What is a Rockwell Hardness Rating?

The Rockwell Hardness Rating (sometimes known as a HR) is a unit of measurement for calculating the strength of a variety of materials. It’s a fantastic way of understanding how strong an item is and focuses on something called ‘indentation hardness’. Based on this, the material is given a number on the Rockwell Scale and you can use this to compare two items and understand which one is better quality.

COMPARING THE HARDNESS OF TWO KNIVES

If we compare two chefs knives from the Nisbets range, we can see how the Rockwell scale can help us pick the best knife for our needs. Knife 1 is from the Hygiplas range, whilst Knife 2 is from the premium Tsuki range.

KNIFE 1

Blade length: 8½"
Blade Material: Stainless steel
Rockwell Hardness Rating: 52°

KNIFE 2

Blade length: 8½"
Blade Material: Stainless steel
Rockwell Hardness Rating: 60°

In this case, although the two knives seem very similar and have the same length blade, Knife 2 is made of steel with a higher indentation hardness. Based on this, the Rockwell hardness rating suggests knife 2 is made of stronger and harder stainless steel.

So, does that mean this is ‘a better knife’ for you? It isn’t necessarily that simple!

WHAT IS A GOOD ROCKWELL SCALE RATING FOR KITCHEN KNIVES?

knife which chopped herbs on wooden chopping board

A high Rockwell Hardness Rating is often associated with durable blades that keep an edge. A high rating means the steel is hard, retains its edge for a long time, but may also be slightly more brittle than other knives. The Tsuki knife range is hardened up to a 60° hardness rating, making it a firm favourite of experienced chefs wanting high quality knives which retain their edge.

By comparison, knives which may encounter more rough treatment or be used by student chefs may benefit from a lower Rockwell rating. This means that whilst blades will lose their edge slightly more quickly, they are less likely to chip or become damaged when stored incorrectly or used on unsuitable surfaces such as stainless steel worktops. The Hygiplas range of affordable chef knives are perfect for this, with a slightly lower HRC rating of around 50°.

HOW DOES THE ROCKWELL SCALE WORK?

The Rockwell scale is over 100 years old and works by using a small piece of incredibly hard material (usually diamond) called an indenter to deliberately dent the material being tested. This dent gets measured, with a smaller dent meaning the material is more tough and gets a higher Rockwell hardness rating.

If you’re interested in the maths, here’s the equation used to calculate the rating:

Where d is the depth of the dent and N and S are known as scaling factors.

Ultimately, however, the main thing to remember is that the higher the Rockwell rating, the harder steel your knife is made from.

WHICH ROCKWELL HARDNESS RATING IS USED FOR KNIVES?

black handled chefs knife resting on stone

HRC is the hardness rating used for commercial knives and other metal utensils.

Confusingly, there are different types of Rockwell or HR rating. These run from HRA through to HRG and the only differences between each rating system are the size/material of the indenter and how hard it is pushed.

When thinking about chefs knives, we use HRC, where a 120° diamond indenter is pushed into the metal with 150 kilogram-force.