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Reserved Instance Capacity Needs Estimator

The Reserved Instance Capacity Needs Estimator estimates how much baseline capacity to reserve for a future planning period. It starts with current peak demand, applies expected demand growth, selects the share of that projected demand you want covered by reserved capacity, and then adjusts for a target reservation utilization rate.

The result can help frame a reservation purchase before comparing commitment prices. Reserving every unit of projected peak demand may create unused commitments when workloads vary, while reserving too little can leave a larger share exposed to on-demand pricing. The coverage percentage lets you choose how much projected demand belongs in the reserved baseline, and the utilization target adds a buffer against overcommitting. This is a sizing model only; actual reserved-instance eligibility, scope, instance-family rules, and commitment flexibility vary by provider and should be checked separately.

Inputs

units
%
%
%
Result
—
Recommended reserved capacity
Projected peak demand—
Demand targeted for reservation—
Reservation buffer—
Projected demand not covered—

1. Enter current peak demand
Use a peak demand measure in the same capacity unit you would reserve.

2. Apply expected growth
Enter the change in demand expected by the time the reservation is intended to cover.

3. Choose reserved coverage
Set the percentage of projected demand you want handled by reserved capacity rather than flexible capacity.

4. Set target reservation utilization
Use a target below 100% if you want the modeled reservation size to include some utilization buffer.

5. Review reserved and uncovered demand
The result shows the estimated reservation size and the projected demand left outside the coverage target.

Projected peak demand = Current peak demand × (1 + Growth rate) Covered demand = Projected peak demand × Coverage rate Recommended reserved capacity = Covered demand ÷ Target reservation utilization

The reservation buffer equals Recommended reserved capacity − Covered demand. Projected demand not covered equals Projected peak demand − Covered demand.

What the result means

The main result estimates the amount of reserved capacity needed to cover the selected share of projected demand at the chosen utilization target.

This is a planning estimate. Provider-specific reservation granularity and eligibility can require rounding or a different final purchase quantity.

Given: Current peak demand = 120 units; expected growth = 20%; reserved coverage = 70%; target reservation utilization = 90%.

Calculation: Projected peak = 120 × 1.20 = 144 units. Covered demand = 144 × 0.70 = 100.8 units. Recommended reserved capacity = 100.8 ÷ 0.90 = 112 units.

Result: The model recommends about 112 reserved capacity units, leaving 43.2 units of projected peak demand outside the reserved coverage target.

Why not reserve 100% of projected peak demand?

You can set coverage to 100%, but many workloads have variable or uncertain demand. A lower coverage target can preserve flexibility for on-demand or other capacity types.

What does target reservation utilization do?

It controls how tightly the reserved pool is sized around the covered demand. A 90% target means the covered demand is planned to consume 90% of the modeled reserved capacity.

Should current demand be average or peak?

Use peak demand for this sizing model because the goal is to determine a capacity quantity. If your reservation strategy is intentionally based on steady baseline demand, use a peak or percentile measure that represents that baseline policy.

Can expected growth be negative?

Yes. A negative growth assumption can model declining demand, provided the resulting projected demand remains nonnegative.

What should I do after estimating capacity?

Use the estimated reservation quantity to evaluate unit cost, total commitment cost, utilization, and savings relative to flexible pricing before making a purchase decision.