The real cost of Microsoft 365 extra storage: $0.20/GB/month, and why nobody budgets for it
Microsoft 365 additional storage cost is $0.20/GB/month, on SharePoint alone. A worked 500-user example, and why finance sees the bill before IT does.
Microsoft 365 extra file storage costs $0.20 per GB per month. It is one of the few Microsoft prices that is genuinely simple, and it is almost never in anybody's budget.
The reason is not that finance teams are careless. It is that the charge appears a long way from the decision that caused it, in a different system, under a name that does not mention SharePoint.
What you actually get before you pay anything
Three workloads, three completely different allowance models. This is the part that makes the bill hard to anticipate — there is no single "Microsoft 365 storage" meter to watch.
| Workload | How it is allocated | What you get | How you add more |
|---|---|---|---|
| SharePoint | Pooled across the tenant | 1 TB + 10 GB per licence; max 25 TB per site | Office 365 Extra File Storage, sold per GB at $0.20/GB/month, in 1 GB increments |
| OneDrive | Per user | 1 TB per user (Business Basic/Standard/Premium, E3, E5); 5 TB available on E3 and E5; 2 GB on F3 | Not sold per GB — raise the per-user quota on a qualifying plan, then via your Microsoft representative |
| Exchange | Per mailbox | 100 GB primary (Business Standard/Premium, E3, E5, Plan 2); 50 GB (Business Basic, Plan 1); 2 GB (F3) | Not sold per GB — archive mailbox, 50 GB on Plan 1 or auto-expanding to 1.5 TB on Plan 2/E3/E5 |
Figures from Microsoft's SharePoint, OneDrive and Exchange Online service descriptions, and the add storage space guidance.
Worth being precise about one thing, because it is widely misreported: only SharePoint has a per-gigabyte meter. OneDrive and Exchange expand through licensing — a higher plan, a bigger per-user quota, an archiving add-on. If somebody quotes you a $/GB rate for mailbox storage, they are quoting the SharePoint rate for the wrong workload.
Note also that SharePoint storage is calculated in binary gigabytes, where 1 GB = 230 bytes. It makes a few per cent of difference at terabyte scale, which matters when you are the one signing off the number.
A worked example on a 500-user tenant
Take a 500-user organisation on E3. Their SharePoint allowance is:
1 TB + (500 × 10 GB) = 1 TB + 5,000 GB ≈ 5.9 TB
Say SharePoint is holding 11 TB — an entirely ordinary figure for a business that has migrated file shares, run Teams for five years and never set a version limit. The overage is roughly 5.1 TB, which is 5,222 GB:
5,222 GB × $0.20 = $1,044 per month
$1,044 × 12 = $12,533 per year
Twelve and a half thousand dollars a year, recurring, for storage nobody chose to consume. And because the underlying growth has not stopped, next year's number is larger.
Put it another way: a terabyte of unmanaged SharePoint content costs about $2,458 a year (1,024 GB × $0.20 × 12). That is the number to carry into the conversation, because it converts an abstract storage problem into a per-terabyte price that a budget holder can act on.
Why the number grows even when nothing happens
The uncomfortable property of storage overage is that it compounds without anybody doing anything. Three mechanisms drive it, and all three are invisible day to day.
Headcount moves the line in both directions. The pool is 10 GB per licence, so every leaver you offboard shrinks the allowance by 10 GB while their OneDrive carries on consuming space during its retention window. A restructure that removes fifty licences removes 500 GB of allowance on the day it completes.
Versions scale with activity, not with content. A library that gains no new documents still grows, because the documents already in it keep being edited. This is why storage charts rise during quiet quarters and surprise people.
Nothing deletes itself by default. Teams recordings, completed project sites, the second copy of a migrated share — none of these expire unless somebody configures them to. Absent a lifecycle policy, the only direction is up.
Which means an overage bill set in January is a floor, not a ceiling. If you budget the current number and change nothing else, you will under-budget.
Why finance sees it and IT doesn't
The extra storage add-on is a subscription on the tenant's billing account. It renews, it appears on the invoice, and it is visible to whoever reconciles the Microsoft bill.
It is not visible in the SharePoint admin centre as a cost. The SharePoint admin sees capacity — a bar that is comfortably under the line, because somebody bought headroom. The purchase that created the headroom happened in a different console, usually during an incident, often months earlier.
So you get two accurate but incompatible views of the same situation. Finance sees a line item that grows every year and cannot tell whether it is justified. IT sees a tenant that is not full and has no prompt to investigate. Nobody is wrong and nobody is looking at the whole thing.
The fix is not a tool, it is a habit: put storage consumption and the storage add-on on the same page, once a quarter, with a trend line rather than a snapshot.
What the number should prompt you to ask
Before approving another increment, three questions are worth more than the purchase:
- How much of this is versions? Capping version history and trimming existing versions is a configuration change, not a deletion project, and it is frequently the largest single line in a reclaim estimate.
- How much of this is duplicated? Byte-identical copies across libraries are pure waste. You cannot find them by browsing; you need hashes.
- How much belongs to people who have left? Leavers' OneDrives and the sites of long-finished projects are usually the least contentious thing to remove.
If the answers add up to more than the overage, you do not have a storage problem. You have a housekeeping problem that is being billed monthly.
Getting the answers
Tenant Storage Optimisation exists to produce that reclaim number quickly. One read-only pass across SharePoint, OneDrive, Exchange and Teams; a hash-based duplicate sweep; a file-versions scan with the reclaim estimate attached; your largest files ranked; and scored recommendations. The PDF and Excel exports are deliberately designed to survive being forwarded to finance.
Read-only Microsoft Graph permissions only — it cannot delete or modify anything — hosted in Azure UK South. Pricing is published, and the first scan costs nothing.
The TSO team's own write-up of what you are really paying for M365 storage goes further into the billing mechanics. On this site, why your tenant is running out of storage covers where the growth comes from, and buy or clean? turns this cost into a decision.
