What Size Drill Bit for a 1 4 Pipe Tap

It's almost impossible to know what size tap drill to use for your screw or bolts without a good tap drill chart at hand.

That's because there are so many variables to consider; from your screw size, its thread pitch, the substrate you're drilling into and even whether you require a close fit or free fit connection.

With all this, you need a quick and handy reference that takes all these specs into account and lays it all out in a single organized table.

Because it can be complex however, a helpful chart is only of any assistance if you can understand it and read it quickly.

But how?

What follows is some practical instruction on how to use a chart for tap drill sizes, including a useful guide for each section of the matrix.

The goal is to make this an easy reference for you, whether you're a casual DIY'er, hobbyist, student or experienced machinist working in a shop using top of the line Starrett tools.

Speaking of easy, we even look at a helpful trick, which uses a quick work-around calculation to find tap drill sizes you need. Keep reading!

TIP: Check out our full chart below to find all tap drill size equivalents for SAE and metric.

Otherwise, let's run through an overview of the chart so you can use it without confusion.

How to Read a Tap Drill Chart

In a nutshell, a good tap drill chart like the one below or a printable PDF version will have three main columns or sections.

The first column is the size of the screw or bolt you're using. The second column will list the size of the tap drill you need to use. The third and final column tell you what size drill bit to use to cut a hole to tap into.

Let's review each in more detail so you can use the chart efficiently.

Start with the Bolt and Screw Size You Need

When using a tap drill conversion chart, make sure you know what size fastener you're using. Screw sizes come in two different measurement classifications called Standard (SAE) and Metric.

Most of us are all to familiar with Standard sizes since North American uses these primarily from furniture to machines and other equipment. Of course, you'll find yourself working with metric bolts often enough.

Standard sizes are broken up into two different classes: a small diameter set and a larger diameter set.

Here's the breakdown for Standard sizes:

Sizes Less than 1/4″:

For sizes less than 1/4″, fractional measurements are not used. Instead, a numbering system has been created and ranges from 0-12. "0" of course is the smallest with "12" being the size just before you get to 1/4″.

Sizes 1/4″ and Larger:

For sizes greater than 1/4″, fractional sizing based on the actual diameter of the screw is used. For the sake of our tap drill chart, we'll see this range go from 1/4″ up to 1″ to correlate over to our tap drill sizing.

That's not all though.

When finding a tap drill size for your fastener in a tap drill chart you need a little more information about your fastener.Knowing the size is only the first part of the puzzle.

You'll also want to know the bolt's correspondingMajor Diameter. The good news is that each screw size has just one major diameter. This is merely the distance between the two outer ridges of the threading, and is included in the tap drill chart for easy reference.

You're almost there!

You'll also want to know the Pitch of your screw. This is just the thread count, or threads per inch (TPI) that the screw you're using is designed for.

This may seem to complicate things but again, a good tap drill chart will list all the possible thread counts to help you match the appropriate tap drill size.

Each fastener size in the Standard specification has two possible threads-per-inch values for screw sizes #1-to-#10. For sizes #12 and larger, there are three thread count sizes.

These different pitch values simply relate to how coarse or fine each threading is. In our tap drill chart for Standard sizes, the smaller pitch number on top is the coarser thread type with the other one (or two) being finer.

Specifically, the first number is called the UNC (Unified National Coarse) pitch. The second number is the UNF (Unified National Fine) pitch. And the bottom number in the stack is UNSF for "Super Fine".

In the chart, you simply find the number that relates to your fastener. Here's an example:

In this snapshot, the number "20" refers to the UNC, "28" refers to the UNF and "32" is UNSF or "Super Fine".

To learn more about pitches beyond what's included in the drill and tap chart, you can download a PDF covering bolt/screw pitches.

For example, a 7/16″ screw size can have a 14, 20, or 28 threads per inch as listed in the appropriate column on the chart.

One final note is that depending on the number of threads, your fastener can have different values for itsMinor Diameter. Not to worry though, these are also standard based on the number of threads. Basically, the more threads, the bigger the minor diameter number.

Now the Tap Drill Sizes for a Perfect Fit

The next set of columns in the tap drill chart simply correspond with the size of your screw listed on the left side of the chart (and just covered above). This info tells you what size tap to use for each fastener size.

What is tapping?

Simply put, if you tap a hole you're cutting threads evenly into it. A tap drill cuts the female threads and is placed down into a pre-drilled hole and turned with a special wrench.

Every tap is tapered for the first few threads to help create an easier start to the cutting process. You can buy taps (and dies for cutting outer or male threads), in kits that come with the required wrench.

In recent years, manufacturers have developed combination drill-tap bits for use in power drills. More about these later – and a tap chart is still going to be helpful!

When you buy a tap or die, it will be marked with three distinct numbers: 1. its diameter, 2. its threads per inch (pitch) and 3. the type of threads it cuts.

Tap and die set showing fastener size, thread type and diameter

So what the chart shows you here is the proper tap drill size that you can use to match the bolt or screw you're cutting threads for.

It's just like selecting drill bit sizes for regular drilling, except that tap drill sizes are assigned to match industry standard sized holes for tapping.

It gets a little complicated though, so our tap drill chart simplifies it for us, thankfully.

In our drill tap chart, we designate two different sizes based on how strong the substrate is (material you're tapping). We divide these up into soft and hard materials.

Soft materials include copper, aluminum and all types of plastic. Harder substances are found in industrial use and typically refer to stainless steel, steel, iron and titanium.

The chart will list two values for tap drill sizes; the standard drill size and it's decimal equivalent in inches. For example, a 5/16 drill size equates to 0.313 inches for soft metal drilling.

Getting Your Pre-Dilled Hole to the Right Size

The last segment of the tap drill chart are the columns on the right third of our table. This is the clearance drill size conversion, or the more plainly, the size drill bit you need to cut your initial hole before carrying out the rest of your tooling.

Two classes of sizes are list in these columns. One for closer or tight fits and one for looser fits.

Depending on the project and job of the fastener you may need it to screw tightly into its shaft. Or, you may want it to fit less snug and have a little clearance.

Each value listed here corresponds correctly to the tap drill sizes on to the left, which of course match to the size of your bolt or screw and thread type.

An Example to Help You Master the Drill and Tap Chart

Sometimes the best way to understand something completely is by an example. Let's look at one briefly.

Let's say you need to repair something on your car, or maybe you're building a machine or structure. You need to bolt together two difference pieces made of hard metal like steel.

First, know what bolts you're using.

You research and determine an SAE "12-28" bolt will do the job of securing your pieces together.

Now what?

It's easy, check the chart by finding the No. 12 size bolt on the far left column.

Snapshot of chart showing our example bolt size

From here, simply slide over and find the corresponding tap drill size in the row that lines up. You'll see that for heavy duty materials like steel, the correct tap drill size is "10" (or a 0.194 inch diameter tap).

Columns showing tap drill sizes for No. 12-28 bolt going into steel

Finally, determine if a tight fit or one with a more "open" clearance is adequate. Consider the material you're securing your attached piece to. Find the drill bit size that matches up in the row the same way you just found the tap drill value.

It's easy once you're familiar with reading the chart from left to right.

What about our metric tap drill chart?

You can read it the same way as the SAE chart. Just keep in mind that metric drills don't use a "thread per millimeter" for the thread pitch. They simply follow the formula of 1 divided by the number of threads per millimeter.

The metric chart will also list the tap drill size in both millimeters and also the site that is equivalent or approximate to North American drill sizes. You'll see one column for each.

This is really helpful if your toolbox or workshop doesn't have a lot of the metric sizes available.

Standard Tap Drill Chart (SAE)

Standard (SAE) Screw/Bolt Size Needed				Tap Drill Size				Pre- Drill Size
Screw Size	Major Diameter	Threads Per Inch	Minor Diameter	75% Thread for Soft Materials		50% Thread for Hard Materials		Tight Fit		Loose Fit
				Fractional	Decimal	Fractional	Decimal	Fractional	Decimal	Fractional	Decimal
0	0.060	80	0.045	3/64	0.047	55	0.052	52	0.064	50	0.070
1	0.073	64	0.054	53	0.060	1/16	0.063	48	0.076	46	0.081
		72	0.056	53	0.060	52	0.064
2	0.086	56	0.064	50	0.070	49	0.073	43	0.089	41	0.096
		64	0.067	50	0.070	48	0.076
3	0.099	48	0.073	47	0.079	44	0.086	37	0.104	35	0.110
		56	0.077	45	0.082	43	0.089
4	0.112	40	0.081	43	0.089	41	0.096	32	0.116	30	0.129
		48	0.086	42	0.094	40	0.098
5	0.125	40	0.094	38	0.102	7/64	0.109	30	0.129	29	0.136
		44	0.097	37	0.104	35	0.110
6	0.138	32	0.100	36	0.107	32	0.116	27	0.144	25	0.150
		40	0.107	33	0.113	31	0.120
8	0.164	32	0.126	29	0.136	27	0.144	18	0.170	16	0.177
		36	0.130	29	0.136	26	0.147
10	0.190	24	0.139	22	0.157	20	0.161	9	0.196	7	0.201
		32	0.152	21	0.159	18	0.170
12	0.216	24	0.165	16	0.177	12	0.189	2	0.221	1	0.228
		28	0.172	14	0.182	10	0.194
		32	0.178	13	0.185	9	0.196
1/4	0.250	20	0.189	7	0.201	7/32	0.219	F	0.257	H	0.266
		28	0.206	3	0.213	1	0.228
		32	0.212	7/32	0.219	1	0.228
5/16	0.313	18	0.244	F	0.257	J	0.277	P	0.323	Q	0.332
		24	0.261	I	0.272	9/32	0.281
		32	0.274	9/32	0.281	L	0.290
3/8	0.375	16	0.298	5/16	0.313	Q	0.332	W	0.386	X	0.397
		24	0.324	Q	0.332	S	0.348
		32	0.337	11/32	0.344	T	0.358
7/16	0.438	14	0.350	U	0.368	25/64	0.391	29/64	0.453	15/32	0.469
		20	0.376	25/64	0.391	13/32	0.406
		28	0.394	Y	0.404	Z	0.413
1/2	0.500	13	0.406	27/64	0.422	29/64	0.453	33/64	0.516	17/32	0.531
		20	0.439	29/64	0.453	15/32	0.469
		28	0.456	15/32	0.469	15/32	0.469
9/16	0.563	12	0.460	31/64	0.484	33/64	0.516	37/64	0.578	19/32	0.594
		18	0.494	33/64	0.516	17/32	0.531
		24	0.511	33/64	0.516	17/32	0.531
5/8	0.625	11	0.514	17/32	0.531	9/16	0.563	41/64	0.641	21/32	0.656
		18	0.557	37/64	0.578	19/32	0.594
		24	0.574	37/64	0.578	19/32	0.594
11/16	0.688	24	0.636	41/64	0.641	21/32	0.656	45/64	0.703	23/32	0.719
3/4	0.750	10	0.627	21/32	0.656	11/16	0.688	49/64	0.766	25/32	0.781
		16	0.673	11/16	0.688	45/64	0.703
		20	0.689	45/64	0.703	23/32	0.719
13/16	0.813	20	0.751	49/64	0.766	25/32	0.781	53/64	0.828	27/32	0.844
7/8	0.875	9	0.739	49/64	0.766	51/64	0.797	57/64	0.8906	29/32	0.9062
		14	0.787	13/16	0.813	53/64	0.828
		20	0.814	53/64	0.828	27/32	0.844
15/16	0.938	20	0.876	57/64	0.891	29/32	0.906	61/64	0.953	31/32	0.969
1	1	8	0.847	7/8	0.875	59/64	0.922	1-1/64	1.0156	1-1/32	1.0313
		12	0.898	15/16	0.938	61/64	0.953
		20	0.939	61/64	0.953	31/32	0.969

Metric Tap Drill Chart

Metric Screw/Bolt Size Needed		Tap Drill Size				Pre-Drill Size
		75% Thread for Soft Materials		50% Thread for Hard Materials		Tight Fit		Loose Fit
Screw Size (mm)	Thread Pitch (mm)	Drill Size (mm)	Approx. US Drill	Drill Size (mm)	Approx. US Drill	Drill Size (mm)	Approx. US Drill	Drill Size (mm)	Approx. US Drill
M1.5	0.35	1.15	56	1.25	55	1.60	1/16	1.65	52
M1.6	0.35	1.25	55	1.35	54	1.70	51	1.75	50
M1.8	0.35	1.45	53	1.55	1/16	1.90	49	2.00	5/64
M2	0.45	1.55	1/16	1.70	51	2.10	45	2.20	44
	0.40	1.60	52	1.75	50
M2.2	0.45	1.75	50	1.90	48	2.30	3/32	2.40	41
M2.5	0.45	2.05	46	2.20	44	2.65	37	2.75	7/64
M3	0.60	2.40	41	2.60	37	3.15	1/8	3.30	30
	0.50	2.50	39	2.70	36
M3.5	0.60	2.90	32	3.10	31	3.70	27	3.85	24
M4	0.75	3.25	30	3.50	28	4.20	19	4.40	17
	0.70	3.30	30	3.50	28
M4.5	0.75	3.75	25	4.00	22	4.75	13	5.00	9
M5	1.00	4.00	21	4.40	11/64	5.25	5	5.50	0.22
	0.90	4.10	20	4.40	17
	0.80	4.20	19	4.50	16
M5.5	0.90	4.60	14	4.90	10	5.80	1	6.10	B
M6	1.00	5.00	8	5.40	4	6.30	E	6.60	G
	0.75	5.25	4	5.50	7/32
M7	1.00	6.00	B	6.40	E	7.40	L	7.70	N
	0.75	6.25	D	6.50	F
M8	1.25	6.80	H	7.20	J	8.40	Q	8.80	S
	1.00	7.00	J	7.40	L
M9	1.25	7.80	N	8.20	P	9.50	3/8	9.90	25/64
	1.00	8.00	O	8.40	21/64
M10	1.50	8.50	R	9.00	T	10.50	Z	11.00	0.4375
	1.25	8.80	11/32	9.20	23/64
	1.00	9.00	T	9.40	U
M11	1.50	9.50	3/8	10.00	X	11.60	29/64	12.10	15/32
M12	1.75	10.30	13/32	10.90	27/64	12.60	1/2	13.20	33/64
	1.50	10.50	Z	11.00	7/16
	1.25	10.80	27/64	11.20	7/16
M14	2.00	12.10	15/32	12.70	1/2	14.75	37/64	15.50	39/64
	1.50	12.50	1/2	13.00	33/64
	1.25	12.80	1/2	13.20	33/64
M15	1.50	13.50	17/32	14.00	35/64	15.75	5/8	16.50	21/32
M16	2.00	14.00	35/64	14.75	37/64	16.75	21/32	17.50	0.6875
	1.50	14.50	37/64	15.00	19/32
M17	1.50	15.50	39/64	16.00	5/8	18.00	45/64	18.50	47/64
	2.50	15.50	39/64	16.50	41/64
M18	2.00	16.00	5/8	16.75	21/32	19.00	3/4	20.00	25/32
	1.50	16.50	21/32	17.00	43/64
M19	2.50	16.50	21/32	17.50	11/16	20.00	25/32	21.00	53/64
	2.50	17.50	11/16	18.50	23/32
M20	2.00	18.00	45/64	18.50	47/64	21.00	53/64	22.00	55/64
	1.50	18.50	47/64	19.00	3/4

There's an old shortcut you can use to quickly find metric tap drill sizes – with the help of a little grade school math.

If you take your screw size (in mm) and subtract it's pitch value, you get your answer.

So for example, take a screw size of M12 with a thread count (pitch) of 1.5. "12mm – 1.5mm = 10.5mm"

Bear in mind, this does not take into account coarse or fine thread counts for any given size, but it does give you an approximate size.

It's still best to stick to the chart!

Tapered Pipe NPT Tap Drill Chart

Of course screw-type fasteners are not the only hardware you can tap for. Pipes are the other common application for tapping.

Joining pipes together with threaded connections requires similar knowledge when it comes to creating pipe threads.

There are two things to note when tooling pipes: 1. Standard pipe threads vs dry fit, and 2. Tapered vs straight pipe tapping.

What are the differences?

Standard pipes (shortened to NPT) is the American National Standard Pipe Threading (ANSI) distinction. An NPT thread requires a sealing agent such as lubricant or tape in order not to leak any fluid it conveys.

The American National Standard "Dry Seal" Pipe Threading (NPTF) is self-sealing. The main physical differences between NPT and NPTF are the flatness of the peaks and valleys of the threads.

For the sake of our NPT tap drill chart, you simply need to know the distinction between an NPT fitting and NPTF. The chart below is for NPT pipes.

Lastly…

Pipe tapping can either be tapered or straight. This is pretty self explanatory, and again you'll see the differentiation on the pipe tap drill chart.

If you want to learn on these differences, this video explains the above in much greater detail:

And now the NPT tap drill chart…

This NPT (American National Standard Pipe Threading) chart is very easy to read. For example, reading across you'll find that a 1/8 NPT tap will require an "R" size tap drill. A 3/8 NPT tap will use a 37/64 size and so on.

Pipe Size	Threads per Inch	Tap Drill Size
1/16″	27
1/8″	27	R
1/4″	18	7/16
3/8″	18	37/64
1/2″	14	23/32
3/4″	14	59/64
1″	11-1/2	1-5/32
1-1/4″	11-1/2	1-1/2
1-1/2″	11-1/2	1-47/64
2″	11-1/2	2-7/32
2-1/2″	8	2-5/8
3″	8	3-1/4
3-1/2″	8	3-3/4
4″	8	4-1/4
4 1/2″	8	4-3/4
5″	8	5-9/32
6″	8	6-11/32
8″	8
10″	8
12″	8
14″	8
16″	8

What Size Drill Bit for a 1 4 Pipe Tap

Source: https://homerepairgeek.com/define/tap-drill-chart.html

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