How to tell if a knife is sharp

No glint, clean paper, no pressure on the skin: that is a sharp knife

Three tests reveal the state of an edge — the light check, the paper test, and the tomato test. Run them in that order. The light check covers the whole edge at once and takes seconds. The paper test shows whether the edge bites. The tomato test is the final word on whether the knife is ready for the counter.

All three together require nothing but a lamp, a sheet of printer paper, and a ripe tomato.

Look along the edge toward a light source

Hold the knife by the handle, tip pointing away from you, edge up. Bring it under a bright lamp or face a window. Instead of looking down onto the flat of the blade, look along the edge itself, heel to tip, the way you would sight down the spine of a book.

Sharp steel is ground to a point: there is no flat surface to catch the light, so the edge looks dark. A stretch that has worn flat or bent slightly creates a small horizontal face that reflects the lamp straight back toward your eye as a bright spot, like the glint on a coin's edge. WebstaurantStore's knife sharpness guide includes it among home sharpness tests.

Draw your eye from heel to tip. Every glint marks one stretch that needs work. A blade with no glints from end to end passes this check.

The paper test: draw the blade through a hanging sheet

Take a sheet of printer paper, pinch one corner, and let it hang loosely. With your other hand on the knife handle, draw the blade through the page from heel to tip in a single smooth stroke, edge leading.

A sharp knife cuts a clean line the full length of the blade. America's Test Kitchen uses it when reviewing knives; Zwilling, Victorinox, and Cangshan all recommend it to owners. Any snag, drag, or tear points to a dull stretch — and where it tears is exactly where the light check will find a glint.

Run the whole blade, not only the middle. The heel is the stretch most likely to be missed near a full bolster, and the tip is the one most often skipped on a sharpener. A knife that passes through the middle but snags at either end is not fully sharp.

The paper test checks the full length of the blade without touching the edge itself.

The tomato test: the knife goes through the skin on its own

Set the blade on the skin of a ripe tomato, edge down, and release all downward pressure. Let the knife rest on its own weight. If the edge is sharp, it passes through the skin at the first contact. If it stalls and you have to press down or start a sawing motion before it cuts, the edge has gone dull enough to sharpen.

Shun, Victorinox, and Messermeister all use this test. The tomato skin is taut enough to show the difference between cutting and crushing. Once it goes through on its own, the edge is ready.

What to do when the knife fails a test

A failed test tells you what to fix, not only that something is wrong.

A knife that barely stalls on the tomato but still passes the paper test often needs only the fine side. Starting on the 3000 grit ceramic side of Rollkeen's rolling sharpener spares the blade the steel removal of a coarse grind; the guide on over-sharpening a knife explains when the coarse end is actually needed. If the knife tears the paper or shows glints along most of the edge, the coarse 400 grit disc is the right start. After any session, run the light test again: every glint that is gone marks a stretch the edge has cleared.

If the knife fails all three tests and sharpening is not bringing it back, the guide on why a knife stays dull after sharpening covers each tool and each cause, with a fix for every one. How often these tests come up positive depends on how the knife is washed, stored, and what it cuts on — the guide on why knives go dull covers each factor. For a rough interval between checks, the guide on how often to sharpen kitchen knives covers what knife makers say and why their answers run from twice a month to once a year.