Tap Drill Size Chart Guide: How to Read It

Finding the right tap drill means matching three things: the thread size (like 1/4-20), the material you’re drilling (aluminum vs. Stainless steel), and the thread percentage you need (usually 65% to 75%). Charts give you the quick answer, but knowing why the numbers differ lets you adjust for your specific job.

That last part, the material and the percentage, is where most shop-floor problems start. A chart says “use a #7 drill for 1/4-20,” and it’s correct. But it doesn’t tell you that in soft aluminum, that same #7 drill might snap your tap because the material squeezes the threads tighter, or that in gummy stainless steel, you might need to open the hole up to an F drill to get the tap to cut without breaking. This is why you need a tap drill size chart guide that accounts for material and thread percentage. The numbers on the page are fixed; your workpiece isn’t.

What follows is a breakdown of how to read any tap drill size chart guide, the key variables that change the numbers, and the formulas to calculate your own size when a chart falls short. By the end, you’ll be able to look at a fastener, feel the material, and know whether to follow the chart exactly or reach for the next size up.

Key Takeaways

• Always start with the thread callout (e.g., 1/4-20 UNC) to find the correct row on a chart. The tap drill size column gives you the target drill bit.
• For most general-purpose work in mild steel or brass, many charts aim for 75% thread engagement, though some sources like OSG recommend a lower 55% to 65% for sufficient strength and better tool life.
• Softer metals (Aluminum) often need a slightly larger drill (aim for 65% thread) to prevent tap breakage. Harder metals (Stainless Steel) sometimes require an even larger drill (down to 50% thread) to reduce cutting force.
• The “major diameter minus pitch” rule works for metric threads. For inch threads, the formula is Drill Size = Major Diameter – (0.01299 × % Thread) / TPI.
• Clearance drill sizes are different. They are larger than tap drills and allow a bolt to pass through a hole without threading.

The Standard (UNC/UNF) Tap Drill Chart

Ever look at a tap drill size chart guide and wonder why there are so many columns for one thread size? A standard Unified National tap drill chart isn’t just a list; it’s a decision tree. Each column represents a different thread depth, and picking the right one depends on what you’re making and what you’re making it from.

The core layout is consistent. You find your thread, like 1/4-20 UNC, in the leftmost column. Next to it, you’ll see columns for different thread percentages, often 60%, 65%, 70%, and 75%. The number in the cell is the drill size you need, shown as a fraction, a numbered drill (#7), a lettered drill (F), or a decimal. For a 1/4-20 thread, the OSG Tapping Guide and the Allied Machine chart both list a #7 drill (0.201 inch) for 75% thread, which is the industry standard starting point.

For a 1/4-20 UNC thread aiming for 75% thread engagement, a common recommendation from sources like Allied Machine and eFunda is a #7 drill bit, measuring 0.2010 inches in diameter. This size produces a minor diameter within the Class 2B thread limits and provides an optimal balance of thread strength and tapping force.

But here’s the first curveball. Look at the JL Smith Co. Chart. It splits recommendations by material. For that same 1/4-20 thread in aluminum or brass, it agrees: use a #7 drill for 75% thread. But for stainless steel or iron, it recommends an F drill (0.2570 inch) for only 50% thread. That’s a massive jump, over 0.056 inches larger. This isn’t an error; it’s a critical material adjustment. A tap fighting through tough stainless needs less metal to cut, or it will snap.

Metric Tap Drill Sizes and the “Major Minus Pitch” Rule

Metric charts look different, but the logic is simpler. You won’t find columns for 75% or 65% thread as often. Instead, the most common rule, and the one codified in the ISO 2306-1972 standard, is major diameter minus pitch.

For example, you have an M10 x 1.5 bolt. The major diameter is 10 mm. The pitch is 1.5 mm. Your tap drill is 10 – 1.5 = 8.5 mm. That’s it. This formula provides a standard drill size; the resulting thread percentage varies, but it is a common starting point for metric threads. For an M6 x 1.0 bolt, the drill is 6 – 1 = 5.0 mm.

Where this goes sideways: Assuming this rule applies to fine-pitch metric threads. The Rivcut guide points out that metric fine pitch threads need a larger tap drill. Since the threads are shallower, the “major minus pitch” calculation still works, but the resulting thread percentage is higher, which can increase tapping difficulty. You might need to manually calculate for a lower percentage.

Thread Percentage: The 75% vs. 50% vs. 65% Debate

So what is thread percentage, and why do charts disagree? It’s the amount of the theoretical full thread height that is actually formed. A 100% thread is extremely strong but brutally hard to tap. A 50% thread is much easier to cut but has less strength. The sweet spot is somewhere in between, but the exact location depends on who you ask.

Source	General Recommendation	Stainless Steel	Aluminum	Key Takeaway
Wikipedia	60% to 75% thread desired.	Not specified.	Not specified.	The broad, consensus range.
OSG Tapping Guide	55% to 65% is sufficient for most.	60% – 70% for average work.	65% – 70% for average work.	Favors lower percentages for tool life.
Rivcut Guide	75% is the standard for charts.	60-65% to reduce force.	65% to extend tap life.	Adjusts based on material hardness.
JL Smith Co.	Splits by material.	Recommends 50% thread.	Recommends 75% thread.	Most aggressive material-based adjustment.

The OSG guide explicitly states its philosophy: “use the largest drill possible… Consistent with adequate strength.” This prioritizes tap life and ease of tapping over maximizing thread strength. They claim a 55% to 65% thread is enough for most jobs. In contrast, many generic charts default to 75%; the Rivcut guide notes that 75% thread gives about 90% of a full thread’s strength.

The shop-floor truth is this: For a home workshop drilling mild steel or brass, 75% is fine. When you move to stainless, listen to the Rivcut or JL Smith advice and open the hole up. Your taps will thank you.

How to Read a Tap Drill Chart (Column by Column)

A tap drill chart is a dense reference, not a narrative. Here’s what each common column means and when you need to look at it:

1. Thread Size / TPI: The anchor. Find your thread (e.g., 1/4) and its threads per inch (20). This is your row.
2. Tap Drill (75%): This is your default target column. If the chart has only one “tap drill” column, it’s almost always for 75% thread.
3. Tap Drill (65%, 70%, etc.): Use these when adjusting for material. Go lower percentage (like 65%) for a larger hole in soft or gummy materials. Go higher percentage (like 85%) for a smaller hole only in thin sheet metal where you need maximum thread depth.
4. Decimal Equivalent: The drill size in inches (e.g., 0.2010). This is essential for using a digital caliper or a drill press with a dial indicator.
5. Clearance Drill (Close Fit / Free Fit): Crucially, these are NOT for tapping. These are the drill sizes for the hole the bolt’s unthreaded shank passes through. “Close fit” (e.g., F drill for 1/4 bolt) gives minimal play for precise alignment. “Free fit” (e.g., H drill) gives more clearance for faster assembly.

I keep my own drill bit sizes chart laminated on the wall, but I always cross-check it against a manufacturer’s guide like the OSG PDF for tricky materials. The wall chart gets me close; the manufacturer data keeps me out of trouble.

Formulas: When You Need to Calculate Your Own Tap Drill

Charts cover standard threads. What about a custom 5/16-24 thread, or needing a 55% thread on a special bolt? You need the formulas.

For inch-based threads, one formula from the Victor Machinery reference is: Drill Size = Major Diameter – (0.975 / Threads Per Inch). For more precision, the OSG formula is: Drill Size = Major Diameter – (0.01299 × % Thread) / TPI.

For example, you need a 1/2-13 thread at 65% engagement in aluminum. Major Diameter = 0.5000″. Calculation: 0.01299 × 65 = 0.84435. Divide by 13 TPI = 0.06495. Drill Size = 0.5000 – 0.06495 = 0.4351 inches. A 0.4375″ (7/16) drill is the closest standard size and is perfect.

Common mistake: Using the formula’s raw decimal result and forcing a non-standard drill. Always round to the nearest standard drill bit size from your comprehensive drill bit size reference. The difference of a few thousandths of an inch won’t matter for strength, but trying to custom-grind a drill bit will.

The reverse formula is just as useful for troubleshooting: % Thread = TPI × (Major Diameter – Drill Diameter) / 0.01299. If a tapped hole strips out, plug in the drill size you used. You might find you accidentally drilled for only 50% thread.

Material Considerations: Why One Size Doesn’t Fit All

The metal in your hands talks. Hard, gummy stainless steel chatters and squeals when drilled. Soft, sticky aluminum grabs at the drill flutes and can weld to the tap. These behaviors demand different drill sizes, even for the same thread.

The ISO 2306-1972 standard notes this quietly: “When tapping relatively soft material, the minor diameter of the tapped hole may become smaller than the drill used due to material squeezing.” This is the engineering way of saying aluminum will pinch your threads. If you drill an 8.5mm hole for an M10x1.5 tap in aluminum, the resulting threads might be tighter than expected, increasing torque and the risk of tap breakage. The solution? Use the chart’s recommendation for a lower thread percentage, like 65%, which means a slightly larger drill.

Material	Challenge	Thread % Advice	Result
Aluminum, Soft Brass	Material grabs tap; chips can weld.	65% – 70% (OSG, Rivcut)	Larger hole reduces friction, extends tap life.
Stainless Steel, Titanium	High strength, work-hardens.	50% – 65% (JL Smith, Rivcut)	Larger hole drastically reduces cutting force.
Thin Sheet Metal	Need maximum thread engagement in minimal material.	75% – 85% (OSG)	Smaller hole ensures enough threads to hold.
Cast Iron, Mild Steel	Predictable, chips break cleanly.	70% – 75% (Standard)	Follow the standard chart.

For stainless, some machinists follow the JL Smith Co. recommendation of 50% thread, while others prefer the 60-70% range suggested by OSG or 60-65% from Rivcut. It seems extreme until you’ve snapped a pricey tap in a deep hole. That larger clearance gives the tap room to cut without overloading. You’re trading a marginal amount of tensile strength for a dramatically higher chance of success. For jobs like this, having a dedicated set of drill bits for metal with a sharp 135° split point is essential.

Clearance Holes vs. Tap Drills: Don’t Mix Them Up

This is the most frequent error in beginner workshops. A tap drill creates the hole you will thread. A clearance drill creates the larger, unthreaded hole a bolt slides through. Using a tap drill for a clearance hole means the bolt won’t fit. Using a clearance drill for a tap hole means your threads will be shallow and weak.

Most charts, like the one from eFunda.com, include both. Look at the row for a 1/4-20 bolt. The tap drill is #7 (0.201″). The “Close Fit” clearance drill is F (0.257″), and the “Free Fit” is H (0.266″). If you’re making a bracket where two plates bolt together, you’d drill one plate with the tap drill and tap it. The matching plate gets drilled with the clearance drill.

What most people get wrong: Drilling both parts with the clearance hole size and then trying to tap one. There’s simply not enough material left to form proper threads, and the fastener will pull right through.

Always mark your drill bits or organize your drill and tap sets so the clearance drills are physically separated. The tactile difference matters when you’re reaching for a bit mid-project.

Step-by-Step: Using a Chart to Drill and Tap a Hole

Let’s walk through a real example, end-to-end. You’re installing a 1/4-20 threaded insert into a steel block.

1. Identify the Thread: It’s 1/4-20 UNC.
2. Consult the Chart: Find 1/4-20. The 75% thread column says #7 drill (0.201″). Since it’s mild steel, we’ll stick with 75%.
3. Select the Drill: Secure a #7 drill bit. Check its drill bit markings to confirm.
4. Drill the Hole: Use cutting fluid. Drill to the required depth, adding a little extra for the tap’s point.
5. Tap the Hole: Align the 1/4-20 tap square to the hole. Start with forward pressure, then back off every half-turn to break the chip. Use plenty of lubricant.
6. Clean and Test: Blow out the hole. Test-fit the bolt. It should screw in smoothly with firm, even resistance.

If the tap feels excessively hard to turn, stop. You likely need a sharper tap, more lubricant, or, if the material is harder than expected, a larger drill. Forcing it is a direct path to a broken tap and a ruined workpiece.

Frequently Asked Questions

What is the most common tap drill size?

For the ubiquitous 1/4-20 UNC thread, the most common tap drill is a #7 drill bit (0.2010 inch), which targets a 75% thread engagement. This is the standard reference point across most charts from sources like Allied Machine and eFunda.

Can I use a regular drill bit for tapping?

Yes, but with a caveat. The drill bit must be the exact size specified by the tap drill chart. A “regular” twist drill bit is fine if it’s the right diameter. However, for best results in metal, use a bit designed for the material, like a cobalt or metal cutting drill bits with a 135° point, to ensure a clean, accurate hole.

How do I know if my tap drill is too small?

If your tap drill is too small, the tap will be very difficult to turn, produce a high-pitched squealing sound, and generate excessive heat. The chips may come out as a fine, dusty powder instead of small curls. This creates extreme stress on the tap, greatly increasing the chance it will snap in the hole.

What’s the difference between a tap drill and a clearance hole?

Tap drill creates a hole slightly smaller than the bolt’s threads, leaving material for the tap to cut. A clearance hole is larger than the bolt’s threads, allowing the bolt’s smooth shank to pass through freely. Never use a clearance drill size for a hole you intend to tap.

Do I need a different tap drill for stainless steel?

Often, yes. Stainless steel is harder and gummier than mild steel. To reduce tapping force and prevent tap breakage, most guides recommend a larger drill bit, which creates a lower thread percentage. The JL Smith Co. chart recommends a 50% thread for stainless, while the OSG guide suggests 60-70%. Start with a chart that specifies material.

Before You Go

Forget memorizing the chart for a 1/4-20 thread. The real skill is knowing how to use the chart. Match the thread, then let the material decide the column. Mild steel? Take the 75% default. Aluminum? Glance over to the 65% column. Stainless? Be bold and consider the 50% or 60% recommendation. When in doubt, drill a test hole in a scrap piece of the same material and try the tap. Those two minutes save a $15 tap and an hour of frustration.

The formulas are your backup plan for oddball threads, but standard charts exist for a reason. Bookmark a reliable standardized tap drill chart and a detailed engineering department drill PDF for your shop wall. The goal isn’t to calculate every hole, it’s to drill the right one the first time.