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topic 151

Where to find the meaning of German finish note RZ

A discussion started in 1996 but continuing through 2018


Q. Need info on German finish notes. Where can I get it? Example Rz80, Rz40, Rz20.




A. In the US, surface texture is described by the use of Ra, which is the arithmetic average of the surface texture, typically determined by a profilometer. German usage, instead, frequently reports Rz, which is the arithmetic average of the five extremes in any set of data measured. Thus simple conversion is impossible; the instrument must be programmed properly for the correct values to be determined. German instrument makers provide equipment capable of both types of measurements, no surprise, I suspect.

Wolf Penzel

November 11, 2008

thumbs up signThanks, Wolf.

Indeed, Michael, a real conversion isn't possible. As a parallel, think of trying to convert horsepower to torque. Small engines generate little torque and horsepower whereas large engines generate high torque and horsepower, so the two parameters "sort of" "tend to" track each other, but conversion is impossible.

It is sometimes theoretically possible to do calculations like "what is the maximum Ra we can have if we must meet an Rz of so and so?", but the problem is that you need a really super smooth profile in one system to assure even a moderate smoothness in the other, because the measurement methods are different, so it's not practical.

See letter 20490 for more on this issue. Good luck.


Ted Mooney, Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey

Correlation between Rz and Ra


Q. I am trying to quote a german print in DIN. I am being given a surface finish of Rz10. I understand there is no direct correlation between the Rz and Ra measurements but is there a conversion that states that Rz10=Ra16 or Rz30=Ra63? These are just random numbers that I have stated, is there any legitimate conversion existing?

Vernon Finfrock
- Mishawaka, Indiana, USA


A. Hello Vernon!

You want to surf on over to and read what they have to say about how Ra and Rz are calculated, and then you'll see why there is difficulty in correlating them.

Good luck!

lee gearhart
Lee Gearhart

To minimize searching and offer multiple viewpoints, we've combined multiple threads into the dialog you're viewing. Please forgive any resultant repetition or failures of chronological order.

Convert Rz ro Ra


Q. I need help converting the Rz finish call out on some German part drawings. I don't know what factor to use to convert them to Ra. Thanks for your help. Brian

Brian Kirchner
- Clinton Twp., Michigan, USA


A. Brian: Rz is not a Ra reading. According to the German DIN standard, is just another name for Rtm in the American nomenclature. (over five cutoffs)

Rtm is the average of the maximum peak-to-valley depth of each sampling length (usually five cutoffs) over the evaluation length.

A good website for surface measurement terms and parameters is Good luck. Bill

Bill Boatright
- Raleigh, North Carolina


A. Ra and Rz are different parameters of roughness.

Ra is the average roughness of a surface.

Rz is the difference between the tallest "peak" and the deepest "valley" in the surface.

You must to verify is your instrument (rugosimeter) is able to show you the Rz.

There is not one parameter to convert from one unit to the other, because the Rz depends of the way you are obtaining the roughness (machining, grit blasting, polishing, etc.)

Jordi Pujol
- Barcelona, Spain


A. The response by Bill Boatright is more precise. I have seen conversion charts but don't know how well they correspond to reality.

Don Whitbeck
- Meadville, Pennsylvania, USA

Chart to Convert Rz to Ra

February 1, 2018

Q. Hi, I do not believe that there is no comparison between Rz and Ra. Maybe there is not direct conversion between those two, but today is possible to measure the surface in Rz first then measure the same surface in Ra. Then someone could make a table of direct comparison between those two.


Bret Durisin
- mayfield Village, Ohio USA

February 2018

A. Hi Bret. It is indeed a simple thing to convert inches to meters, or pounds to kilograms, or Centigrade to Fahrenheit because they are measuring the same things.

But you can study travel time by air vs. travel time by rail between thousands of cities, make a huge table, and calculate that travel by air is usually about 3X faster on average ... but even if you know that travel by air from Point A to Point B will take 3 hours, you would be crazy to count on the train taking 9 hours :-)

Similarly, Rz and Ra are not measuring the same thing; so there is some general correlation but no 'conversion' is possible.


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

February 1, 2018

thumbs up sign Hi Ted,

Thank you for your reply. I understand that there is not a simple conversion between Rz and Ra. But to me finish is a finish and roughness and a roughness. It does not matter what we call each.

I am just saying that engineers are like attorneys. Some can take a simple thing and complicated so bad that it will become a nightmare.

Many engineers and/or machinists today have a hard time to to do conversions between millimeters and inches. They are asking how much is 10 millimeters or so. To me when working with millimeters take a metric scale and when with inches take scale which have graduation in inches. Very simple.

best regards


Bret Durisin [returning]
- Mayfield Village, Ohio, USA

February 2018

Hi Bret. I like your idea of just using the right scale instead of converting. So why not just do that for Ra and for Rz? :-)

I have to disagree about finish is finish and roughness is roughness: for example, we in the plating industry know that you can take a roughly polished piece of steel, do self-leveling nickel plating on it, and get a nice shiny mirror finish. But if that steel has a deep scratch, the plating does not level it; rather, the plating amplifies the scratch, making it more obvious and more ugly. To us the depth of the single deepest scratch is the whole story, the difference between a plateable part and a non-plateable one; neither Ra nor Rz tell us what we need to know.


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

February 3, 2018

A. Hi Bret!

The National Physical Laboratory of the UK has published a nice FREE guide to measurement of surface finish using stylus instruments, and you can find it at
If you read the appropriate sections, you'll realize why the conversion chart you seek is more complicated than it may seem at first glance.

All the best!

lee gearhart
Lee Gearhart

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