How to test that a knife is sharp enough?

I usually notice when cutting onions and tomatoes. With a very sharp knife cutting an onion doesn't cause much tears at all. As it dulls though it does more crushing than slicing which releases more gas into the air, which makes you cry more. Ripe tomatoes help because they are so tender. If it becomes difficult to slice a tomato without crushing it, your knife is too dull.

A well used, properly cared for knife should require sharpening every 6-18 months, depending on usage. Mine get sharpened yearly.

You may find it beneficial to read these related questions, and their answers:

• How can I safely improve my cutting technique
• How should I care for my knives?
• Why not put knives in the dishwasher?

The most readily evident way of determining if a knife needs to be sharpened is when you notice that you're having to apply more force than normal.

When you start out with a sharp knife you will become accustomed to how it glides through food. Over time you're going to notice that you are having to apply more pressure than normal and that's when it's time to have it sharpened.

Factors that affect how frequently knives need to be sharpened include:

-The type of knife itself: Forged knives, if properly cared for will typically hold and edge longer than stamped knives.

-Care for the knife: Washing knives in dishwashers wears down the edge quicker. Storing loose in drawers without a blade guard will also cause them to dull quicker.

-The manner in which you use the knife: The "Whack" "Whack" noise that so many people associate with cutting is an audible clue that you're cutting incorrectly. The "Whacking" of the blade against the board is caused by pushing the blade downward rather than forward. Cutting straight down against the board dulls the blade through the blunt force pressure against the cutting surface and it also results in smashing and crushing the food instead of providing a clean cut.

-The surface that you're cutting on: The harder the surface the more damage it will do to your blade. Don't cut on surfaces composed of tempered glass, stone (natural or man-made products), solid surfaces such as Corian, metal, or hard plastic.

-The frequency of use: The more often a knife is used, the more frequent it will need to be sharpened.

I usually hold up a single sheet of newspaper and poke the point of the knife through. If you can make a downward cut without the paper tearing, the knife is sharp.

There's the old boy-scout test where you see if it will catch on your fingernail. It's not exactly sanitary for commercial kitchens, but if it tacks on your nail, you're golden!

Hold the edge up to a light source. If the light glints off the edge, it isn't sharp enough. Back to the grind stone...

Hold the knife straight out with the sharp-edge upwards.

Drop a silk scarf on it.

The silk scarf should be cut neatly in half as it falls to the ground.

If it grabs the hair on your arm and shaves it, then it is very sharp (almost too sharp for many uses). Like your ordinary utility work knife. I usually keep one blade this sharp in my pocket knife and the others sharp but not that sharp. A quick slip and you're off to the emergency room. I do find that I can only get it super sharp with honing steel rather than diamond steel.

• If you have a supply of cheap sponges, try if you can easily cut into them (not into the scrubbing side!), or even slice corners off.

• Tomato sideways: take a (preferrably damaged, useless) tomato, cut enough off so it can stand on a surface without being easily toppled. Try to slice off slices without holding the tomato in place. Refrain from the tomato drop test - it doesn't work on all blade geometries regardless of how sharp they are, and can also damage the edge.

• As mentioned above, leg or arm hair shave tests - DANGEROUS but valid for judging the EDGE, but will not show geometry problems (too thick behind the edge, unfavourable shoulders...)

Sharpening (without going to a finer stone) more than needed when already as sharp as the stone you are using can get it, by the way, will make most types of knives WORSE in performance because you are thickening the geometry.