Drill Bit Sizes Explained: How to Choose the Right Bit Without Guesswork

If we’ve ever stood in a hardware aisle staring at a wall of drill bits, wondering which one we actually need, we’re definitely not alone. Drill bit sizes can feel confusing at first. Fractions, decimals, letters, millimeters… sometimes all mixed together in one set.

The good news is this: we don’t need to be experts to make the right choice. With a basic understanding of how drill bit sizes work, most everyday drilling tasks become much simpler, safer, and far less frustrating.

In this guide, we’ll walk through drill bit sizes step by step. We’ll explain what the numbers mean, how different sizing systems work, common mistakes people make, and how to choose the right size for typical home and DIY projects. No technical overload, no sales talk—just clear, practical explanations.

At its core, a drill bit’s size refers to its diameter.
In other words, how wide the hole will be once we drill it.

This measurement is taken across the cutting edges at the widest point of the bit. Even a small difference in size can matter, especially when we’re working with screws, anchors, or precise holes.

A bit that’s too small can split wood or make driving a screw difficult.
A bit that’s too large can leave us with a loose fit that won’t hold properly.

That’s why understanding sizes isn’t just a technical detail—it directly affects the result of our work.

Drill bit sizes are usually labeled using one of three systems. Sometimes, especially in larger sets, we’ll see more than one.

Fractional Sizes (Imperial)

Fractional sizes are very common in the U.S.
These are measured in inches and written as fractions, such as:

• 1/16″

• 1/8″

• 3/16″

• 1/4″

They’re straightforward and easy to recognize, which makes them popular for general household projects. The downside is that the steps between sizes can feel uneven, especially with smaller bits.

Decimal Sizes

Decimal sizes are also measured in inches, but shown as decimals instead of fractions.

For example:

• 0.125″ instead of 1/8″

• 0.250″ instead of 1/4″

These sizes are often used when precision matters more, but for everyday DIY work, we usually don’t need to think in decimals unless we’re matching an exact specification.

Metric Sizes

Metric sizes are measured in millimeters and are widely used around the world.

Common examples include:

• 3 mm

• 5 mm

• 8 mm

Metric bits increase in logical, even steps, which many people find easier to work with. It’s not unusual to have both imperial and metric bits in the same toolbox, especially if we follow instructions from different sources.

It’s tempting to grab a bit that looks “close enough” and start drilling. Sometimes that works. Often, it doesn’t.

Here’s why size really matters:

• Screw fit – Pilot holes need to match the screw’s core, not the threads.

• Material protection – The right size helps prevent cracking or splitting.

• Accuracy – A properly sized hole keeps parts aligned and stable.

• Clean results – Correct sizing reduces tear-out and rough edges.

Taking a moment to choose the right size usually saves time in the long run.

Most household drill bit sets cover a predictable range. Knowing what’s typical helps us understand what we already have—and what we might be missing.

Small Bits (Under 1/8″ or 3 mm)

These are often used for:

• Pilot holes

• Small screws

• Light materials like thin wood or plastic

They’re easy to break if we apply too much pressure, so patience matters here.

Medium Bits (1/8″–3/8″ or 3–10 mm)

This is where most DIY drilling happens.

Typical uses include:

• Shelves and brackets

• Furniture assembly

• General home repairs

If we’re only going to own one basic set, this range is the most useful.

Large Bits (Over 3/8″ or 10 mm)

Larger bits are used less often but still come in handy for:

• Anchors

• Larger bolts

• Running cables or pipes through wood

These bits require slower speeds and a steadier hand.

Sometimes the markings are worn off, or bits get mixed together. When that happens, we still have options.

Here’s what usually works:

• Measure the diameter with a ruler or caliper

• Compare visually against known sizes

• Test-fit carefully in a scrap piece of material

A drill bit gauge can make this easier, but even without one, we can usually figure it out with a bit of patience.

Even experienced DIYers fall into these traps. Being aware of them helps us avoid unnecessary problems.

“Close Enough Is Good Enough”

This is probably the most common mistake.
A slightly wrong size can lead to stripped screws, loose anchors, or damaged materials.

Mixing Metric and Imperial Without Noticing

Metric and imperial sizes can look similar but aren’t always interchangeable. A 5 mm bit is close to 3/16″, but not identical.

Using the Same Size for Every Material

Wood, metal, plastic, and masonry all behave differently. Even if the size is correct, the drilling approach may need to change.

Drilling Too Fast With Small Bits

Smaller bits need slower speeds and lighter pressure. Rushing often leads to broken tips or crooked holes.

Let’s keep this practical. Here’s how we usually approach everyday situations.

Drilling Pilot Holes for Screws

• Match the bit size to the core of the screw, not the threads

• Softer wood → slightly smaller pilot hole

• Harder wood → slightly larger pilot hole

When in doubt, test on a scrap piece first.

Drilling Holes for Wall Anchors

• The hole should match the anchor’s body diameter

• Too small → anchor won’t fit

• Too large → anchor won’t hold

Always double-check before drilling into finished walls.

Drilling Through Wood

Wood is forgiving, but it can split if the bit is too small or we drill too close to an edge. Choosing the right size and drilling steadily makes a big difference.

Drilling Through Metal

Metal needs:

• Accurate sizing

• Slower speeds

• Firm, controlled pressure

Here, even small size differences matter more than they do in wood.

Over time, we tend to develop habits that make drilling less stressful. These tips come up again and again for a reason.

• Label or organize bits so sizes are easy to spot

• Test on scrap material whenever possible

• Let the bit do the work instead of forcing it

• Keep bits clean and sharp for accurate holes

• Slow down with smaller sizes

These small habits often matter more than having the “perfect” bit set.

Not every project requires exact measurements. For rough work, temporary fixtures, or hidden areas, precision isn’t always critical.

That said, understanding drill bit sizes gives us confidence. We know when precision matters—and when it doesn’t.

Quick Summary: Drill Bit Sizes at a Glance

• Drill bit size refers to the diameter of the hole

• Common systems include fractional, decimal, and metric

• Choosing the right size improves fit, strength, and appearance

• Mixing up sizes is a common DIY mistake

• Testing and slowing down often leads to better results

Understanding drill bit sizes isn’t about memorizing charts or becoming technical. It’s about making everyday projects smoother and more predictable. Once we get familiar with the basics, choosing the right bit becomes second nature—and drilling stops feeling like guesswork.