What angle should I sharpen at, and how do I know when it is sharp?

Why the angle matters less than its consistency, what a burr is and why it is the only reliable test, and the working angles for common tools.

Difficulty: beginner
Time: 25 min
Read: 5 min
Safety: caution

Short answer

Pick an angle and keep it — consistency matters far more than the exact number. Work one side until you can feel a fine wire of steel, the burr, along the whole length of the edge, then work the other side until the burr flips, then remove it with progressively lighter strokes. The burr is the only proof you have reached the edge.

People get stuck on the number of degrees. The number matters much less than doing the same number every stroke, because grinding at a slightly different angle each pass simply rounds the edge over and no amount of effort will then make it sharp. Understanding the burr is what turns sharpening from guesswork into a procedure with a definite end point.

Safety

Sharpening makes a blade far more dangerous than the blunt one you started with, and it cuts before you feel it. Work with the edge moving away from you, keep the hand holding the work behind the edge and never in its path, and clamp anything that can move. Feel for a burr by drawing a thumb across the blade away from the edge, never along it. Test sharpness on paper, not on skin. Wear cut-resistant gloves for awkward tools, and never sharpen a blade you are holding in mid-air.

What you'll need

A sharpening stone or plate
A honing guide (optional)
Water or oil as the stone requires
A strop (optional)
Paper to test on

The sharpening angle and the burr that tells you it is done

Step by step

Understand what blunt looks like.A sharp edge is two faces meeting at a line, which reflects no light. A blunt edge has a small flat between them. Hold the blade edge-up under a lamp and look straight down at it: a line of light along the edge is the flat, and that is what you are removing.
Choose an angle appropriate to the tool.Roughly 15 degrees per side for a fine kitchen knife, 20 to 25 per side for a general-purpose or outdoor knife, 25 to 30 for a chisel or plane iron, and steeper still for an axe or a machete. A shallower angle cuts better; a steeper one lasts longer under impact.
Or simply follow the bevel that is already there.Rest the blade flat on the stone and rock it up until the bevel sits flush — you can feel it settle. That is the angle the tool was made at, and matching it is almost always the right choice.
Hold the angle constant.A honing guide for chisels and plane irons, or a repeatable body position for a knife. Marking the bevel with a permanent marker and taking a couple of strokes shows exactly where you are removing metal: ink gone at the edge means you are on it, ink gone at the top means the angle is too high.
Work one side until a burr forms.Steady strokes with light pressure, keeping the whole bevel in contact. Continue until you can feel a fine wire of steel folded over onto the far side of the edge, along its entire length. This is the burr, and it is the only thing that proves you have removed metal all the way to the edge.
Feel for the burr safely.Draw a thumb gently across the blade, away from the edge, at several points along its length. Never run a finger along the edge. If the burr is present in the middle and absent at the heel or the tip, keep working those areas.
Turn the blade over and raise the burr the other way.The same number of strokes on the second side, until you feel the burr flip to the first side. On a chisel or plane iron with a flat back, you only work the bevel and then remove the burr from the flat back with the back held perfectly flat on the stone.
Move to a finer stone and repeat.Fewer strokes, lighter pressure. The finer stone refines the scratch pattern and produces a smaller, more easily removed burr.
Remove the burr with alternating light strokes.One stroke each side, progressively lighter, until it breaks away. A burr left on feels sharp for the first cut and then folds over, which is why a knife can seem sharp and go blunt within a minute.
Strop if you have one.A dozen strokes on leather with compound, edge trailing, removes the last of the burr and polishes the two faces. It makes a noticeable difference for very little effort.
Test on paper, not skin.Hold a sheet of paper by one edge and slice down through it. A sharp blade cuts cleanly with no snagging. A blade that tears or slips has either a burr left on or has not reached the edge.
Clean up and put it away safely.Wipe the blade, and put it back in its sheath, block or guard immediately. Far more people are cut by a freshly sharpened tool left on a bench than by using one.

Tips

The marker pen trick is the fastest way to learn a consistent angle. Two minutes of it teaches more than an hour of guessing.
Light pressure and more strokes beats heavy pressure and fewer. Pressure rounds the edge and digs into the stone.
If a blade will not take an edge at all, it may be chipped, or overheated at some point so the temper is lost. Both need reshaping on a coarse stone.

Common mistakes

Changing the angle slightly with every stroke — It grinds a rounded, convex edge instead of two flat faces meeting at a line. That edge feels dull no matter how much work goes into it, and this is the single reason most people conclude they cannot sharpen.
Stopping before a burr has formed along the whole edge — No burr means you have not reached the edge, so you have merely polished the shoulders of the bevel. The blade goes back in the drawer no sharper than it started, and the effort is wasted.

If it doesn't work

Burr forms in the middle but not at the tip

Cause: Not enough contact at the tip during the stroke — Fix: Lift the handle slightly as you sweep towards the tip on a curved blade, so the bevel stays in contact all the way round.

Blade feels sharp then dulls within a minute

Cause: Burr left on, folding over in use — Fix: Alternate light strokes each side until the burr breaks off, then strop.

Blade will not take an edge at all

Cause: Chipped edge, or the temper lost from overheating — Fix: Reshape on a coarse stone until the damage is gone. If it never holds an edge again, the steel has been overheated and softened.

Scratches getting deeper rather than finer

Cause: Stone loaded with metal, or too much pressure — Fix: Clean the stone, use water or oil as appropriate, and lighten the pressure.

Bevel getting wider each time you sharpen

Cause: Angle too low, or too much metal removed — Fix: Hone a small secondary bevel a couple of degrees steeper rather than re-grinding the whole face each time.

Questions people ask

Does the exact angle really matter?

Less than people think. A knife sharpened consistently at 22 degrees will outperform one sharpened inconsistently at a theoretically ideal 15. Choose a sensible angle for the tool and then be rigorous about repeating it.

Why does my knife go blunt so quickly after sharpening?

Almost always a burr that was never removed. It stands up and feels sharp for the first few cuts, then folds over and the edge is gone. Alternating light strokes and a strop cure it.