Should you buy more M365 storage or clean up? A decision framework
M365 storage expansion vs cleanup, settled with arithmetic: a decision tree, a worked 500-user example at $0.20/GB/month, and when buying is the right call.
When a tenant runs out of storage, two reasonable people reach opposite conclusions. IT wants to clean up, because the content is obviously junk. Finance wants to buy, because $1,000 a month is cheaper than three weeks of engineering time. Both are arguing from real numbers; they are just different numbers.
The argument is settleable. It needs one figure neither side usually has.
The only number that decides it
Everything turns on reclaim potential: how many gigabytes you could remove without anyone objecting. Not "how much is in there" — how much could go.
With that figure the comparison is arithmetic, because Microsoft's extra file storage is priced at a flat $0.20 per GB per month (see add storage space):
- Cost of buying = overage GB × $0.20 × 12, every year, rising as the tenant grows.
- Value of cleaning = reclaimable GB × $0.20 × 12, every year, as recurring as the cost it replaces.
- Cost of cleaning = a one-off: internal days, or a fixed-fee engagement.
Which gives the rule:
Clean if the annual value of your reclaim potential exceeds the one-off cost of doing the work. Buy if it doesn't — and buy only the residual, after cleaning.
The "12 months" framing matters because it is the point at which the two sides become comparable. Cleanup is a one-off cost; storage is a perpetual one. Any reclaim that pays for itself inside a year keeps paying every year afterwards.
The decision tree
Is the tenant over its allowance?
├─ No → Set version limits + a recordings lifecycle now.
│ Re-check in 3 months. Buy nothing.
└─ Yes → Run a read-only scan. Get the reclaim number.
│
├─ Reclaim ≥ overage?
│ → CLEAN. You do not have a capacity problem.
│
├─ Reclaim ≥ 40% of overage?
│ → CLEAN FIRST, then buy the residual.
│ Typical case. Cleaning changes what you buy.
│
└─ Reclaim < 40% of overage?
→ BUY, and set limits so it does not recur.
Genuine growth. Stop paying people to fight it.Most tenants we scan land in the middle branch, and most of them expected to be in the bottom one.
Worked example: a 500-user tenant
A 500-user E3 organisation. SharePoint allowance is 1 TB + (500 × 10 GB) ≈ 5.9 TB. SharePoint is holding 11 TB, so the overage is about 5.1 TB, or 5,222 GB.
Option A — buy the lot.
5,222 GB × $0.20 = $1,044/month → $12,533/year, recurring and growing.
Option B — scan first. The scan finds 3.4 TB that can go:
| Source | Reclaimable | Annual value at $0.20/GB/mo |
|---|---|---|
| File versions (cap + trim) | 1.6 TB | $3,932 |
| Byte-identical duplicates | 1.1 TB | $2,703 |
| Leavers' OneDrives | 0.7 TB | $1,720 |
| Total | 3.4 TB | $8,356 |
Residual overage after cleaning is 1.7 TB (1,741 GB), which costs $348/month — $4,177/year.
The comparison. Buying everything costs $12,533 a year forever. Cleaning first and buying the residual costs $4,177 a year, plus a one-off cleanup. The cleanup has to cost more than $8,356 to be the wrong call — and capping versions and releasing leaver accounts is days of work, not weeks.
Note what has also happened: the $8,356 is avoided every year, and the tenant is now growing from a smaller base.
Costing the cleanup honestly
The framework only works if the one-off side of it is a real number rather than "some of Dave's time". Price it the way you would price any other piece of work.
The three actions that produce most of the reclaim are not equally expensive:
- Emptying recycle bins is close to free. An afternoon, no policy change, no review needed, because somebody already decided that content should go.
- Capping and trimming versions is a configuration change plus a report to read. Budget a day or two, most of which is deciding the policy rather than applying it.
- Releasing leavers' OneDrives is the expensive one, because it needs a human review per account and a conversation with HR about retention. This is where cleanup projects overrun.
Sequence them in that order and you can stop whenever the remaining reclaim stops justifying the remaining effort. That is the real advantage of having the breakdown before you start: you are not committing to a project, you are working down a ranked list and choosing where to get off.
When buying is genuinely the right answer
It often is, and pretending otherwise wastes everyone's time:
- The content is real and regulated. If you are retaining under a statutory obligation, that storage is a cost of doing business. Buy it and move on.
- Reclaim potential is under 40% of the overage. The cleanup will not change the purchase enough to justify the disruption.
- The growth is a project, not a pattern. A one-off ingest with a known end date is a capacity decision, not a hygiene one.
- You have already cleaned. If version limits are set, recordings expire and leaver accounts are released, you are at the floor. Buy the rest.
What makes the decision bad is not buying. It is buying without the reclaim number, because then you are not making a decision at all — you are paying to avoid making one.
Getting the number
Tenant Storage Optimisation produces exactly the figure this framework needs: one read-only pass across SharePoint, OneDrive, Exchange and Teams, a hash-based duplicate sweep, a file-versions scan with reclaim estimate, your largest files ranked, and scored recommendations. Export to PDF for the finance conversation and Excel for the work itself.
Read-only Microsoft Graph permissions — it cannot change anything — hosted in Azure UK South. The first scan is free, which means you can settle the buy-or-clean argument before spending anything on either.
More on this from the TSO team: from scan to savings in 90 days and duplicates, the tax you didn't know you were paying. On this site: what extra storage actually costs, what the quota alert means, and before you buy more storage, reclaim what you have.
