How to Tell When a Knife Needs Sharpening (and How to Fix It)

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Updated August 2026 · 6 min read · UK Japanese-steel knife specialists

A knife rarely goes blunt overnight, so most of us carry on with a dull blade for weeks without realising it. The good news is you don't need any special kit to check: a sheet of paper and a ripe tomato will tell you almost everything. This guide gives you five quick tests, then explains the part that trips people up — whether your knife needs a quick hone or a proper sharpen — and what to reach for.

It matters more than it sounds. A blunt knife is the one that slips, so it's actually the more dangerous tool in the kitchen — and it bruises herbs, crushes tomatoes and tears rather than slices. Keeping an edge keen takes minutes, and our sharpening stones and honing steels are made for the hard Japanese steel most of our knives are ground from.

Five ways to tell your knife is blunt

No single test is definitive, but together they give a reliable picture. Run through them whenever a knife starts to feel like hard work.

1. The paper test

Hold a sheet of printer paper up by one edge and slice downward through it. A sharp knife glides straight through and leaves a clean cut. A blunt one snags, tears the paper or folds it over instead of biting. This is the single most useful check because the paper shows up dull spots along the edge.

2. The tomato test

A ripe tomato is unforgiving. Rest the blade on the skin and draw it back with almost no downward pressure. A sharp edge bites in and slices under its own weight; a blunt one skids across the surface or squashes the tomato before it cuts. If you find yourself pressing down, the edge has gone.

3. Look at the edge in the light

Point the edge up towards a bright light and look straight down at it. A truly sharp edge is so thin it disappears — it reflects almost nothing. Dull or rolled sections catch the light as tiny bright glints or a faint white line. This tells you exactly where the edge needs work.

4. The thumbnail test

Rest the edge gently on your thumbnail at a shallow angle — carefully, and without any slicing motion. A sharp edge grips and won't slide; a blunt one slips straight off. It's an old workshop trick and it works, but keep the pressure feather-light.

5. How it feels in use

Trust your hands. If you're sawing back and forth, pressing harder than you used to, struggling with onion skins and pepper, or your cuts are crushing rather than parting cleanly, the knife is asking to be sharpened. A keen Japanese-steel blade should feel almost effortless.

Hone or sharpen? They're not the same thing

This is where most people go wrong. Honing straightens an edge that has rolled over microscopically with use — it removes little or no metal and takes a few seconds on a steel. Sharpening grinds a fresh edge on a stone when honing no longer brings the knife back, and it does remove a small amount of metal.

The rule of thumb: hone little and often, sharpen occasionally. For a home cook that usually means a quick hone every few uses and a full sharpen on a whetstone every few months. If a hone no longer restores a paper-cutting edge, reach for the stone. Our guide on how often to sharpen a Japanese-steel knife goes into the timing in more detail.

One important point for Japanese-steel knives specifically: the VG10 and AUS-10 blades we sell are hard (around 60–61 HRC) and ground to a fine ~15° per side, rather than the ~20° of a typical Western knife. That thin, hard edge holds its sharpness longer but wants gentle treatment — a smooth or fine diamond rod for honing, and a finishing stone in the 3000–8000 grit range to bring back a polished edge. Avoid coarse, ribbed butcher's steels, which can chip a hard edge. If you want the full method, see how to sharpen a knife on a whetstone and the correct sharpening angle.

What to reach for

Two tools cover almost everything: a honing steel for regular upkeep, and a whetstone for when the edge genuinely needs re-grinding. Both of the below are made for hard Japanese steel.

Tool	What it does	When to use it	Price
Honing steel	Realigns a rolled edge; removes almost no metal	Every few uses	£54.99
Whetstone	Grinds a fresh, polished edge	Every few months, or when honing stops working	£59.99

FAQ

How often should I sharpen a Japanese-steel knife?

For a home cook, hone every few uses and give the knife a full sharpen on a whetstone roughly every three to six months. It depends on how much you cook and what board you use — a hard board dulls an edge faster than end-grain wood.

Can I just use a honing steel and skip the whetstone?

Not indefinitely. A steel realigns the edge but can't rebuild it. Once honing no longer restores a clean paper-cutting edge, the knife needs a whetstone to grind a fresh edge.

Will a pull-through sharpener work on a Japanese knife?

It's best avoided. Most pull-through sharpeners grind at a fixed ~20° and are aggressive, which can spoil the thin ~15° edge of a hard Japanese-steel blade. A whetstone gives you control over the angle and a far better finish.

What angle should I sharpen at?

Around 15° per side for most double-bevel Japanese-steel knives — thinner than a Western knife. Keeping that angle steady matters more than anything; see our guide to the correct sharpening angle.

How do I know the knife is sharp again?

Re-run the paper and tomato tests. A freshly sharpened knife should slice printer paper cleanly and bite into a tomato skin under its own weight with no sawing.

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