Reserved Instance Break-Even Calculator
Whether committing to a reserved or savings-plan rate beats paying on demand, given how many hours a month you actually expect to run — the break-even utilization, the saving or loss over the term, and the months of unused commitment that would wipe it out.
A reservation is a discount for paying whether or not you use the capacity.
How the reserved instance break-even calculator works
A reservation is a discount for paying whether or not you use the capacity. It wins when utilization is above the break-even — the reserved rate over the on-demand rate — and loses below it. At a 40% discount the break-even is 60% utilization: a server that runs sixteen hours a day should be reserved; one that runs eight should not.
The risk row asks the other question: if the workload disappears part way through the term, how many idle months erase the saving.
Formula: break-even utilisation = reserved rate / on-demand rate; monthly saving = hours × on-demand − 730 × reserved
Worked examples
| Inputs | Monthly saving from reserving (all instances) | Note |
|---|---|---|
| 500 hours a month at a 40% discount | 119.04 | reserve |
| A part-time workload | -264.96 | stay on demand |
| Always on | 560.64 | the full discount |
FAQFrequently asked questions
What is a reserved instance?
A commitment to pay for capacity for one or three years in exchange for a discount of 30 to 70% on the on-demand rate. Savings plans are the same idea applied to spend rather than a specific instance.
What is the break-even?
The utilization at which the reserved cost equals what you would have paid on demand — simply one minus the discount. Above it, reserve; below it, do not.
Why does the term matter?
Because you pay for every month of it whether the workload survives or not. The idle-months row says how much of the saving a cancelled project would burn.
Should I reserve everything that is always on?
The stable baseline, yes. Keep the variable part on demand or spot, and revisit the reservation before it renews — instance families change.
What about upfront payment?
All-upfront usually adds a few more points of discount. Treat the upfront sum as the term's reserved cost paid early, and compare on the same monthly basis.
Where these figures come from
- IEC 80000-13 — Information science and technology (quantities and units) — the decimal (kB, MB) versus binary (KiB, MiB) prefixes used throughout
- RFC 4632 — Classless Inter-domain Routing (CIDR) — the address-plan arithmetic behind the subnet calculator
- RFC 1918 — Address Allocation for Private Internets — the private ranges the subnet calculator recognises
- NIST SP 800-63B — Digital Identity Guidelines, Authentication — length over composition rules; the basis of the password guidance here
- NIST SP 800-57 Part 1 — Recommendation for Key Management — key-strength comparisons used by the key-space calculator
- NIST — Digital Identity Guidelines (SP 800-63) — US federal authentication standard
Last checked: September 2026. Units follow the SI decimal convention (IEC 80000-13 defines the binary alternatives); network and security figures cite the defining standard.