About this tool
Drill bits are sold in three languages at once: fractions of an inch, decimal inches, and millimetres. A workshop drawer may hold a 1/4 inch bit next to a 6.5 millimetre bit, and a project plan may call for a 3/8 inch hole while the only set in the shop is metric. A drill bit size converter speaks all three languages, so the right bit is found by number rather than by trial and error.
The conversion itself is simple arithmetic. A fraction becomes a decimal by dividing the top number by the bottom number, so 3/8 becomes 0.375. Decimal inches become millimetres by multiplying by 25.4, because one inch is exactly 25.4 millimetres. So 0.375 inches is 9.525 millimetres. DrillBit does both steps at once and rounds the metric result to three decimal places, which is more precision than any hand drill can use but enough for a machinist's chart.
Fractional sizes dominate woodworking in countries that use imperial measurements, because traditional drill sets step through 64ths of an inch. A set that runs from 1/16 to 1/2 inch in 64ths gives a fine ladder of sizes, and each step is small enough that the nearest bit is usually close enough for wood. Metalwork adds number gauge bits, numbered from 80 down to 1, and letter gauge bits from A to Z, which fill the gaps between fractions for tapping and reaming.
Metric bits are simpler to reason about, because the size is the diameter in millimetres, often in half-millimetre steps. A 6 millimetre bit drills a 6 millimetre hole. There is no fraction to decode. That is why many makers keep a metric set even in an imperial workshop: when a plan gives a metric hole, the metric bit is the direct answer, and when it gives a fraction, the converter tells you the nearest metric bit without guessing.
The second half of DrillBit is the pilot hole, and it is the part that saves projects. Driving a screw into wood without a pilot hole wedges the fibres apart, and near the end of a board that wedging becomes a split that runs along the grain. A pilot hole removes just enough material that the screw's threads can bite without forcing the wood apart. The hole should be smaller than the screw, because the threads still need wood to grip.
The usual rule is to drill a pilot hole about 70 to 80 percent of the screw's shank diameter in softwood. The shank is the solid core of the screw, not the outer thread diameter. In hardwood, which splits more easily and resists screws more strongly, a slightly larger pilot hole is safer, especially near edges. The exact best size depends on the screw gauge, the wood species, and how close the screw is to the end grain, so testing on a scrap piece of the same wood is always worth the minute it takes.
Clearance holes are a related idea that beginners sometimes confuse with pilot holes. When joining two boards, the top board often gets a clearance hole large enough for the screw to pass through freely, while the bottom board gets the pilot hole. That way the screw pulls the two boards together instead of holding them apart by its threads. Countersinking, which cuts a shallow cone so the screw head sits flush, is another companion operation that makes a finished joint look professional.
Choosing the bit itself matters as much as choosing the size. Twist bits are the general-purpose choice for wood, plastic, and metal. Brad-point bits have a centre spur that keeps the bit from wandering on wood and leaves a clean entry hole. Forstner bits cut flat-bottomed holes for hinges and dowels. Spade bits cut large holes quickly in framing timber but leave a rougher finish. Using a twist bit where a brad-point bit belongs is a common cause of a hole that starts in the wrong place.
Technique completes the picture. Mark the depth with tape wrapped around the bit when the hole must not go through. Drill at a speed suited to the material: fast for small bits in wood, slower for large bits and for metal, where heat dulls the edge. Withdraw the bit periodically to clear chips from deep holes, and back the workpiece with scrap wood to prevent tear-out where the bit exits. A sharp bit cuts with light pressure; if you must push hard, the bit is dull or the speed is wrong.
Keeping a conversion chart near the drill press turns all of this into habit. Note the fractional, decimal, and metric sizes of the bits you own, and note the pilot sizes that worked for your most common screws in your most common woods. DrillBit is that chart in interactive form: enter any size, get every equivalent, and get a pilot-hole starting point. The result is fewer split boards, fewer stripped screws, and holes that are the size the plan actually asked for.
Drill bit materials are a subject that repays a little study. High-speed steel is the standard for general work, balancing toughness, heat resistance, and price. Cobalt steel adds heat resistance for hard metals such as stainless steel, at a higher price and with more brittleness. Titanium-coated bits are high-speed steel with a hard coating that reduces friction, but the coating wears off when the bit is sharpened, so the benefit is temporary. Carbide bits are extremely hard and excellent for masonry and tile, but they are brittle and should not be used where they may flex.
Speed and feed are the two variables that control drilling quality, and both depend on bit size and material. As a rule, smaller bits run faster and larger bits run slower, because the cutting edge at the rim of a large bit travels much faster than the edge of a small bit at the same rotation speed. Metal drilling needs cutting fluid to carry away heat, while wood drilling needs chip clearance to prevent burning. A bit that squeals, smokes, or produces dust instead of shavings is telling you that speed, pressure, or sharpness is wrong.
Sharpening is a skill that pays for itself quickly. A dull bit requires force, wanders on entry, and produces ragged holes, while a sharp bit cuts with light pressure and leaves clean edges. Twist bits can be sharpened on a bench grinder or with a dedicated sharpener, restoring the cutting angle to about 118 degrees for general work. Brad-point bits are harder to sharpen because of their spur, and many woodworkers replace them when dull. Learning to recognise a dull bit, by the sound and the feel, is more valuable than any particular sharpening jig.
Tapping is the precision application where drill size matters most. When a hole is to be threaded for a bolt, the tap drill size must leave exactly the right amount of material for the thread, typically about 75 percent thread engagement. Too large a tap drill leaves a weak thread that strips, while too small a tap drill makes tapping difficult and risks breaking the tap in the hole. Tap drill charts, which list the correct bit for each thread size, are essential companions to a conversion chart in any metalworking shop.
Safety deserves emphasis, because drills injure more hobbyists than almost any other power tool. Always clamp the workpiece rather than holding it by hand, because a bit that catches can spin the work violently. Wear eye protection, since chips fly at speed, and avoid loose clothing and gloves near a rotating bit, which can catch and pull a hand in. Unplug the drill when changing bits, and never force a bit that has jammed: reverse it out, clear the hole, and start again with less pressure.
Organising bits by size saves more time than any single technique. A labelled index, with bits returned to their slots after use, means the right size is found in seconds rather than by measuring every bit in a jumbled box. Many makers keep a printed conversion chart inside the lid of the index, with fractional, decimal, and metric sizes side by side, and with their most-used pilot sizes noted in pencil. DrillBit is that chart in digital form, but a paper copy in the workshop still earns its place on the wall.
Frequently asked questions
How do I convert a fractional drill size to millimetres?
Divide the fraction to get decimal inches, then multiply by 25.4. For example, 3/8 inch is 0.375 inches, and 0.375 times 25.4 is 9.525 millimetres.
What is a pilot hole and why do I need one?
A pilot hole is a smaller hole drilled before driving a screw. It guides the screw, reduces the force needed, and stops wood from splitting, especially near edges and ends.
What size pilot hole should I drill for a wood screw?
As a starting point, use about 70 to 80 percent of the screw's shank diameter in softwood, and slightly larger in hardwood. When in doubt, test on a scrap piece of the same wood.
Are drill bit sizes the same as screw sizes?
No. Drill bit size is the diameter of the hole, while screw size usually refers to the screw's gauge and length. A pilot hole is deliberately smaller than the screw itself.
Why do some drill sets skip sizes?
Fractional sets step by 64ths or 32nds, which leaves gaps. Number and letter gauge sets fill those gaps for precision work such as metal drilling and tapping.
Can I use a metric bit in place of a fractional one?
Often yes, if the metric size is within a few hundredths of a millimetre. Check the converted size first, because a bit that is too large leaves a loose hole and a bit that is too small forces the screw.