Amortized vs Unblended Cost: What Each Column Shows

Unblended cost is the cost of usage as it is charged, on the date it is charged. Amortized cost takes the upfront and recurring fees of reservations and savings plans and spreads them across the period they cover, so each day or resource shows its effective rate. The two columns only diverge when you own commitments. With no reservations or savings plans, they are the same number.

They are easy to confuse because both appear side by side in AWS Cost Explorer and Azure Cost Management, and because unblended cost is sometimes described, wrongly, as a per-resource effective rate. It is not amortized: it shows fees when they are billed.

AWS definitions

AWS Cost Explorer’s “Aggregate costs by” options define the family precisely:

• Unblended costs “reflect the cost of the usage.” Grouped by charge type, they separate discounts into their own line items. A Reserved Instance fee shows as a spike on the day you are charged.
• Amortized costs “reflect the effective cost of the upfront and monthly reservation fees spread across the billing period.” AWS estimates them by combining unblended costs with the amortized portion of upfront and recurring reservation fees.
• Blended costs reflect “the average cost of usage across the consolidated billing family”, available when you use consolidated billing in AWS Organizations.
• Net unblended and net amortized costs are the same views after discounts (net amortized includes discounts such as RI volume discounts).

AWS’s own example: a one-year Partial Upfront t2.micro RI bought for $30 upfront with a $2.48 monthly fee shows as a spike on the first of the month in unblended view. Switched to amortized for a 30-day month, the chart shows an effective daily rate of $0.165. AWS notes amortized costs are not available for billing periods before 2018, and that grouping by purchase option shows how much of a reservation went unused.

Azure definitions

Azure splits the same distinction into two datasets. Actual Cost “provides data to reconcile with your monthly bill”: it includes reservation purchase records, and the effective price of usage that received a reservation discount is zero. Amortized Cost is similar, except the effective price on discounted usage is “the prorated cost of the reservation (instead of being zero)”; it has no purchase records but does include unused reservation hours.

That zero is why Azure actual cost is a poor view for per-team or per-resource reporting: every reserved VM appears free, and the whole purchase lands on one day.

Which one to use

For showback and chargeback, amortized cost is the fairer basis because it gives each consumer its share of the commitment instead of charging the whole purchase to whoever owns the billing account. For finance reconciliation, stay with the billed column.

How ZopNight picks a column

When billing sync is configured, ZopNight overlays billed cost on its calculated rack-rate cost per resource per day. For Azure it reads amortized cost, so reserved and savings-plan VMs carry their real effective daily cost instead of $0; for AWS and GCP it uses actual billed cost, and GCP figures are net of credits. A resource-day with no billing row falls back to the rack rate, and the summary is labelled as billed or rack-rate so you know which you are reading. Each cost line also records its purchase type (On-Demand, Reservation, Savings Plan, Spot). The wider billing picture is in cloud billing.