Spot Instance Savings Calculator

spot rate = on-demand × (1 − discount); effective discount = 1 − (spot cost ÷ useful hours) ÷ on-demand rate, where useful hours exclude work lost to interruptions. Spot bills at a floating market rate for spare capacity, so the headline saving is straightforward — but you also pay for the partial work destroyed when AWS reclaims an instance with two minutes' notice. Dividing the Spot bill by the hours that actually produced finished work gives the effective discount, which is the figure to compare against a Savings Plan.

Spot only pays off for interruption-tolerant work: at a few interruptions a month the effective discount barely moves, but on a long non-checkpointed job the rework can erase the entire saving.

This calculator takes 6 inputs: On-demand rate per hour, Spot discount vs on-demand, Instances in the Spot fleet, Hours running per month, Interruptions per instance per month, Work lost per interruption. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.

Anything that cannot be killed with two minutes' notice: single-node stateful databases, licence-locked software, and long batch jobs without checkpointing. Diversifying across many instance types and Availability Zones in a Spot fleet is the standard mitigation for everything else.