What's your standard for determining blade sharpness?

I’m not hung up on needing my knife to be able to shave hair off my forearm. I look directly down on the edge to see if any lights reflecting off it that would indicate there’s more work to be done.

I cut rope, plastic wrappers, mostly cardboard in day to day use. If I’m not struggling doing that I’m happy enough.
 
Last edited:
not the best, and I am sure yours is better, I run it across my thumb nail, which helps feel bad spots. I also do various paper. again not saying I am right, hundred ways to do it.
 
I see if it catches my thumbnail after sharpening. If it is shaving sharp (on my forearm). If the edge is "grabby" when I run my fingers along the blade after sharpening. And edge life. Edge life takes a while to prove out.
 
I just slide it through a sheet of paper to verify it doesn't have a burr, an edge that's rolled over, or a nick. I'd also look at the edge for shiney areas like Frulk mentioned. I can do better than that but don't need to. I'm going to cut tape and cardboard with my pocket knife. My wife insists on smashing the edge of the kitchen knives into plastic cutting boards and washing them in the dishwasher (and I don't dissent). For the same reasons I don't carry a fancy custom knife or buy costly kitchen knives. I don't have junk. I carry Opinel or Case or Mora, and my kitchen knives are some 40-year-old Henckels. If I buy anything, it will be like Mercer or Tojiro, not even Shun or Wusthof or Yoshihiro or Miyabi. Those are a little nicer, but I would consider my use to be abusive.
 
Cutting printer paper isn't a bad test. It will let you feel any imperfections or dull spots on the blade as you draw the edge through the paper. The next level up is cutting something like receipt paper that is much thinner and more flexible than printer paper. Same as the printer paper test, but requires a bit more sharpness to get good results. I wouldn't feel bad if I had to use paper as a testing medium.

Just be aware that there is a certain amount of technique involved in cutting paper with a knife, and one can make a knife look sharper, or more dull than it really is without much effort. Starting the cut a long way from the point where the paper is held will make the knife seem more dull. Trying to push cut as opposed to drawing the knife through the paper will make it look more dull than it really is. Drawing the knife through the paper very slowly will make it seem more dull than it is. Not starting the cut with the line of the cutting edge perpendicular to the line of the edge of the paper will make it seem like the knife isn't catching the paper edge and therefore make it seem like the knife is more dull than it really is. Trying to cut straight into the piece of paper as opposed to cutting slivers off it will make the knife seem more dull than it really is.

If you just want to feel for imperfections, take a piece of plastic with a sharp corner/edge and lightly pull the cutting edge of the knife across it as if you were sawing into the piece of the plastic. Don't actually apply cutting force-you're just trying to feel if the cutting edge moves smoothly over the plastic. If there are chips or rolls or dull spots, you will feel them as hitches or jerks as the cutting edge slides over the plastic.

Shaving arm hairs is a decent test. This one was all I used for many years. Splitting hairs (hair whittling) is kind of an interesting test. This is sort of like shaving with the edge above the skin so the edge has to catch on the hair without anything "backing" the hair. To me, this is more of a bragging rights thing than it is practical. I can't tell the difference between an edge that will split hairs and an edge that will shave conventionally when I cut something practical.

The ripe tomato test is a reasonably practical method of seeing if a knife is sharp.

Cutting something like a carrot is a useful test since it also depends on the knife geometry being pretty decent. It's one thing to have a sharp edge, but if the bevel/grind tapers up too fast, you're still going to be wedging whatever you're cutting apart and that will make the knife feel more dull.
 
I pass the knife 1/4" above my forearm and if the hairs jump off my arm in screaming terror, I know the blade is sharp enough!

In real life, I just hold the edge to catch the light and look for shiny spots like others have mentioned. Also, I lightly drag my thumb across the edge each way as a quick check for a rolled edge.
 
If I can easily split a hair ... it's sharp !

You sharpen knives long enough and you can feel the sharp edge with the ball of your thumb ...
Another trick is the blade edge will not slide across your finger / thumb nail but rather dig in and grab the nail ... if dull the blade will slide over the nail surface and not "catch" .
Gary
 

This guy’s videos are great. Hair whittling sharpness is attainable.
 
I push the pad of my left thumb AGAINST the cutting edge, perpendicular to the knife's spine, and feel how the edge resists my skin. When I observe someone do the same thing, but pushing WITH the cutting edge I know they know nothing at all about knives. I hold my tongue and just take my newly-sharpened blade back from them.
 
I think it's probably worth noting that most of the sharpness tests can be "gamed" to some extent, or can provide varying results if they aren't administered properly.

For example:
  • There are different grades and types of paper, and as I mentioned, technique plays a role in how well the knife will cut the paper. The tester can take a sharp knife and make it look duller than it is. The tester can take a dull knife and make it look sharper than it is.
  • Hair whittling depends somewhat on the type of hair (curly/thick hair is easier, thin/straight hair is harder), on the speed that the knife is moved over the hair, and, according to some, on the direction of movement of the blade (i.e. from root to tip or vice versa) and on how far from the finger the blade is touched to the hair.
  • Shaving depends somewhat on hair type and on pressure. A knife that isn't quite shaving sharp can sometimes be "coerced" into shaving with a bit more pressure.
  • The Edge On Up testers can be gamed a little by the speed at which the pressure is applied as well as by introducing sawing motion or tilting the blade as it is pushed into the test fiber.
Feeling for sharpness with a thumb or finger is subjective.
Checking for rolls/chips/dull spots with a thumbnail or piece of plastic will tell you if the edge isn't uniform but doesn't provide a lot of information about sharpness.

Also, even if the edge is really fine/sharp, if the grind/thickness behind the bevel is wrong, pushing the knife through a non-yielding medium (cutting hard stuff) can still be quite difficult. Maybe even more difficult than pushing a very thin blade through the medium even if the edge isn't that sharp. So some actual cutting tests may give misleading results.

The other thing to keep in mind when testing to see if a knife has been dulled is that dulling isn't necessarily a uniform effect. It's actually kind of rare to use the entire blade edge to cut something, so it follows that the edge isn't going to be dulled the same all along its length. That means that in tests where an action is taken to dull the blade between successive sharpening tests, the dulling action needs to happen in a controlled part of the blade and the sharpness test will have to be administered to the part of the blade that is being dulled in the test.
 
I bought my brother one of those for Christmas, he’s been a knife sharpening guru for decades now. He loves the thing, I knew he would, his knives were all between 100 & 300 when I gave it to him, his 2 main pocket knives were both below 200. Now he’s playing with different stones and such competing with himself.

If something shaves, it’s good enough for me. I use the little work sharp belt sander, which my brother calls cheating, but it sure is easy.
 
Back
Top