Noob question about blade size

Hi everyone,

Just joined. Have question about blade size. Have done due diligence in research but apologies in advance if this has already been discussed to death.

I've just come into a Dubl Duck Satinedge in what appears to be all original condition. Measured the blade. It's 18mm from top of spine to edge. In the "The Straight Razor" Wiki 2.2, it says: "an 18mm large blade is called a 5/8." 18 mm = 0.7 inches, whereas 5/8 (0.625) inches = 15.8mm. So why is an 18mm blade not a 6/8 ( = 19mm)???

Attachment 87024

Would it be because a 6/8 is .75?

Many would also say it is a 11/16 blade :)

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Originally Posted by ace

Would it be because a 6/8 is .75?

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Exactly the problem.

5/8 = 0.625
6/8 = 0.75
18mm = 0.7

So 18mm is "closer" to a 6/8, being only 0.05 inches off rather than 0.075 inches...which is what prompted the question.

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Originally Posted by Zephyr

Many would also say it is a 11/16 blade :)

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Thanks Zephyr. I hadn't seen blade sizes commonly expressed in 16th of an inch...guess I missed it.

You also have to consider that different people and razor companies measure differently. Some measure from the top of the spine and others measure from where the spine touches the stone.

I've found it much simpler to read the tape measure myself. Ofc that's not helpful, but I'm a little confused by numbers, and wonder why you wouldn't just measure it. Are you curious if the wiki is wrong? Is that the question? Good eye by the way, it could have said 2 inches equals 9/8ths, and I'd have said ok. Lucky for me, the tape measure is never wrong.

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Originally Posted by regularjoe

I've found it much simpler to read the tape measure myself.

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Which is what I did. The blade measures 18mm. Which is 0.7 inches. Which to my mind is closer to 6/8 (0.75")...but then I read the Wiki, and it said 18mm is a 5/8 (0.625").

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Originally Posted by regularjoe

Are you curious if the wiki is wrong?

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I'm curious what the proper measure is for this blade. Is it a 5/8 or a 6/8?

I'm not doubting the Wiki, b/c you all know waaaaaay more about razors than I ever will. But I am wondering why an 18mm blade is considered a 5/8, when it's closer to a 6/8. Is that just convention, b/c it's always been that way? Or what?

OK, sorry, I don't know. I read a tape measure that shows eights of an inch. 3/4ths is 6/8ths, 5/8ths is 5/8ths. It'd either be a light 6/8ths, or a heavy 6/8ths, depending on where the tape landed. I didn't contribute to the wiki, and I don't measure in millimeters unless it's a coticule.

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Originally Posted by ace

Would it be because a 6/8 is .75?

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0.75" is 19mm

@OP not all 6/8 blades were same dimension there would always be some discrepancy in size. Same for all other sizes 5/8, 7/8 etc.

Because not all Razor companies expressed razor sizes in 16th's DD is one that did...

But the person that wrote the Wiki article might not have realized that...

Also as Maniaman/Stefan just wrote, there is no hard and fast rule for measurements there are a ton of theories floating about but near everyone of them has a flaw....

+1 To Glenn,
Consider also that a .7" blade is NOT a full .75" and the way measurements USUALLY work is you can go over, but you can't go under. A gallon of gas must be at least a gallon of gas, a 12 oz can must contain at least 12 oz, etc. They can over fill and put 12.2oz in a can, but if they get caught putting 11.8oz in they're in trouble.

So you could apply similar logic when measuring blades today. Particularly when looking at resale practices it avoids a headache to designate a .7" blade as 5/8 instead of 6/8.

Of course if you've ever bought lumber all of this goes out the window...

FWIW
Jim

It is of course very difficult to represent metric measurements in inches as rational numbers, particularly when traditionally we only really use 8 or 16 as the denominator. You can, of course, keep expanding the denominator geometrically (so, go to 32ths) but for some reason that doesn't seem to occur with straights, or at least it is very uncommon. Even if you did, you'd still run into the same issue.

So some data loss is necessary when converting from metric to inches (when you constrain the inches data to be rational). The data loss can take one of 3 basic forms:
(1) Rounding to the nearest lowest unit (eg 16th);
(2) Floor: take the number back to the largest previous integer (in the numerator) (eg: 13/32 becomes 3/8);
(3) Ceiling: take the number up to the smallest following integer (in the numerator) (eg: 13/32 becomes 7/16).

In my travels I have noticed that people tend to get upset if you overestimate or round up the width of a blade. Therefore what we most often see is the Floor operator being employed, at least in my experience.

So, for your particular example, 18mm = 0.7086614 inches. This is somewhere between 22/32 and 23/32. Using the Floor function, we take 18mm to be 22/32 = 11/16. So, if you are happy to talk about 16ths of an inch you could leave it there. If, on the other hand, you are prepared to go to 8ths, applying the Floor function again yields 5/8, as appears in the Wiki.

Considering that 1/16th of an inch is 1.5875mm, and assuming most people use a ruler to measure the width, and taking into account the various definitions of what "razor width" actually is, and given we really only care about these things in ballpark terms anyway, I'd say the 5/8ths of an inch definition in the Wiki is pretty OK, and in fact I would go further and say it is actually the most conservative, and hence honest, appraisal of the inch equivalent to 18mm. However, if you wanted to get pedantic about it, and you were very very sure about both the accuracy of your measurement device and the way you measured the width (as in, the definition of razor width - which, by the way, is not set in stone) then you might want to adopt the 11/16ths conversion.

James.

Where's Heisenberg when you need him?:hmmm:

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Originally Posted by FoxAndFish

Measured the blade.

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'Size doesn't matter it's the way that you use it' However, one of my female friends did say that nine out of ten women would disagree with that statement.

Thank you much to everyone who contributed to this thread and to my better understanding of blade size.