What Is the Spot Instance Savings Calculator?
Cloud providers like AWS, Google Cloud, and Azure offer spare compute capacity at steep discounts through spot (or preemptible) instances. This calculator helps you quantify exactly how much you save by running workloads on spot pricing instead of standard on-demand pricing. Enter the two hourly rates and your expected usage hours, and it returns both your savings percentage and the total dollars saved.
How to Use It
Enter three values: the on-demand price per hour (the standard rate), the spot price per hour (the discounted rate), and the number of usage hours over the period you care about. A typical full month of continuous use is about 730 hours. The tool instantly shows your percentage discount along with on-demand cost, spot cost, and total savings.
The Formula Explained
The savings percentage is the difference between the two prices divided by the on-demand price: \(\frac{\text{On-Demand} - \text{Spot}}{\text{On-Demand}} \times 100\). The total dollars saved multiplies the per-hour price difference by the number of hours: \(\left(\text{On-Demand} - \text{Spot}\right) \times \text{Hours}\). These two figures together tell you both the relative discount and the real budget impact.
$$\text{Savings \%} = \frac{\text{On-Demand} - \text{Spot}}{\text{On-Demand}} \times 100$$$$\text{Total Saved} = \left(\text{On-Demand} - \text{Spot}\right) \times \text{Hours}$$
Worked Example
Suppose an on-demand instance costs $0.10/hour and the equivalent spot instance costs $0.03/hour, running for 730 hours a month. The savings percentage is $$(0.10 - 0.03) \div 0.10 \times 100 = \mathbf{70\%}.$$ The total saved is $$(0.10 - 0.03) \times 730 = \mathbf{\$51.10}$$ for that month, with on-demand costing $73.00 versus only $21.90 on spot.
FAQ
Are spot prices guaranteed? No. Spot prices fluctuate with supply and demand, and instances can be reclaimed with little notice, so use them for fault-tolerant or interruptible workloads.
What does 730 hours mean? It is the average number of hours in a calendar month (\(8{,}760 \div 12\)), commonly used for monthly cloud cost estimates.
Does this include data transfer or storage? No, it covers only compute instance hourly pricing. Add storage, networking, and other charges separately for a full bill estimate.
