Basic formula
Conversion rate = conversions divided by relevant visits, multiplied by 100. Define whether you use sessions, users or another traffic unit and stay consistent.
Avoid benchmark obsession
A 'good' rate depends on traffic source, offer, price, audience intent and conversion event. Compare like with like before using a generic benchmark.
Segment before diagnosing
Break performance down by source, device, campaign and audience when possible. An overall rate can hide a strong segment and a weak one.
Use rate with volume
A higher percentage on tiny traffic may not matter more than a slightly lower percentage at meaningful scale. Pair conversion rate with absolute conversions and economics.