The trial conversion rate is the share of trials that became paying customers, measured per cohort of trials that started in the same month.
Trial conversion = trials of the cohort that paid at some point ÷ decided trials of the cohort
Measure by cohort. Payments in June divided by trials in June mixes May's trials with June's and moves with the calendar, not with the product.
Trials that are still running are undecided and belong in neither side of the fraction.
sumwerk's funnel shows, per month a trial started, how many converted, are still running or did not convert, and the median days to the first payment. Trials can come from Stripe or from your product through the API.
A restricted, read-only key is all it takes. The first import runs in a few minutes; every number you see opens into the customers and Stripe references behind it.