Azure Cost Management reports the same reservation in two ways, and neither number is wrong. Which one you use decides whether a chargeback survives being challenged.
Cost Management offers two metrics over the same data: actual cost and amortized cost. They differ in exactly one respect: when a commitment purchase is recorded, and against which resource. That single difference decides whether a monthly report reconciles to the invoice or explains what a team consumed.
1. Actual cost: what the invoice says
Buy a one-year reservation for CHF 12'000 paid upfront and the full CHF 12'000 lands in the purchase month. Nothing follows for the next eleven months. The virtual machine benefiting from that reservation runs at zero cost in this view: the effective price of the covered usage is zero.
For reconciling against the invoice this is ideal. For answering what a team consumed in March, it is close to useless. One caveat: Cost Management covers usage and purchases, but not tax, support charges or credits. Actual cost mirrors the invoice's cost lines, not the invoice total. Anyone who has promised finance an exact match should know that in advance.
2. Amortized cost: what the resource consumed
Amortized cost takes a different route. Azure divides the purchase by the number of days in the term and books the daily portion against the resources that actually drew on the benefit. Two consequences to catch:
- The monthly amounts are not equal. Amortization is daily, so a month is worth its own number of days. CHF 12'000 ÷ 365 = CHF 32.88 per day, which makes January CHF 1'019 and February CHF 921.
- The unused share becomes visible. If 30% of the reservation goes unused on a given day, that share of the daily cost carries the charge type UnusedReservation (UnusedSavingsPlan for savings plans) and the rest is attributed to the virtual machine. It is deliberately not attributed to the subscription that made the purchase, because it belongs to no resource.
Under CSP, though, the amortized metric does not carry the commitment through: the reservation cost is dropped rather than spread across the resources that used it. Over the term that leaves the amortized view short by the full commitment, and in any single month the commitment’s daily portion is simply missing. The Zation Platform rebuilds that view for CSP customers, allocating the commitment to the resources and service families that actually consumed it while keeping the total on the invoice.
3. What that looks like in a report
A one-year reservation for CHF 12'000, bought on 1 January, paid upfront. Fully used in January and February, and 70% used in March.
| Month | Actual cost | Amortized cost | Of which unused |
|---|---|---|---|
| January | 12'000 | 1'019 | 0 |
| February | 0 | 921 | 0 |
| March | 0 | 1'019 | 306 |
| April | 0 | 986 | 0 |
| … | … | … | … |
| Full term | 12'000 | 12'000 | 306 |
So the two views report different monthly totals, and that is not an error. The amortized view departs from the invoice deliberately, because it moves cost to where the benefit arises. Over the full term both arrive at the same number.
4. Chargeback: the case where only one view works
Two departments share that reservation. In March, of the amortized CHF 1'019, Retail drew 60% and Logistics 10%, and 30% went unused:
- Retail: CHF 611. Attributed to its own virtual machines, at resource level, checkable line by line.
- Logistics: CHF 102. Same mechanism, smaller share.
- Unused: CHF 306. Attributable to no resource, and therefore to no department.
The actual cost view of the same March shows CHF 0 for both departments and CHF 12'000 sitting in January, in whichever subscription happened to place the order. There is no defensible way to derive a department's share from that. Without amortized, resource-level figures you cannot put a number on a department's share of a jointly used reservation and have it hold up.
The CHF 306 is the more interesting line. It is the only place where a commitment decision that turned out wrong becomes visible in money, and it belongs to whoever made the commitment. A team that gets it allocated pro rata is being charged for someone else's forecast.
5. Where amortization does not apply
Amortization does not reach everywhere, and a missing spread is not automatically a defect:
- Marketplace purchases are not amortized. Virtual machine software usage reservations sold through the Marketplace are the exception.
- Pay-as-you-go subscriptions are not supported. You can buy a reservation on MS-AZR-0003P, but cost analysis will not show it amortized.
- Commitment purchases only appear in EA and MCA data. Under MOSA and the legacy offers the usage carries the discount, but the purchase itself is not in the dataset, so there is nothing to spread.
6. How FinOps teams work with the two views
A working practice fixes which view answers which question, and then leaves that decision alone.
One default lens. Everything internal runs on amortized: showback, chargeback, unit economics, budget tracking, anomaly detection at resource level. Actual is kept for two jobs: reconciling with finance against the invoice, and cash planning, because an upfront commitment is a cash event in one month no matter how it is reported.
Never both in one chart. A monthly trend that silently mixes the metrics produces a spike in the purchase month and a dip after it. Someone will eventually explain that spike as an incident, and spend a week on it.
CSP needs the view rebuilt. Azure does not deliver a usable amortized view under CSP, so a FinOps practice on a partner-billed tenant either reconstructs the allocation itself or runs on a platform that already does. That is what the Zation Platform provides.
Unused gets a KPI and an owner. UnusedReservation and UnusedSavingsPlan are the numbers commitment decisions are measured on. They sit with the commitment owner, alongside coverage and utilisation, and they are reviewed on the same monthly cadence as the rest of rate optimisation.
In FinOps Framework terms: the amortized view is the precondition for the Allocation and Reporting & Analytics capabilities, while the actual view serves Invoicing & Chargeback reconciliation. Neither replaces the other.
Bottom line
Actual cost answers “what did we pay”. Amortized cost answers “what did we use”. An organisation needs both, and most of the trouble comes from using one to answer the other's question. Amortized for everything internal, actual for the invoice.