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by Nathan.Fouarge on Nov 12, 2026, 8:00:02 AM
Each backup job requires space for one full backup, plus the amount of changed data per run, multiplied by the number of runs that are kept. Since change rates vary widely between environments, measure the actual rate before sizing the backup target.
The question, "How big does my backup drive need to be?" comes up regularly in our support tickets, and you need the answer before you can provide a client with a hardware quote. Here's how to calculate it.
Table of Contents
- How Do You Calculate Backup Storage?
- The Setup: 30 Daily and 12 Monthly Restore Points
- Two Examples: Low and High Change Rate
- Measure First, Then Size
- What to Do When You Outgrow the Backup Drive
- FAQ
How Do You Calculate Backup Storage?
Since each backup job is its own backup set with its own initial full backup, size each job separately. Each job requires space for one full backup and all subsequent changes.
The formula
Space per job ≈ full backup + (changed data per backup × restore points kept)
The size of the full backup depends on how much data you select. The changed data per backup refers to the data sent to the target by the job after the initial full backup. The number of restore points kept comes from the job's retention settings. For instance, a daily job with a retention period of 30 days keeps 30 daily restore points in addition to the initial full backup.
One full backup per job is sufficient because NovaBACKUP uses an incremental forever scheme. After the initial full backup, each subsequent backup captures changes at the file or block level. When the retention period ends, outdated versions and blocks are removed, and the latest version of every unchanged file remains in the backup set. There are no recurring full backups to plan for. Our guide to Data Retention Best Practices explains how retention handles each file.
The Setup: 30 Daily and 12 Monthly Restore Points
Consider a client who needs daily restore points for the last 30 days and monthly restore points for the last 12 months. This would require setting up two jobs:
Job 1
A daily backup job with 30 days of retention.
Job 2
A monthly backup job that runs on the first day of each month with 12 months of retention.
Each schedule is in its own job, so you can change one without affecting the other. If you're still deciding how long to keep backups, read our blog post Creating a Backup Plan? Ask These Questions First covers what to decide before setting retention.
The examples below illustrate the daily job. The monthly job follows the same formula, keeping 12 runs, each of which holds a month of changes.
Two Examples: Low and High Change Rate
The input that varies most between environments is changed data per backup. Both examples use a 500 GB machine and the aforementioned daily job.
Low Change Rate: About 3 GB per Day
In one of our tests, 1 TB of data containing four million files sent approximately 6 GB of changes over the course of one day, which is roughly 0.6%. At that rate, a 500 GB machine would send about 3 GB per day.
500GB + (3GB x 30) = 590GB
High Change Rate: About 50 GB per Day
Machines with heavier daily activity generate far more data. Our blog post explaining how the Incremental Forever Backup scheme works assumes a daily change of 10%. For a 500 GB machine, that equates to 50 GB per day.
500GB + (50GB x 30) = 2,000GB
| Example | Daily change | Daily job (30 runs kept) | Daily job vs. source data |
|---|---|---|---|
| Low change rate | ~3 GB (0.6%) | 590 GB | 1.2× |
| High change rate | 50 GB (10%) | 2,000 GB | 4× |
The daily backup job alone can require anywhere from 1.2 to four times the size of the machine. A fixed multiplier cannot cover that range.
Measure First, Then Size
For a new client, run the job for one week and record the amount of change per backup job from the job history. Plug that number into the formula for each job, adding room for data growth.
After a few weeks, check how much the target holds and make adjustments as needed. Storage capacity and utilization should be included in your regular monitoring; our blog post, Measuring Backup Performance: Essential KPIs to Track explains which metrics to watch.
Free tool
Check your own numbers
Use our backup storage calculator to check your own numbers.
Open the backup storage calculatorBackup target
Size, change rate and retention in. Drive size out.
For cloud targets, the estimate provides the storage line for your quote. NovaBACKUP Cloud storage costs $0.05 per GB per month. Therefore, the 590 GB daily job from the low change example costs about $29.50 per month.
What to Do When You Outgrow the Backup Drive
When a target is full, you have two options. The appropriate option depends on whether you need the existing restore points on the new disk.
Keep Your Backup History
Copy or move the backup device folder to the new disk, then assign the new disk the same drive letter as the old one. NovaBACKUP will find the existing backup files and continue from the last backup, rather than running a new full backup. Make sure to copy the folder when no job is scheduled to run so that its contents don't change during the process.
Start Fresh on a New Disk
Add the new disk as a device and assign jobs to it. The jobs will start over with a new full backup, and your earlier restore points will stay on the old disk. Keep the old disk until you no longer need the restore points it contains.
If the new target is a NAS, refer to the Essential Guide to NAS as Backup Storage for SMBs for setup and access control instructions. Whichever disk ends up holding the restore points, monitor its health. Don't Let Your Backups Fool You explains what to monitor.
FAQ
FAQ
Do I need to create a new full backup when I switch to a larger hard drive?
No. If you copy the backup device folder to the new disk and assign it the same drive letter, NovaBACKUP will continue from the last backup. A new full backup only runs when you add the disk as a new device.
FAQ
How often should I check how much space my backups use?
Check after the first few weeks of a new job once you have real change rates, and then check it as part of your regular backup monitoring.
FAQ
How do I size local and cloud targets for a hybrid job?
Size each target separately. In a hybrid backup job, each location has its own initial full backup and retention settings, so apply the formula to each target. Keep in mind that the formula applies to each target's backup job.
Try NovaBACKUP free for 30 days
Run Your Own Numbers
Use the backup storage calculator to determine the appropriate size for your backup storage by entering your client's data size, change rate, and retention.
Request a free 30-day trial to see per-job retention and incremental forever at work.
Worth Reading
How Much Storage Do You Need for Backups? A Sizing Formula for MSPs
Backup Restore Options: Every Way to Restore Files, Systems, and VMs
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