When you look at the front of a fire extinguisher, you will see a large, bold code printed above the instructions. It usually looks something like 13A, 21A, or 55B.
Most business owners assume that buying a “6 Litre Foam” extinguisher is enough to tick the safety box. However, the physical size of the canister is actually the least important factor. What matters is the Rating.
The rating tells you exactly how powerful that extinguisher is. A high-performance 3-litre unit can often outperform a cheap 9-litre unit. If you do not understand these ratings, you risk spending money on heavy equipment that is legally insufficient to protect your floor space.
At Fire Response UK Ltd, we ensure that every extinguisher we specify meets the strict mathematical requirements of BS 5306. This guide explains what those numbers mean and how to use them to calculate your true fire safety needs.
The Quick Answer
The code on an extinguisher consists of a Number and a Letter.
The Letter indicates the Class of Fire (Fuel Type) it can extinguish (e.g., A = Wood/Paper, B = Flammable Liquids).
The Number indicates the Size of Fire (Power) it can extinguish based on controlled laboratory tests. For example, a 13A rating means the extinguisher can put out a specifically sized stack of wood. The higher the number, the bigger the fire it can handle. For legal compliance, you need a total “A-Rating” of 26A for every 400 square metres of floor space.
The Letters: What Fuel are You Fighting?
Before looking at the power (the number), you must ensure you have the right tool for the fuel (the letter).
Class A: Organic solids. This covers 90% of office risks (paper, cardboard, wood, furniture, textiles).
Class B: Flammable liquids. This covers petrol, diesel, oils, and paints.
Class C: Flammable gases (butane, propane). Note: Extinguishers are rarely rated for this in numeric terms, as you should usually isolate the gas supply first.
Class F: Cooking oils and fats (deep fat fryers).
You will notice there is no “Class E” for electrical fires. We will explain why later in this article.
The Numbers: How Powerful is it?
This is where the confusion usually lies. The number is determined by independent testing houses (like BSI) using strict test fires.
Class A Ratings (The Wood Crib Test)
To get an “A” rating, the extinguisher must put out a burning crib (stack) of wooden sticks. The number represents the length of that wood stack in decimetres.
13A: Can extinguish a stack of wood 1.3 metres long.
21A: Can extinguish a stack of wood 2.1 metres long.
34A: Can extinguish a stack of wood 3.4 metres long.
Why this matters: If you buy a cheap “budget” 6L foam extinguisher, it might only have a 13A rating. If you buy a premium high-performance 6L foam extinguisher (with better chemical additives), it might have a 21A or even 34A rating. You get more firefighting power in the same-sized can.
Class B Ratings (The Liquid Tray Test)
To get a “B” rating, the extinguisher must put out a circular tray full of flammable liquid (usually a mix of fuel and water). The number represents the volume of liquid in litres.
34B: Can extinguish 34 litres of burning liquid.
55B: Can extinguish 55 litres of burning liquid.
Safety Note: Just because an extinguisher is rated 55B does not mean an untrained novice can tackle a 55-litre petrol fire safely. The rating proves what an expert can do in a test. For safety, we usually recommend covering only a fraction of the rated risk.
Class F Ratings (The Fryer Test)
For kitchens, the rating corresponds directly to the size of the fryer.
75F: Can tackle a deep fat fryer containing 75 litres of oil.
Calculating Your Requirements: The “0.065” Rule
Under British Standard 5306-8, you cannot just guess how many extinguishers you need. You have to calculate it based on the floor area.
For Class A risks (general offices, shops, warehouses), the formula is:
Floor Area (m2) x 0.065 = Total A-Rating Required
Example:
You have an office that is 200 square metres.
200 x 0.065 = 13.
You need a total rating of 13A on that floor.
How to achieve it:
Option 1: One 6-Litre Foam extinguisher (rated 13A).
Option 2: Two smaller 3-Litre Foam extinguishers (rated 8A each) giving a total of 16A.
This is why ratings are critical. If you rely on 2kg CO2 extinguishers (which often have NO Class A rating), you have technically provided zero protection for the building structure, regardless of how many you buy.
You must also ensure that:
There is a minimum of 26A total rating per floor (even if the floor is tiny).
No person should have to travel more than 30 metres to reach an extinguisher.
The “Electrical” Rating Myth
Clients often ask us for an “E-Rated” extinguisher.
There is no such thing. Electricity itself does not burn; it is the source of ignition for other materials (plastic casing, circuit boards). Once you turn the power off, it becomes a standard Class A or B fire.
However, you will see a symbol of a Lightning Bolt on many extinguishers.
This does not mean it is designed for electricity. It means it has passed a dielectric test (usually 35,000 Volts) and is safe to use if you accidentally hit a live cable.
CO2 (Black): The standard choice for electrical risks as it causes no damage.
Powder (Blue): Safe on electricity, but ruins the equipment.
Foam (Cream): Most modern foam units have the lightning bolt symbol (pass the 35kV test), but they should still not be used directly on live equipment unless necessary.
Why “High Performance” Units Save Money
Understanding ratings can actually save your business money.
Imagine you have a large warehouse that requires a total rating of 210A.
Scenario A (Cheap Units): You buy budget water extinguishers rated at 13A. You would need 17 extinguishers. That is 17 units to buy, 17 to commission, and 17 to service every year.
Scenario B (High Performance): You buy advanced additives extinguishers rated at 34A. You would only need 7 extinguishers to cover the same legal requirement.
Although the individual units cost more, you save massively on installation and long-term maintenance costs.
Conclusion
Fire extinguisher ratings are the scientific proof behind the safety equipment. They allow us to move away from guesswork and towards a calculated, compliant fire safety strategy.
When you purchase extinguishers, look beyond the price tag and the litre capacity. Look at the number before the letter.
If you are unsure whether your current provision meets the “0.065” calculation, or if you are worried you have the wrong ratings for your specific risks (like solvents or lithium batteries), contact Fire Response UK Ltd. Our engineers can conduct a full Fire Risk Assessment to ensure your maths adds up to safety.
For more information on the testing standards, you can view the guidelines from the British Standards Institution (BSI).
