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A more robust explanation of the whats and whys of the conversion rate.
Note: I'm talking about a conversion rate in marketing here, usually in online marketing, where you want to compare the rate at which people buy or achieve a goal. Not currency exchange.
(goal / pool ) x 100 = conversion rate
The goal is usually a sale, but not always. It can also be a free signup, a lead, clicking a certain button, calling a phone number, etc.
The pool is the number of people who could have achieved your goal, but might or might not have.
And the conversion rate is the de facto standard for comparing how well a piece of marketing is performing, mostly used in internet marketing but actually pretty useful in most marketing or sales environments. You can read more about how conversion rates in marketing are used.