Free tools
Free CRO calculators
Plan a test, check the result, and size the revenue. Every calculator runs in your browser, needs no signup, and shows the formula it uses.
Plan a test
Sample size calculator
How many visitors per variant you need to detect a given lift at your chosen confidence and power.
Open calculatorTest duration calculator
How many days a test should run, given your traffic, baseline rate, and the lift you want to detect.
Open calculatorMDE calculator
The smallest lift a test can reliably detect, given your traffic and how long you can run it.
Open calculatorMeasure performance
Turn a single test into a running program
These calculators size a single test. Spar runs the whole loop: it audits your storefront, ranks the gaps by revenue impact, drafts the variations, and measures the result. Start a 14-day free trial.