Availability Zone Redundancy Cost Calculator

Work out availability zone redundancy cost instantly with clear inputs, formula shown and shareable results.

About Availability Zone Redundancy Cost Calculator

Surviving the loss of one availability zone means the remaining zones must carry the whole load, so instance count multiplies by z/(z-1): 50 percent overhead with two zones, 33 percent with three, 25 percent with four. That is the quantitative case for three zones. Cross-zone traffic is charged in both directions and is easy to overlook.

Formula

Zone redundancy

instances = ceil(capacity x zones / (zones - 1)) rounded up per zone; overhead = (instances - capacity) / capacity

Worked example

With the default values below, Availability Zone Redundancy Cost Calculator returns Monthly cost with zone redundancy of $2,680.00. Change any input above and every figure here is recalculated in your browser.

Inputs used

Instances needed for capacityminimum 1: 12
Availability zonesaccepts 1–6: 3
Monthly cost per instanceminimum 0: 140
Cross-zone traffic per monthminimum 0 TB: 8 TB

Results

Monthly cost with zone redundancy: $2,680.00
Instances required: 18
Overhead above bare capacity: 50.0%
Cross-zone data transfer: $160.00

Step by step

1Values used
2Zone redundancy
3Monthly cost with zone redundancy= 2,680.00
4Instances required= 18
5Overhead above bare capacity= 50.0
6Cross-zone data transfer= 160.00

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax or compounding frequency into account.
Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why not use two zones to save money?

Two zones means each must carry 100 percent of the load alone, so you pay 50 percent overhead rather than 33 percent for three — and lose more capacity per failure.

How is cross-zone traffic reduced?

Zone-aware routing so requests are served within the zone they arrive in, and topology-aware replica selection for reads. Both cut the charge substantially.