UL (Underwriters Laboratories) Fire-Resistant Ratings Explained

UL (Underwriters Laboratories) Fireproof Ratings Explained

If you are choosing or comparing fireproof safes, you may have seed different fire ratings used to describe the fire-resistant capabilities of such safes. Among all these ratings one of the most common is provided by UL. They can be displayed as as UL Class 125, or UL Certified 350, or UL rated 150, etc. And then followed by a duration of 1-3 hours.

In this article we explain what these UL fireproof ratings are, what those ratings numbers mean, and how they can affect your choice of a fireproof safe.

What UL (Underwriters Laboratories) is

UL or Underwriters Laboratories is an independent, globally-recognized organization that sets safety standards and conducts testing of various products, including fireproof safes. UL has been around for over 125 years and developed their own UL fire resistant ratings, which can be granted to safes that passed their tests.

How UL Fire Resistant Ratings Are Determined

UL fire resistant ratings are determined through a series of tests during which the safes are exposed to high temperatures for a set amount of time. The tests are conducted in specially designed furnaces, that can simulate various real-life situations.

The tested models can be subjected to not only heat but also explosion impact, as well as drop tests. The goal is to check how a specific model can protect its contents from fire and other associated damage. As well as to keep the temperature inside the safe below a certain temperature for the set duration. Only if the items inside the safe remain undamaged and the temperature doesn’t rise above the limit, then the safe passes the test.

UL Fire Resistant Ratings Explained

UL fire-resistant ratings are expressed in Hours and Class. There different UL fire resistant ratings, each providing a certain level of protection.

Depending on the rating a specific type of items can be stored and protected from fire in such a safe. Here are a few of the most common ratings:

  • UL Class 125
  • UL Class 150
  • UL Class 350

What does the number in a fire rating mean?

The number 125, 150 and 350 equals to the maximum temperature level (in Fahrenheit) that the inside of the safe can reach in a fire. Here’s what each UL rating means:

UL Class 125 Fire Rating meaning

UL Class 125 Means that the temperature inside the safe will not rise above 125°F (~52°C).

UL Class 150 Fire Rating meaning

UL Class 150 means that the temperature inside the safe will not rise above 150°F (~65°C).

UL Class 350 Fire Rating meaning

UL Class 350 indicates that the temperature inside the safe will not rise above 350°F (~175°C).

UL Fire Rating duration explained

Then there’s always the duration that is described in the fireproof rating. It can be 30 minutes, 1 hour, 2 hour or more. So what is the 1 Hour UL fire rating? or What does the 2 hour UL fire rating mean?

 It means – for how long (1 hour or 2 hours, or whatever the rating states) the temperature inside the safe will not rise above the certain level. For example:

  • UL Class 350 2-hours rating means that the temperature inside a safe will not go above 350°F(~175°C) for at least 2 hours in case of a fire.
  • UL Class 350 1-hour rating means the temperature should not go higher that 350°F(~175°C) for duration of 1 hour.

Here’s a table that shows various UL fire ratings compared:

UL fire resistant ratings for safes explained

It also shows the max external temperature outside the safe at which is has been tested in order to get the appropriate fireproof rating. On practice it means that if the heat levels during the house fire goes even higher than the max external limit, than the temperature inside the safe may exceed the expected values.

However, according to the US Fire Administration, the average temperature of a house fire is around 1,100 degrees Fahrenheit (593 Celsius). Thus, the max external temp limit set during UL testing should be enough to provide the appropriate protection.


You will not necessarily see the exact definition of a fireproof rating in the safe’s description. The manufacturers may not even display it as UL Class 350 1-hour. Instead, they can simply state that the safe has been tested to keep the inside temperature below 350°F(~175°C) for the duration of 1 hour, for example.

Why understanding UL Fire Ratings Is Important:

Understanding UL fire resistant ratings can help you to choose fireproof safes that will protect your valuables in the event of a fire. The better the UL fire resistant rating, the longer the safe will protect its contents from the heat of a fire.


UL fire resistant ratings are an important factor to consider when choosing a fireproof safe. By understanding what they mean and how they’re determined, you can make an informed decision about which safe is right for your needs.

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