So, you’ve got a drill, but you need to know which drill bit fits the job. How do you tell the size of a drill bit? You tell the size of a drill bit by looking at the markings on the bit itself, using a drill bit gauge, or measuring its diameter with a caliper. This guide will break down all the ways to figure out those crucial drill bit sizes so you can get your project done right.
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Why Knowing Drill Bit Size Matters
Picking the right drill bit size is super important for any drilling task. Use a bit that’s too big, and you’ll make a hole that’s too wide, which can weaken the material. Use a bit that’s too small, and you might struggle to get the fastener (like a screw or bolt) to fit properly. For example, when you’re drilling pilot holes for screws, using a drill bit that’s the right size means the screw will go in smoothly and hold tight. If you’re drilling holes for bolts and nuts, you need a bit that matches the bolt’s diameter so it can pass through easily. This guide covers various drill bit identification methods to ensure accuracy.
Common Ways to Identify Drill Bit Size
There are several reliable ways to determine the size of your drill bits. Each method offers a different level of precision and convenience.
1. Reading the Markings on the Drill Bit
Most drill bits have their size printed or etched directly onto them. This is the quickest and most common method for drill bit identification.
Where to Find the Markings
- Shank: The part of the bit that goes into the drill chuck is called the shank. Often, the size is printed here.
- Flutes: The spiral grooves along the bit are called flutes. Sometimes, markings appear between the flutes.
- Web: The thin central part of the bit that connects the flutes.
What the Markings Mean
- Imperial System: You’ll usually see measurements in inches, like
.125",3/16", or.250". Sometimes, it might be written as1/8or1/4. - Metric System: These bits will be marked in millimeters, such as
3mm,5mm, or8mm.
Important Note: The markings can sometimes be small or worn away, especially on older or frequently used bits.
2. Using a Drill Bit Gauge
A drill bit gauge is a handy tool specifically designed for drill bit measurement. It’s essentially a metal plate with holes of various standard sizes drilled into it.
How to Use a Drill Bit Gauge
- Find the Right Hole: Simply slide the shank of your drill bit into the holes on the gauge.
- Match the Fit: When you find a hole that the shank fits snugly into without wobbling, that hole’s size is your drill bit’s size.
- Check Both Sides: Some gauges have markings on both sides, showing sizes in both imperial and metric.
Benefits of a Drill Bit Gauge
- Accuracy: Gauges are very accurate for identifying common drill bit sizes.
- Organization: They help keep your bits sorted and easy to find.
- Durability: Made of metal, they can withstand shop use.
A drill bit gauge chart is often printed on the gauge itself or can be found online to help you cross-reference sizes.
3. Measuring with a Caliper
For the most precise drill bit measurement, especially if markings are missing or you need to be absolutely certain, a caliper is the best tool. A caliper allows you to measure the drill bit diameter directly.
Types of Calipers
- Vernier Caliper: A traditional caliper with sliding jaws and a scale for reading measurements.
- Digital Caliper: Features an electronic display that shows the measurement in inches or millimeters, making it easier to read.
How to Measure with a Caliper
- Open the Jaws: Gently open the jaws of the caliper.
- Place Around the Bit: Position the jaws on the widest part of the drill bit’s cutting edge, just above the flutes. Ensure the jaws are perpendicular to the bit’s axis.
- Close the Jaws: Slowly close the jaws until they touch the sides of the bit firmly but without excessive force.
- Read the Measurement: Read the measurement displayed on the caliper. This is the drill bit diameter.
Tips for Using a Caliper
- Cleanliness: Make sure the drill bit and caliper jaws are clean.
- Consistency: Always measure at the same point on the bit.
- Zeroing: If using a digital caliper, ensure it’s zeroed before measuring.
Using a caliper provides an exact drill bit measurement, which is invaluable when working with specific material requirements or when precise hole sizing is critical.
Interpreting Drill Bit Size Markings and Charts
Understanding the different systems used for drill bit sizes is key. The two main systems are imperial (inches) and metric (millimeters).
Imperial Sizes
Imperial sizes are common in the United States and some other countries. They are typically expressed in fractions of an inch or decimal inches.
- Fractions:
1/16",3/32",1/8",5/32",3/16",7/32",1/4", etc. - Decimals:
.0625",.09375",.125",.15625",.1875",.21875",.250", etc.
It’s helpful to have a drill bit size chart that shows the decimal equivalents of common fractional sizes.
Metric Sizes
Metric sizes are used in most of the world and are based on millimeters.
- Millimeters:
1mm,1.5mm,2mm,2.5mm,3mm,3.5mm,4mm,4.5mm,5mm,5.5mm,6mm, etc.
Number and Letter Sizes (Imperial)
Some older or specialized sets might use letter sizes (A to Z) or number sizes (1 to 80). These are less common for general DIY but can be found in machinist sets.
- Letter Sizes: ‘A’ is the smallest (0.055 inches), and ‘Z’ is the largest (0.400 inches).
- Number Sizes: Size 1 is 0.0235 inches, and size 80 is 0.0135 inches.
A drill bit gauge chart will typically include these systems for easy conversion.
Drill Bit Size Chart Example (Common Sizes)
| Imperial Fraction | Imperial Decimal | Metric (mm) |
|---|---|---|
| 1/16″ | 0.0625″ | 1.59 |
| 5/64″ | 0.078125″ | 1.98 |
| 3/32″ | 0.09375″ | 2.38 |
| 7/64″ | 0.109375″ | 2.78 |
| 1/8″ | 0.125″ | 3.18 |
| 9/64″ | 0.140625″ | 3.57 |
| 5/32″ | 0.15625″ | 3.97 |
| 3/16″ | 0.1875″ | 4.76 |
| 7/32″ | 0.21875″ | 5.56 |
| 1/4″ | 0.250″ | 6.35 |
| 9/32″ | 0.28125″ | 7.14 |
| 5/16″ | 0.3125″ | 7.94 |
| 11/32″ | 0.34375″ | 8.73 |
| 3/8″ | 0.375″ | 9.53 |
Note: Metric conversions are approximate for exact fractional inch sizes.
Specific Situations and Special Drill Bits
Sometimes, standard markings aren’t enough, or you’re dealing with specialized bits.
1. Masonry and Concrete Bits
These bits often have a carbide tip. While the shank might have size markings, the tip itself usually doesn’t directly indicate size. Rely on the shank markings or a gauge for these. They are designed for drilling into hard materials like brick, stone, and concrete.
2. Wood Bits (Auger Bits, Forstner Bits)
- Auger Bits: These usually have a screw point and spurs. The size is typically stamped on the shank, often in fractions of an inch.
- Forstner Bits: Known for creating clean, flat-bottomed holes. The drill bit diameter is usually clearly marked on the shank or the flat side of the bit.
3. Metalworking Bits (Cobalt, HSS)
High-Speed Steel (HSS) and Cobalt bits are common for drilling metal. They are very likely to have clear markings on the shank, usually in either imperial or metric sizes. For precision drilling in metal, accurately knowing the drill bit diameter is crucial for tap drills and clearance holes.
4. Specialty Bits (Countersink, Step Bits)
- Countersink Bits: Used to create beveled holes for screw heads. The size often relates to the shank diameter or the maximum diameter the bit can create.
- Step Bits: These look like a cone with multiple diameters. They are marked with the specific diameters they can drill at each “step.” For instance, a bit might be marked
4-20mmindicating it can drill holes of 4mm, 6mm, 8mm, …, up to 20mm.
Tips for Keeping Your Drill Bits Organized and Readable
Maintaining your drill bits ensures you can always find the right size quickly.
1. Use a Drill Bit Index or Case
Most drill bit sets come in organized cases. If yours didn’t, invest in a drill bit index. These are usually plastic or metal cases with labeled slots for each bit. This is one of the best ways for drill bit identification and keeping them sorted.
2. Clean Your Bits Regularly
After use, clean off any debris, dust, or metal shavings from your drill bits. Use a brush or a clean rag. This will make markings easier to see and prevent rust.
3. Avoid Damage to Markings
When storing bits, try to prevent them from banging against each other. This can wear down or obscure the size markings.
4. Keep a Reference Chart Handy
A drill bit size chart or a drill bit gauge chart can be kept in your workshop or toolbox for quick reference.
Common Questions About Drill Bit Sizes
Here are answers to frequently asked questions to further help with identifying drill bit size.
What is the difference between drill bit diameter and length?
The diameter is the width of the cutting edge across the widest point of the bit. The length refers to the overall length of the drill bit. For most drilling tasks, the diameter is the critical measurement.
Can I tell the size of a drill bit by its color?
No, the color of a drill bit (e.g., black oxide, titanium nitride coating, cobalt) usually indicates the material it’s made from or its coating, which affects its performance and durability, not its size. Always rely on markings, gauges, or calipers.
How do I know if my drill bit is metric or imperial?
Look at the markings. Imperial sizes will have symbols like " (inches) or fractions like 1/8. Metric sizes will have mm after the number.
What if my drill bit has no markings?
If a drill bit has no visible markings, the best approach is to use a drill bit gauge or a caliper for accurate drill bit measurement.
What are the most common drill bit sizes?
The most common drill bit sizes generally range from 1/16 inch (approx. 1.5mm) up to 1/2 inch (approx. 12.7mm), with many common fractional and metric sizes in between, like 1/8″, 3/16″, 1/4″, 5/16″, 3/8″ and 3mm, 5mm, 6mm, 8mm, 10mm.
What is a drill bit gauge chart used for?
A drill bit gauge chart helps you identify and convert between different systems of drill bit sizes, such as imperial fractions, decimal inches, and metric millimeters. It’s essential for cross-referencing and ensuring you select the correct bit.
By familiarizing yourself with these drill bit identification methods, you can confidently choose the right bit for any project, ensuring cleaner holes and better results. Whether it’s reading the shank, using a gauge, or employing a caliper, mastering drill bit measurement is a fundamental skill for any DIYer or tradesperson.