How this calculator works
Backups take more space than the data they protect, because a good backup doesn't just keep last night's copy. It keeps a series of restore points — snapshots you can go back to. If a file was deleted three weeks ago, or ransomware quietly encrypted things on Tuesday, you need a copy from before that happened. Each restore point you keep takes some space.
The calculator estimates that space in five steps:
- One full copy. Your total data, divided by the compression and deduplication ratio.
- Daily restore points. Each stores roughly the data that changed that day (your daily change rate).
- Weekly, monthly and yearly points. In "incremental forever" mode, each stores the data that changed over that period. Because the same files and databases change again and again, a week's changes are less than five days added together. The calculator assumes daily changes fall within the "share of data that ever changes" you set, and estimates how much of it changed at least once over 5, 21 and 250 working days (a week, a month and a year). In "periodic full" mode, each of these points is a complete copy.
- Growth. The same math for your data size in one and three years, at your growth rate.
- Copies and buffer. Multiplied by the number of backup copies, plus free space so backups don't fail when storage fills up.
All defaults are editable estimates, not measurements. The best numbers come from your own backup reports: after a few weeks, compare actual storage use against this estimate and adjust the change rate and ratio.
Choosing the inputs
| Input | What to enter | Tip |
|---|---|---|
| Total data | Everything your backup protects: server, shared drives, practice-management or accounting database, imaging archive | Check the "used space" on each drive or server, not the drive size |
| Daily change rate | Share of data that changes on a working day | 1–5% is a common planning range; your backup software's job reports show the real figure |
| Compression & dedupe ratio | How much your backup software shrinks the data | Use 1 (no savings) for mostly X-rays, photos or video, which are already compressed |
| Retention | How many daily, weekly, monthly and yearly points to keep | Match your recovery needs and any record-retention rules |
| Copies | Backup copies, not counting the original | 2 for the 3-2-1 rule |
A common retention scheme is called grandfather-father-son (GFS): keep dailies for a couple of weeks, weeklies for a month, monthlies for a year and yearlies for several years. The defaults (14 daily, 4 weekly, 12 monthly, 3 yearly) follow that pattern. Adjust them to your needs.
A worked example
Take the starting numbers in the calculator: 2 TB of data, 2% daily change, 30% of the data ever changing, a 1.5 : 1 compression ratio, and 14 daily, 4 weekly, 12 monthly and 3 yearly restore points. In incremental-forever mode, one backup copy needs about 7 TB today — roughly three and a half times the data, mostly because of the twelve monthly points. With 20% growth a year and a 20% free-space buffer, plan for about 14.6 TB per copy in three years, and double that for two copies.
Switch the same inputs to "periodic full backups" and one copy jumps to about 26 TB today, because each of the 19 weekly, monthly and yearly points is a complete copy. That's why simple USB-drive or NAS rotations rarely keep more than a few weeks of history, and why backup software that stores only changes is worth paying for. The biggest levers on the result are how many monthly and yearly points you keep and how well your data compresses — try changing them to see the effect.
The 3-2-1 rule, explained
The 3-2-1 backup rule is a simple way to make sure no single problem wipes out every copy of your data:
- 3 copies of your data — the original and two backups.
- 2 different types of storage — for example, a local backup appliance or NAS and a cloud service.
- 1 copy off-site — so a fire, flood, theft or hurricane at the office doesn't take the backups with it. In Florida, that matters; see our hurricane IT preparedness guide.
That's why the "copies" default is 2. The local copy gives fast restores; the off-site copy is your insurance. Each copy needs roughly the storage the calculator shows "per backup copy", although cloud services may store data more or less efficiently than local hardware.
Make one copy immutable or offline
Ransomware attackers go after backups first. CISA's #StopRansomware Guide (September 2023) recommends maintaining offline, encrypted backups of critical data and testing them regularly, noting that many ransomware variants try to find and delete or encrypt accessible backups. It also mentions immutable cloud storage as an option, with careful configuration.
An immutable backup is locked for a set period: nobody — including someone who has stolen an administrator password — can change or delete it until the lock expires. If you turn on immutability, remember that locked data can't be cleaned up early, so the buffer in this calculator matters more.
Storage is only half of a backup plan
Knowing how much storage you need is the easy part. Make sure you also know:
- How fast you can restore. Restoring several terabytes from the cloud over a typical office connection can take days. That's your recovery time objective (RTO). Our internet speed calculator helps size your connection.
- How much data you can afford to lose. Backing up nightly means you could lose a day's work — your recovery point objective (RPO).
- That restores actually work. Test a restore at least a few times a year and write down the result.
- That backups are encrypted. Especially for patient or client data, both in transit and at rest.
For HIPAA-covered practices, the Security Rule's contingency plan standard includes a required data backup plan (45 CFR 164.308(a)(7)): procedures to create and maintain retrievable exact copies of electronic protected health information. It also requires a disaster recovery plan to restore lost data.
Next steps
Print or save your results as a PDF to share with whoever buys your backup hardware or cloud storage. For the full picture — what to back up, how often, and how to test it — read our backup and disaster recovery guide and ransomware protection for small businesses. If you'd like us to design and monitor backups for you, see our services or book a free call.