When you’re learning about tapping, you’ll realize how crucial it is to know the right drill bit size for a 1/2” tap. Trust me, it simplifies things a lot. To help you out, I’ve put together a chart that’ll guide you to the perfect ½” tap drill size.
What Size of Drill Bit Do I Use for a 1/2" Tap?
The tap size chart below provides the tap size, drill bit in coarse and fine threads, and the drill bit size you should use for your project. The table also includes the 50% and 75% threads in decimal equivalent.
Tap Size and Pitch | Cutting Taps | Forming Taps | ||
| Drill Size | Decimal Equivalent | Drill Size | Decimal Equivalent | |
| 1/2-13 | 27/64 | 0.4219 | 29/64 | 0.453 |
| 1/2-20 | 29/64 | 0.4531 | 15/32 | 0.469 |
| 1/2-28 | 15/32 | 0.4690 | 15/32 | 0.469 |
The 1/2-inch tap drill size has three different numbers of threads or pitches. The 13-inch being the shortest can produce the shallowest pilot hole among the three. The 20-inch has fine threads and can make deeper pilot holes than the 13-inch.
How Does Tap Sizing Work 1/2"
A tap or die is marked with three distinct numbers:
- Diameter
- Pitch (threads per inch) [1]
- Type of thread it cuts
The tap sizing chart shows the proper tap drill size that you can use to match the screw or bolt that you are cutting threads for. It works almost the same as choosing drill bit sizes for regular drilling.
Tap drill sizes are designed to match industry standard-sized holes for tapping. While the array of numbers can be confusing, the chart above offers a simplified yet accurate version to help you navigate the process.
How to Calculate the Tap Drill Size for a 1/2"
There are two ways to calculate the tap drill size for a ½”. Below are some examples of calculating for cutting taps and forming taps.
For cutting taps, you have to subtract the pitch from the tap basic major diameter to get the tap drill size.
Tap drill size = tap basic major diameter – pitch
To get the drill size, look at the equation below:
Drill Size = Major Diameter – [(0.01299 x desired % of thread) / threads per inch]
To get the drill size in mm, follow the equation below:
Drill Size (mm) = major diameter – (desired % of thread x pitch (mm) / 76.98)
To get the tap drill size for forming taps, divide the pitch by two then subtract it from the major diameter. You can also follow the other calculations below to get the drill size:
Drill Size = major diameter – [(0.0068 x desired % of thread) / threads per inch]
Drill Size (mm) = major diameter – (desired % of thread x pitch (mm) / 147.06)
Why Do 13” and 20” Require Different Sizing?
Even though the 13 and 20 have the same diameter, which is 1/2″, 13” and 20” pertain to the number of threads per inch, in other words, the tap’s thread pitch. The tap length is influenced by the number of threads per inch.
The -13 tap has a coarser and lesser number of threads. While the -20 tap is longer and can make a deeper hole than 13.
That is why they need different sizing requirements. Because of this, they also need different drill bit sizes even though they have the same diameter.
FAQ
What can I use if I don't have a 1/2-inch drill bit?
If you don’t have a ½-inch drill bit, you can use a copper piece to drill holes into a plastic. You can also use self-drilling screws or a normal screwdriver to make a hole.
How do you drill a 1/2-inch hole in wood?
To drill a ½-inch hole in wood, you need to attach a piece of scrap wood to the back of your workpiece. Start drilling slowly, then gradually increase the speed. When you feel the bottom, go slow again to prevent breakage.
Conclusion
Using the chart to pinpoint the 1/2 tap drill size can make your project go smoother. It not only helps you get the math right but also cuts down on extra tools and materials you might waste otherwise.
Robert Johnson is a passionate furniture maker & carpenter, sought after for his knowledge on the craft.
You’ve probably seen his down-to-earth wisdom in USA Today, Bobvila, Family Handyman, and The Spruce, where he has shared commentary and guidance on various woodworking topics.
Robert is the brain behind Sawinery, where he aims to share tips, tricks, and a passion for all things carpentry.
