For a 5/16 tap, the correct drill bit is usually F (0.257″) for a 5/16-18 UNC tap and I (0.272″) for a 5/16-24 UNF tap. The right size matters because a hole that is too small can break the tap, while a hole that is too large can leave weak threads.
I get this question a lot because tap sizes can look simple until you actually need to drill the hole. Then the thread count, letter drill sizes, and coarse vs. fine threads start to get confusing fast. The good news is that this is an easy fix once you know what you are looking at. I’ll walk you through the exact drill sizes, how to choose between UNC and UNF, how to calculate tap drill size, and how to tap a clean 5/16 hole without ruining the tap.
Quick Answer: Drill Bit Size for 5/16 Tap
If you are tapping a 5/16-18 UNC thread, use an F drill bit, which measures 0.257 inch. If you are tapping a 5/16-24 UNF thread, use an I drill bit, which measures 0.272 inch. Standard tap drill charts from sources like American Fastener and JC Fasteners match these sizes.
This is the fastest way to remember it: coarse thread gets the smaller drill, and fine thread gets the slightly larger drill. That difference may look tiny on paper, but it makes a big difference when you start cutting threads.
5/16 Tap Drill Size Chart (UNC vs UNF)
| Thread Type | Thread Size | Tap Drill Size | Decimal | Common Use |
|---|---|---|---|---|
| UNC | 5/16-18 | F | 0.257″ | General-purpose fastening, softer materials, everyday shop work |
| UNF | 5/16-24 | I | 0.272″ | Finer thread engagement, tighter assemblies, vibration-sensitive setups |
When people search for the 5/16 tap drill size, they usually mean one of these two thread standards. That is why the thread count matters just as much as the 5/16 size itself. If you skip that detail, it is very easy to grab the wrong drill bit.
Understanding UNC vs UNF Threads
What Is UNC (Coarse Thread)?
UNC stands for Unified National Coarse. A 5/16-18 UNC fastener has 18 threads per inch. Coarse threads are common for general shop use because they are easier to start, less likely to cross-thread, and usually better in softer materials. If I am working on basic steel, wood-backed fixtures, or general repair jobs, coarse threads are often the safer pick.
What Is UNF (Fine Thread)?
UNF stands for Unified National Fine. A 5/16-24 UNF fastener has 24 threads per inch. Fine threads give you more threads in the same length, which can improve holding in some applications and help with adjustment precision. They are often used where vibration resistance and tighter clamping feel matter more.
Which One Should You Choose?
If you want a simple rule, go with UNC for general use and UNF when the application specifically calls for fine threads. If you are matching an existing bolt, always match the original thread. If you are creating new threads from scratch, think about the material, the load, and how easy you want the installation to be. In most beginner-friendly situations, 5/16-18 UNC is the more forgiving choice.
How to Calculate Tap Drill Size (Simple Formula)
A common shortcut is this: Drill Size = Major Diameter – Thread Pitch. It is not the only method used in machining, but it is a very practical way to understand why tap drill sizes land where they do.
For a 5/16-18 UNC tap, the major diameter is 0.3125 inch. The pitch is 1 ÷ 18, which equals about 0.0556. Subtract that from 0.3125 and you get about 0.2569, which rounds right to an F drill bit at 0.257 inch.
For a 5/16-24 UNF tap, the pitch is 1 ÷ 24, or about 0.0417. Subtract that from 0.3125 and you get about 0.2708, which is very close to an I drill bit at 0.272 inch.
This is why the numbers make sense. You are not just memorizing random drill letters. You are choosing a hole size that leaves enough material for the tap to cut solid threads without forcing it too hard.
Why Choosing the Right Drill Bit Size Matters
The correct drill bit size affects almost everything in the tapping process. If the hole is too small, the tap has to remove too much material. That raises friction, builds heat, and sharply increases the risk of a broken tap. Anyone who has snapped a tap inside a workpiece knows that this turns a quick job into a bad afternoon.
If the hole is too large, the tap cuts shallow threads. The fastener may still go in, but the threads will not have the same holding strength. That can be a real problem on parts that see repeated use, vibration, or load.
The right size gives you the sweet spot: clean thread engagement, lower tap stress, and better finished results. Good technique matters too. Shops and tooling manufacturers also stress the importance of proper lubrication and chip control during tapping, especially in metalworking. A helpful overview from Sandvik Coromant explains why coolant and lubrication affect thread quality and tool life.
Step-by-Step: How to Drill and Tap a 5/16 Hole
Tools You Need
Before you start, gather the basics:
- 5/16 tap in the correct thread type
- Correct drill bit size
- Drill or drill press
- Tap handle or tap wrench
- Cutting fluid or tapping lubricant
- Center punch
- Safety glasses
If you are tapping metal, I strongly prefer using a drill press when possible. It helps keep the hole straight and makes the whole process more controlled.
Step 1: Drill the Hole
Mark the spot with a center punch so the bit does not wander. Drill the hole using the correct tap drill size for your thread. Keep the drill straight and use steady pressure. If you are drilling steel or another harder material, slow down a little and let the bit cut instead of forcing it.
Step 2: Apply Lubrication
Add cutting fluid before tapping. This reduces friction, helps chips clear, and makes the tap cut more smoothly. Dry tapping is one of the easiest ways to shorten tap life and ruin thread quality. The harder the material, the more this matters.
Step 3: Tap the Threads
Start the tap as straight as possible. Turn it slowly and evenly. A good habit is to turn forward a bit, then back off slightly to break the chips. Keep repeating that pattern until the threads are fully cut. Do not rush this step. Broken taps usually come from side pressure, poor alignment, or trying to force the tool.
Step 4: Clean the Threads
Back the tap out carefully and clear away chips. Test the fit with the correct bolt. The bolt should thread in smoothly without wobble or excessive force. If it feels rough, inspect the hole for chips, misalignment, or a damaged thread start.
Common Mistakes to Avoid When Tapping
- Using the wrong drill bit size for the thread count
- Skipping lubrication, especially in steel or stainless
- Starting the tap crooked, which can damage threads fast
- Applying too much force instead of letting the tap cut
- Failing to back off and break chips while tapping
- Mixing up UNC and UNF because both are labeled 5/16
Most tapping problems come down to one of those mistakes. When beginners tell me tapping feels difficult, it is usually not because the process is hard. It is because one small setup detail was off from the start.
Best Drill Bits for Tapping (Recommended Picks)
What to Look for in a Drill Bit
For tapping work, I like drill bits that cut cleanly and hold size well. In most shops, high-speed steel is a solid starting point. If you drill a lot of harder metals, cobalt bits are often worth the upgrade. The biggest thing is not the fancy label. It is consistent sizing, sharp cutting edges, and a bit that does not wander or overheat too easily.
Recommended Drill Bits for 5/16 Tap
Letter Drill Bit Set
A smart choice if you want both F and I sizes in one set. Best for home shops, repair work, and anyone who taps more than one thread size.Check Price
Cobalt F Drill Bit
Great if you mostly need 5/16-18 UNC. A cobalt F bit is a strong pick for tougher metals and repeated shop use.Check Price
Cobalt I Drill Bit
A good match for 5/16-24 UNF. This is useful when you need fine threads and want a bit that stays sharp longer under load.Check Price
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Tap Drill Size Chart (General Reference)
| Tap Size | Drill Size | Decimal |
|---|---|---|
| 1/4-20 | #7 | 0.201″ |
| 5/16-18 | F | 0.257″ |
| 5/16-24 | I | 0.272″ |
| 3/8-16 | 5/16 | 0.3125″ |
I like keeping a quick chart like this nearby because it saves time and reduces mistakes. If you tap threads often, a printed chart in the shop is one of those small things that pays off over and over.
FAQs
What size drill bit for 5/16-18 tap?
Use an F drill bit, which measures 0.257 inch.
What size drill bit for 5/16-24 tap?
Use an I drill bit, which measures 0.272 inch.
Can I use a slightly bigger drill bit?
You can, but it is usually not a good idea unless the application specifically allows for lower thread engagement. A larger hole makes weaker threads.
What happens if drill size is too small?
The tap has to cut too much material. That increases friction and makes it much easier to break the tap or damage the threads.
Is tapping hard for beginners?
No. It gets much easier once you use the correct drill size, keep the tap straight, use lubrication, and back off to break chips. Good setup does most of the work.
Conclusion
If you came here looking for the exact answer, here it is one more time: use an F drill bit for a 5/16-18 UNC tap and an I drill bit for a 5/16-24 UNF tap. That one detail will save you from weak threads, rough cuts, and broken taps.
Once you understand the difference between coarse and fine threads, the rest of the job becomes much more straightforward. Drill the correct hole, use proper lubrication, keep the tap aligned, and take your time. That is the method I use in the shop, and it is still the simplest way to get clean threads that hold up.
