Interest when offering funding is calculated based on the amount you offered, the rate, and the amount of time it was offered for. It also includes funding *fees (15% or 18%, depending on if it was a hidden order).
Daily Interest earning: amount * rate% * (seconds / seconds in a day) * (1 - fee%)
Please see below an example of how interest and fees are calculated when you offer margin funding:
For instance, if you have an active 10,000 USD offer at 0.06%, you would, therefore, earn 6.00 USD per day.
Let's say that the funding was offered for 12 hours exactly, this would earn you 3 USD. On those 3.00 USD, a 15% *fee is charged (0.45 USD) by Bitfinex making your final earning 2.55 USD for that offer.