Drill Bit Size Chart — Fraction, Number, Letter & Metric

Drill bit size chart

This diameter-sorted chart covers common fractional, number, letter, and metric drill series. Search by designation or diameter, filter by system, and sort by the displayed columns. The metric diameter is the canonical value used for comparisons; decimal inches are provided as a convenience.

The chart covers fractional sizes from 1/64 in through 1 in, number drills from #80 through #1, letter drills A through Z, and metric sizes from 0.5 mm through 13.0 mm in 0.1 mm increments. It is a practical reference for common sizes, not a substitute for the drill manufacturer’s tolerance or the requirement on a drawing.

How drill size systems differ

Fractional inch drills

Fractional drills use an inch fraction such as 1/8, 3/16, or 1/4. The denominator is not a universal step size: this chart uses 1/64-in increments through 1 in, while drill sets can include finer fractional steps or omit sizes.

Number drills

Number drills use designations from #80 through #1 in this chart. The diameter increases as the number decreases, so #80 is smaller than #21 and #1 is larger than both. The number itself is an identification label, not a measurement you can safely treat as a decimal without a reference table.

Letter drills

Letter drills use A through Z. The diameter increases from A toward Z, but letter sizes are not interchangeable with a letter grade or a tap designation. Use the actual diameter shown in the chart when comparing a letter drill with an inch or metric option.

Metric drills

Metric drills state the nominal diameter directly in millimetres, such as 6.0 mm or 6.5 mm. Metric sizes may be sold in different increments, so confirm that the increment and tolerance suit the tool, material, and required hole.

Exact size versus nearest size

An exact match means the selected designation has that source diameter. A nearest result is a comparison across systems; it is the closest listed diameter, not a claim that the two designations are interchangeable. For example, 1/4 in is exactly 6.35 mm, while a 6.0 mm drill is a nearby but smaller metric choice.

When two candidates are equally close, the lookup uses a deterministic order: the smaller diameter wins, followed by the declared system order and then the designation. This keeps repeated searches consistent. The chart also exposes the nearest listed sizes below and above the requested diameter when a reverse lookup is useful.

How to choose up or down

Do not choose the larger or smaller nearest drill by diameter alone. The correct direction depends on the operation:

• For a tapped hole, start with the thread designation, tap type, material, and required percentage of thread. Use the appropriate tap-drill recommendation.
• For a clearance hole, start with the fastener diameter and the required fit. A nearby drill size from another system might be too tight or too loose.
• For reaming, boring, dowel work, or a specified press or slip fit, drill allowance and final-size tolerance control the choice.
• For general drilling, account for tool runout, material behavior, burrs, and the hole tolerance rather than treating the nearest chart entry as the finished size.

How to use the chart for reverse lookup

1. Measure or read the required diameter in millimetres or decimal inches.
2. Search the chart for the value or enter the closest designation you know.
3. Check the system filter and compare the nearest result in the intended system.
4. Confirm whether the result is exact or nearest, then check the operation-specific requirement before drilling.

Related drill and hardware charts

• Tap Drill Size Chart — Metric, UNC & UNF — select a starting drill for a specified thread and tapping process.
• Clearance Hole Size Chart — choose a hole size for a screw or bolt when the hole is not tapped.
• PEM Hardware Hole Size Chart — check the recommended hole for a self-clinching fastener.

Source note

The dataset uses the ASME B94.11M twist-drill standard landing page and Guhring’s public decimal-equivalents reference for the listed drill series. Inch-to-millimetre and millimetre-to-inch values use the named 25.4 conversion formulas. Verify the current tool catalogue, tolerance, and process requirement for production-critical work.

Frequently asked questions

What size is a 1/4-inch drill in millimetres?

A 1/4-in drill has an exact nominal diameter of 6.35 mm. A 6.0 mm metric drill is nearby but is 0.35 mm smaller.

What size is a #21 drill?

A #21 drill is 0.1590 in, or about 4.0386 mm, in the reference data. Use the application requirement to decide whether that is the correct drill or whether a different size is required.

What is an F drill in millimetres?

An F drill is 0.2570 in, or about 6.5278 mm. The nearest common metric entry in this chart is 6.5 mm, but nearest does not mean interchangeable.

Should I use the nearest fractional or metric drill?

Use the system required by the drawing, fastener, tap, tool, and hole tolerance. A nearest cross-system result is a comparison aid, not a general recommendation.