Michael Woloszynowicz does some great leg work to back into what needs to happen for Dropbox to be profitable.

My guess is that the low usage numbers for free users, and average usage number for paid users are the closest to reality. With that in mind it looks like Dropbox needs a conversion rate somewhere in the 1.8% to 2% range. Since freemium conversion rates run anywhere from 0.5% to 5% this is neither a low nor high hurdle, and is certainly attainable. Dropbox’s challenge is that these numbers will constantly shift as they depend heavily on the speed at which free users convert vs. how quickly new free users sign up and consume their space. If the rate of new sign-ups drastically outpaces conversions, then Dropbox will need to seek further venture financing. The final thing to consider is that Dropbox’s total funding is in the $7.2M – $10M range. Given the high burn rate of $3.1M-$5.8M per month, Dropbox must have a good base of paid users already as they wouldn’t be able to survive on outside financing alone.