There are two main measurements for web fonts. Points and Pixels (pt and px). The ratio between the two is 0.75:1. This means each pixel is equivalent to 0.75 pt. This means that a font of size 8px would be equal to a font of 6pt. I have created a simple chart or table below which represents most common pixel to point conversions:

Points | Pixels | Ems | Percent |

6pt | 8px | 0.5em | 50% |

7pt | 9px | 0.55em | 55% |

7.5pt | 10px | 0.625em | 62.5% |

8pt | 11px | 0.7em | 70% |

9pt | 12px | 0.75em | 75% |

10pt | 13px | 0.8em | 80% |

10.5pt | 14px | 0.875em | 87.5% |

11pt | 15px | 0.95em | 95% |

12pt | 16px | 1em | 100% |

13pt | 17px | 1.05em | 105% |

13.5pt | 18px | 1.125em | 112.5% |

14pt | 19px | 1.2em | 120% |

14.5pt | 20px | 1.25em | 125% |

15pt | 21px | 1.3em | 130% |

16pt | 22px | 1.4em | 140% |

17pt | 23px | 1.45em | 145% |

18pt | 24px | 1.5em | 150% |

20pt | 26px | 1.6em | 160% |

22pt | 29px | 1.8em | 180% |

24pt | 32px | 2em | 200% |

26pt | 35px | 2.2em | 220% |

27pt | 36px | 2.25em | 225% |

28pt | 37px | 2.3em | 230% |

29pt | 38px | 2.35em | 235% |

30pt | 40px | 2.45em | 245% |

32pt | 42px | 2.55em | 255% |

34pt | 45px | 2.75em | 275% |

36pt | 48px | 3em | 300% |

Hope this reference helps and saves you from having to do a manual conversion ;)