Drill Bit Size Chart: Decimal Equivalents, Free PDF

Q: What size is a #7 drill bit?

A #7 drill bit is 0.2010 inches (5.105 mm) in diameter. It is one of the most commonly referenced number drills because it is the standard tap drill for 1/4-20 UNC threads at 75% thread engagement.

Q: What size is a letter F drill bit?

A letter F drill bit is 0.2570 inches (6.528 mm) in diameter. Letter F is commonly used as the tap drill for 5/16-18 UNC threads. Letter drills range from A (0.2340") to Z (0.4130") and fill the gap between number drills and fractional drills.

Q: What is the decimal equivalent of a 5/16 drill?

The decimal equivalent of a 5/16 inch drill bit is 0.3125 inches (7.938 mm). To convert any fractional drill size to decimal, divide the numerator by the denominator: 5 divided by 16 equals 0.3125.

Q: What is the difference between letter and number drills?

Number drills range from #1 (0.2280") down to #80 (0.0135") and cover smaller hole sizes. Letter drills range from A (0.2340") up to Z (0.4130") and cover medium hole sizes. Together they fill the gaps between standard fractional drill sizes, giving machinists more precise diameter options.

Q: What drill bit sizes are available in metric?

Standard metric drill bits are available from 1 mm to 25 mm, typically in 0.5 mm increments (1.0, 1.5, 2.0, 2.5, etc.). Smaller metric drills are also available in 0.1 mm increments for precision work. Common sizes include 3 mm, 5 mm, 6.8 mm (M8 tap drill), 8.5 mm (M10 tap drill), and 10.2 mm (M12 tap drill).

Q: How do I convert a fractional drill size to decimal?

To convert a fractional drill size to decimal, divide the numerator by the denominator. For example, 5/16 = 5 divided by 16 = 0.3125 inches. For quick reference: 1/8 = 0.125, 1/4 = 0.250, 3/8 = 0.375, 1/2 = 0.500, 3/4 = 0.750, and 1 inch = 1.000.

Q: What is the most common drill bit size?

The most common drill bit size is 1/4 inch (0.250 inches / 6.35 mm). It is widely used for general-purpose drilling, clearance holes for #10 and #12 screws, and as a starting point for many projects. Other very common sizes include 1/8", 3/16", 5/16", 3/8", and 1/2".

Q: What size drill bit do I need for a 1/4-20 tap?

For a 1/4-20 UNC tap, use a #7 drill bit (0.2010 inches). This gives approximately 75% thread engagement, which is the standard for most CNC machining and general-purpose tapping. If you do not have a #7 drill, a 13/64 inch drill (0.2031") is a close substitute.

Q: Are metric and fractional drill sizes interchangeable?

Metric and fractional drill sizes are sometimes close enough to substitute, but they are not exact equivalents. For example, a 6.5 mm drill (0.2559") is close to a letter F drill (0.2570"), and a 10 mm drill (0.3937") is close to a 25/64" drill (0.3906"). For critical applications like tap drilling, always use the exact size specified. For general clearance holes, a close substitute usually works fine.

Drill Size ▲	Decimal (in) ▲	Metric (mm) ▲	Closest Fractional ▲	Category ▲
#80	0.0135	0.343	1/64″	Number
#79	0.0145	0.368	1/64″	Number
1/64″	0.0156	0.397	1/64″	Fractional
#78	0.0160	0.406	1/64″	Number
#77	0.0180	0.457	1/64″	Number
#76	0.0200	0.508	1/32″	Number
#75	0.0210	0.533	1/32″	Number
#74	0.0225	0.572	1/32″	Number
#73	0.0240	0.610	1/32″	Number
#72	0.0250	0.635	1/32″	Number
#71	0.0260	0.660	1/32″	Number
#70	0.0280	0.711	1/32″	Number
#69	0.0292	0.742	1/32″	Number
#68	0.0310	0.787	1/32″	Number
1/32″	0.0312	0.794	1/32″	Fractional
#67	0.0320	0.813	1/32″	Number
#66	0.0330	0.838	1/32″	Number
#65	0.0350	0.889	1/32″	Number
#64	0.0360	0.914	1/32″	Number
#63	0.0370	0.940	1/32″	Number
#62	0.0380	0.965	3/64″	Number
#61	0.0390	0.991	3/64″	Number
1.0 mm	0.0394	1.000	3/64″	Metric
#60	0.0400	1.016	3/64″	Number
#59	0.0410	1.041	3/64″	Number
#58	0.0420	1.067	3/64″	Number
#57	0.0430	1.092	3/64″	Number
#56	0.0465	1.181	3/64″	Number
3/64″	0.0469	1.191	3/64″	Fractional
1.5 mm	0.0591	1.500	1/16″	Metric
#55	0.0520	1.321	1/16″	Number
#54	0.0550	1.397	1/16″	Number
#53	0.0595	1.511	1/16″	Number
#52	0.0635	1.613	1/16″	Number
#51	0.0670	1.702	1/16″	Number
#50	0.0700	1.778	1/16″	Number
1/16″	0.0625	1.588	1/16″	Fractional
#49	0.0730	1.854	5/64″	Number
#48	0.0760	1.930	5/64″	Number
#47	0.0785	1.994	5/64″	Number
5/64″	0.0781	1.984	5/64″	Fractional
2.0 mm	0.0787	2.000	5/64″	Metric
#46	0.0810	2.057	5/64″	Number
#45	0.0820	2.083	5/64″	Number
#44	0.0860	2.184	3/32″	Number
#43	0.0890	2.261	3/32″	Number
#42	0.0935	2.375	3/32″	Number
3/32″	0.0938	2.381	3/32″	Fractional
2.5 mm	0.0984	2.500	3/32″	Metric
#41	0.0960	2.438	3/32″	Number
#40	0.0980	2.489	3/32″	Number
#39	0.0995	2.527	7/64″	Number
#38	0.1015	2.578	7/64″	Number
#37	0.1040	2.642	7/64″	Number
#36	0.1065	2.705	7/64″	Number
7/64″	0.1094	2.778	7/64″	Fractional
3.0 mm	0.1181	3.000	1/8″	Metric
#35	0.1100	2.794	7/64″	Number
#34	0.1110	2.819	7/64″	Number
#33	0.1130	2.870	7/64″	Number
#32	0.1160	2.946	1/8″	Number
#31	0.1200	3.048	1/8″	Number
1/8″	0.1250	3.175	1/8″	Fractional
#30	0.1285	3.264	1/8″	Number
#29	0.1360	3.454	9/64″	Number
9/64″	0.1406	3.572	9/64″	Fractional
3.5 mm	0.1378	3.500	9/64″	Metric
#28	0.1405	3.569	9/64″	Number
#27	0.1440	3.658	9/64″	Number
#26	0.1470	3.734	5/32″	Number
#25	0.1495	3.797	5/32″	Number
#24	0.1520	3.861	5/32″	Number
#23	0.1540	3.912	5/32″	Number
5/32″	0.1562	3.969	5/32″	Fractional
4.0 mm	0.1575	4.000	5/32″	Metric
#22	0.1570	3.988	5/32″	Number
#21	0.1590	4.039	5/32″	Number
#20	0.1610	4.089	11/64″	Number
#19	0.1660	4.216	11/64″	Number
#18	0.1695	4.305	11/64″	Number
11/64″	0.1719	4.366	11/64″	Fractional
4.5 mm	0.1772	4.500	11/64″	Metric
#17	0.1730	4.394	11/64″	Number
#16	0.1770	4.496	11/64″	Number
#15	0.1800	4.572	3/16″	Number
#14	0.1820	4.623	3/16″	Number
#13	0.1850	4.699	3/16″	Number
3/16″	0.1875	4.762	3/16″	Fractional
#12	0.1890	4.801	3/16″	Number
#11	0.1910	4.851	3/16″	Number
#10	0.1935	4.915	13/64″	Number
#9	0.1960	4.978	13/64″	Number
#8	0.1990	5.055	13/64″	Number
13/64″	0.2031	5.159	13/64″	Fractional
5.0 mm	0.1969	5.000	13/64″	Metric
#7	0.2010	5.105	13/64″	Number
#6	0.2040	5.182	13/64″	Number
#5	0.2055	5.220	13/64″	Number
#4	0.2090	5.309	13/64″	Number
#3	0.2130	5.410	7/32″	Number
7/32″	0.2188	5.556	7/32″	Fractional
#2	0.2210	5.613	7/32″	Number
#1	0.2280	5.791	15/64″	Number
5.5 mm	0.2165	5.500	7/32″	Metric
A	0.2340	5.944	15/64″	Letter
15/64″	0.2344	5.953	15/64″	Fractional
6.0 mm	0.2362	6.000	15/64″	Metric
B	0.2380	6.045	15/64″	Letter
C	0.2420	6.147	1/4″	Letter
D	0.2460	6.248	1/4″	Letter
1/4″	0.2500	6.350	1/4″	Fractional
E	0.2500	6.350	1/4″	Letter
6.5 mm	0.2559	6.500	1/4″	Metric
F	0.2570	6.528	1/4″	Letter
G	0.2610	6.629	17/64″	Letter
17/64″	0.2656	6.747	17/64″	Fractional
7.0 mm	0.2756	7.000	9/32″	Metric
H	0.2660	6.756	17/64″	Letter
I	0.2720	6.909	17/64″	Letter
J	0.2770	7.036	9/32″	Letter
9/32″	0.2812	7.144	9/32″	Fractional
K	0.2810	7.137	9/32″	Letter
7.5 mm	0.2953	7.500	19/64″	Metric
L	0.2900	7.366	19/64″	Letter
19/64″	0.2969	7.541	19/64″	Fractional
M	0.2950	7.493	19/64″	Letter
N	0.3020	7.671	19/64″	Letter
5/16″	0.3125	7.938	5/16″	Fractional
8.0 mm	0.3150	8.000	5/16″	Metric
O	0.3160	8.026	5/16″	Letter
P	0.3230	8.204	21/64″	Letter
21/64″	0.3281	8.334	21/64″	Fractional
Q	0.3320	8.433	21/64″	Letter
8.5 mm	0.3346	8.500	21/64″	Metric
R	0.3390	8.611	11/32″	Letter
11/32″	0.3438	8.731	11/32″	Fractional
S	0.3480	8.839	11/32″	Letter
9.0 mm	0.3543	9.000	23/64″	Metric
T	0.3580	9.093	23/64″	Letter
23/64″	0.3594	9.128	23/64″	Fractional
U	0.3680	9.347	3/8″	Letter
3/8″	0.3750	9.525	3/8″	Fractional
9.5 mm	0.3740	9.500	3/8″	Metric
V	0.3770	9.576	3/8″	Letter
W	0.3860	9.804	25/64″	Letter
25/64″	0.3906	9.922	25/64″	Fractional
10.0 mm	0.3937	10.000	25/64″	Metric
X	0.3970	10.084	25/64″	Letter
Y	0.4040	10.262	13/32″	Letter
13/32″	0.4062	10.319	13/32″	Fractional
Z	0.4130	10.490	27/64″	Letter
27/64″	0.4219	10.716	27/64″	Fractional
10.5 mm	0.4134	10.500	27/64″	Metric
11.0 mm	0.4331	11.000	7/16″	Metric
7/16″	0.4375	11.112	7/16″	Fractional
11.5 mm	0.4528	11.500	29/64″	Metric
29/64″	0.4531	11.509	29/64″	Fractional
15/32″	0.4688	11.906	15/32″	Fractional
12.0 mm	0.4724	12.000	15/32″	Metric
31/64″	0.4844	12.303	31/64″	Fractional
12.5 mm	0.4921	12.500	1/2″	Metric
1/2″	0.5000	12.700	1/2″	Fractional
13.0 mm	0.5118	13.000	33/64″	Metric
33/64″	0.5156	13.097	33/64″	Fractional
13.5 mm	0.5315	13.500	17/32″	Metric
17/32″	0.5312	13.494	17/32″	Fractional
35/64″	0.5469	13.891	35/64″	Fractional
14.0 mm	0.5512	14.000	35/64″	Metric
9/16″	0.5625	14.288	9/16″	Fractional
14.5 mm	0.5709	14.500	37/64″	Metric
37/64″	0.5781	14.684	37/64″	Fractional
15.0 mm	0.5906	15.000	19/32″	Metric
19/32″	0.5938	15.081	19/32″	Fractional
15.5 mm	0.6102	15.500	39/64″	Metric
39/64″	0.6094	15.478	39/64″	Fractional
5/8″	0.6250	15.875	5/8″	Fractional
16.0 mm	0.6299	16.000	5/8″	Metric
41/64″	0.6406	16.272	41/64″	Fractional
16.5 mm	0.6496	16.500	21/32″	Metric
21/32″	0.6562	16.669	21/32″	Fractional
43/64″	0.6719	17.066	43/64″	Fractional
17.0 mm	0.6693	17.000	43/64″	Metric
11/16″	0.6875	17.462	11/16″	Fractional
17.5 mm	0.6890	17.500	11/16″	Metric
45/64″	0.7031	17.859	45/64″	Fractional
18.0 mm	0.7087	18.000	45/64″	Metric
23/32″	0.7188	18.256	23/32″	Fractional
18.5 mm	0.7283	18.500	47/64″	Metric
47/64″	0.7344	18.653	47/64″	Fractional
3/4″	0.7500	19.050	3/4″	Fractional
19.0 mm	0.7480	19.000	3/4″	Metric
19.5 mm	0.7677	19.500	49/64″	Metric
49/64″	0.7656	19.447	49/64″	Fractional
25/32″	0.7812	19.844	25/32″	Fractional
20.0 mm	0.7874	20.000	25/32″	Metric
51/64″	0.7969	20.241	51/64″	Fractional
20.5 mm	0.8071	20.500	13/16″	Metric
13/16″	0.8125	20.638	13/16″	Fractional
21.0 mm	0.8268	21.000	53/64″	Metric
53/64″	0.8281	21.034	53/64″	Fractional
21.5 mm	0.8465	21.500	27/32″	Metric
27/32″	0.8438	21.431	27/32″	Fractional
55/64″	0.8594	21.828	55/64″	Fractional
22.0 mm	0.8661	22.000	55/64″	Metric
7/8″	0.8750	22.225	7/8″	Fractional
22.5 mm	0.8858	22.500	57/64″	Metric
57/64″	0.8906	22.622	57/64″	Fractional
23.0 mm	0.9055	23.000	29/32″	Metric
29/32″	0.9062	23.019	29/32″	Fractional
59/64″	0.9219	23.416	59/64″	Fractional
23.5 mm	0.9252	23.500	59/64″	Metric
15/16″	0.9375	23.812	15/16″	Fractional
24.0 mm	0.9449	24.000	15/16″	Metric
61/64″	0.9531	24.209	61/64″	Fractional
24.5 mm	0.9646	24.500	31/32″	Metric
31/32″	0.9688	24.606	31/32″	Fractional
63/64″	0.9844	25.003	63/64″	Fractional
25.0 mm	0.9843	25.000	63/64″	Metric
1″	1.0000	25.400	1″	Fractional

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Quick Answer: What Is the Decimal Equivalent of a Drill Bit Size?

A drill bit size chart converts every standard drill designation into its exact decimal inch and millimeter diameter. The four standard series are number drills (#1 = 0.2280″ to #80 = 0.0135″), letter drills (A = 0.2340″ to Z = 0.4130″), fractional drills (1/64″ to 1″) and metric drills (1–25 mm). To convert a fraction, divide numerator by denominator, so 5/16 = 0.3125″.

This chart lists 219 drill sizes with: drill designation, decimal inch equivalent, millimeter value and closest fractional size. Inputs include any drill callout, a number (#7), a letter (F), a fraction (5/16) or a metric size (6.5mm). Search by size or filter by series to find an exact match or a close substitute.

Common conversions: #7 → 0.2010″ (5.105 mm) | F → 0.2570″ (6.528 mm) | 5/16″ → 0.3125″ (7.938 mm) | 1/4″ → 0.2500″ (6.35 mm) | 8.5 mm → 0.3346″ | number drills #1–#80, letter drills A–Z, metric 1–25 mm.

Drill	Decimal (in)	Metric (mm)	Closest fraction	Why it is searched
#43	0.0890″	2.261 mm	3/32″	4-40 UNC tap drill
#36	0.1065″	2.705 mm	7/64″	6-32 UNC tap drill
#29	0.1360″	3.454 mm	9/64″	8-32 UNC tap drill
#25	0.1495″	3.797 mm	5/32″	10-24 UNC tap drill
#21	0.1590″	4.039 mm	5/32″	10-32 UNF tap drill
#7	0.2010″	5.105 mm	13/64″	1/4-20 UNC tap drill
#3	0.2130″	5.410 mm	7/32″	1/4-28 UNF tap drill
F	0.2570″	6.528 mm	1/4″	5/16-18 UNC tap drill
I	0.2720″	6.909 mm	17/64″	5/16-24 UNF tap drill
5/16″	0.3125″	7.938 mm	5/16″	3/8-16 UNC tap drill
Q	0.3320″	8.433 mm	21/64″	3/8-24 UNF tap drill
U	0.3680″	9.347 mm	3/8″	7/16-14 UNC tap drill
27/64″	0.4219″	10.716 mm	27/64″	1/2-13 UNC tap drill
29/64″	0.4531″	11.509 mm	29/64″	1/2-20 UNF tap drill

Drill Size	Decimal (in)	Metric (mm)	Common Use
#7	0.2010″	5.105 mm	Tap drill for 1/4-20 UNC (75% thread)
13/64″	0.2031″	5.159 mm	Closest fractional substitute (slightly larger)
5.0 mm	0.1969″	5.000 mm	Closest metric (slightly smaller — higher thread %)

Frequently Asked Questions

What size is a #7 drill bit?

A #7 drill bit is 0.2010 inches (5.105 mm) in diameter. It is one of the most commonly used number drills because it is the standard tap drill for 1/4-20 UNC threads at 75% thread engagement. The closest fractional size is 13/64″ (0.2031″).

What size is a letter F drill bit?

A letter F drill bit is 0.2570 inches (6.528 mm) in diameter. It is commonly used as the tap drill for 5/16-18 UNC threads. The closest fractional equivalent is 1/4″ (0.2500″), but the F drill is 0.0070″ larger.

What is the decimal equivalent of a 5/16 drill?

The decimal equivalent of a 5/16″ drill bit is 0.3125 inches (7.938 mm). To convert any fraction to decimal, divide the top number by the bottom number: 5 ÷ 16 = 0.3125.

What is the difference between letter and number drills?

Number drills range from #1 (0.2280″) down to #80 (0.0135″) and cover smaller hole sizes. The numbers go backwards — higher numbers mean smaller drills. Letter drills range from A (0.2340″) up to Z (0.4130″) and cover medium hole sizes. Together, they fill the gaps between standard fractional drill sizes, giving machinists more precise diameter options for tapping and other operations.

What drill bit sizes are available in metric?

Standard metric drill bits are available from 1 mm to 25 mm, typically in 0.5 mm increments (1.0, 1.5, 2.0, 2.5 mm, etc.). For precision work, metric drills are also available in 0.1 mm increments. Common sizes include 3 mm, 5 mm, 6.8 mm (M8 tap drill), 8.5 mm (M10 tap drill), and 10.2 mm (M12 tap drill).

How do I convert a fractional drill size to decimal?

Divide the numerator (top number) by the denominator (bottom number). For example: 5/16 = 5 ÷ 16 = 0.3125 inches. Quick reference: 1/8 = 0.125, 1/4 = 0.250, 3/8 = 0.375, 1/2 = 0.500, 3/4 = 0.750, 1″ = 1.000.

What is the most common drill bit size?

The most common drill bit size is 1/4″ (0.250 inches / 6.35 mm). It is widely used for general-purpose drilling, clearance holes for #10 and #12 screws, and as a starting point for many projects. Other very common sizes include 1/8″, 3/16″, 5/16″, 3/8″, and 1/2″.

What size drill bit do I need for a 1/4-20 tap?

For a 1/4-20 UNC tap, use a #7 drill bit (0.2010″). This gives approximately 75% thread engagement, the standard for most CNC machining applications. If you do not have a #7 drill, a 13/64″ (0.2031″) is a close substitute. See our tap drill chart for all thread sizes.

Are metric and fractional drill sizes interchangeable?

Sometimes they are close enough to substitute, but they are not exact equivalents. For example, a 6.5 mm drill (0.2559″) is close to a letter F drill (0.2570″), and a 10 mm drill (0.3937″) is close to a 25/64″ drill (0.3906″). For critical applications like tap drilling, always use the exact specified size. For general clearance holes, a close substitute usually works fine.

What are number drill bit sizes?

Number drill bits run from #80 (0.0135 in, the smallest) up to #1 (0.2280 in). A higher number means a smaller drill. They fill the gaps between fractional sizes. Most tap drills for UNC and UNF threads are number drills, for example #7 for 1/4-20 and #36 for 6-32.

What are letter drill bit sizes?

Letter drill bits run from A (0.2340 in) to Z (0.4130 in) and continue where number drills stop. Letter E is exactly 1/4 in. They are common in machine shops for tap and clearance holes between 1/4 in and 13/32 in, for example letter F is the tap drill for 5/16-18.

How do I pick a tap drill bit size?

Look the thread up in a tap drill chart. The tap drill leaves a hole a little smaller than the thread's major diameter so the tap can cut the thread into the wall. A 75% thread is the normal target: strong enough for almost every joint without overloading the tap. Use the exact listed drill rather than the nearest fractional size.

What is the difference between jobber and screw machine length drills?

Jobber length is the standard general-purpose length. Screw machine (stub) length drills are shorter and stiffer, so they deflect less, start more accurately and hold position better in CNC work where rigidity matters. Both come in the same number, letter, fractional and metric sizes.

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