Fire ratings measure the amount of time that a door is able to contain the spread of fire, as tested according to NFPA guidelines. This means a door with a two-hour rating, for example, can be expected to resist combustion for two hours in the event of a fire. Also, a fire door will typically have three-quarters of the rating of the surrounding wall. For instance, a wall with a two-hour rating will usually contain doors with a 90-minute rating. Overall, a fire rating can range from 20 minutes to three hours, but keep in mind that 20-minute doors are designed to block smoke rather than flames. The rating requirements for each type of door and wall in a building are set by a fire marshal or by building codes.