. Both prices are user-supplied list prices to confirm on the AWS Backup pricing page for your region.
Why does Backup Storage matter?
It explains the recurring surprise that protecting 2 TB needs closer to 18 TB of backup storage, and shows that trimming monthly retention moves the bill far more than trimming daily retention.
What values do I need to enter?
This calculator takes 8 inputs: Primary data protected, Daily change rate, Share of changed blocks that change again, Daily recovery points kept, Weekly recovery points kept, Monthly recovery points kept, Warm backup storage price, Cold backup storage price. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.
Why does cutting daily retention barely help?
Each daily point only carries one day of change, while each monthly point carries about thirty. Dropping one monthly point removes as much storage as dropping thirty daily ones.