Trial conversion rate = (converted trials ÷ trials in the cohort) × 100.
Numerator: converted trials
The count of trials that produced a paid subscription within the stated window. Decide in advance whether a paid subscription means a successful first payment, an accepted order, or a signed contract, and whether a discounted or internal account counts. Refunds and immediate cancellations should be removed, otherwise the metric counts money the business never kept.
Denominator: trials in the cohort
Here there are two accepted definitions, and both are in common use. Measured on starts, the denominator is every trial that began during the period. Measured on completions, it is only the trials that ran to the end of the trial period. The starts version answers "how good is the whole funnel". The completions version answers "how good is the closing conversation with people who stuck around". Use whichever you like; say which one you used.
The window
The period after the trial starts within which a payment still counts, for example thirty days from trial start or fourteen days after trial expiry. Fix the window, and do not report a cohort until it has fully matured, or recent months will always look worse than older ones for purely arithmetic reasons.
The unit
Accounts or users. In a team product, five people from one company starting five trials is one commercial opportunity. Counting the users rather than the account depresses conversion and hides what is really happening.