Capacity baseline

Estimate backup capacity from retention, not only current data

A backup destination needs room for the protected data and the retained history. Start with a visible capacity floor, then verify the destination’s versioning and storage behavior.

Keep the current set and retained change separate

The first copy starts from the data you protect now. Retention adds historical change after that. Keeping those quantities separate makes the destination requirement explainable instead of turning an advertised capacity number into a promise.

For a planning baseline, use:

minimum destination capacity = current protected data + retained changed data
retained changed data = monthly change × retained days ÷ 30

The planner uses decimal TB and GB consistently. It deliberately leaves compression and deduplication outside the arithmetic because those outcomes depend on the data mix and the destination implementation.

Retention changes the destination decision

Two plans with the same current data can need different capacity when one keeps a short rollback window and the other keeps several months of historic versions. A lower change rate can also make a longer retention window practical, while a large media ingest or active project archive can consume the same destination much faster.

Do not treat a single full-copy estimate as the retention plan. Record the intended recovery history, then compare it with the exact destination’s version lifecycle and deletion behavior.

Carry a capacity floor into product research

Before comparing an external drive, a second NAS, or a cloud account, carry these values together:

• Current protected data.
• Expected changed data per month.
• Required retention days.
• Minimum destination capacity before destination-specific overhead.

The next decision is not the label on the storage product. It is whether the chosen destination can retain the required history while preserving a recovery path independent from the primary system.