Lichess is a free Chess Server – one of the best out there in fact. It comes with free engine (Stockfish) analysis, and many other nice features. It is well known that Lichess ratings are inflated compared to FIDE / USCF / most other rating systems. The following are some of the best conversion systems to convert Lichess rating to other ratings.

1) Dudeski_robinson’s Formula

FIDE Rating = 187 + Lichess Classical Rating X 0.38 + Lichess Blitz Rating X 0.48

Dudeski_robinson’s formula is pretty scientific, he actually uses linear regression out of real data to produce the above formula.

A rough estimate would be:
Fide ELO = Lichess Classical – 170
or
Fide ELO = Lichess Blitz – 80

Source: Lichess Forum

2) Chess Rating Comparison 2016 (Google Sheets)

This is also pretty scientific, with the added plus that it also compares between USCF and Chess.com, in addition to FIDE.

Source: Google Docs

Discussion: Reddit