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Screw Sizes & Specifications

2026-03-23

Screw dimensions mainly depend on three core parameters: thread diameter, pitch, and length, and are classified into three major systems: metric, imperial, and American standard. Below are the most commonly used categories and specifications.

I. Core Dimension Parameters (Key to Understanding Model Numbers)

Nominal Diameter (Major Diameter)
- Metric: Indicated by M + number, unit in mm, e.g., M3, M4, M5, M6, M8, M10, M12.
- American standard: Indicated by # + number, e.g., #2, #4, #6, #8, #10.
- Imperial: Indicated by fractional inches, e.g., 1/8", 1/4", 5/16", 3/8".
Thread Pitch
- Metric: Distance between adjacent thread crests, unit in mm. Coarse pitch is standard (usually omitted); fine pitch must be marked, e.g., M6×1.0 (coarse), M6×0.75 (fine).
- American / Imperial: Expressed in threads per inch (TPI), e.g., UNC (coarse), UNF (fine), such as 1/4"-20 (20 threads per inch).
Length
- Metric: Unit in mm, e.g., M4×20 (length 20mm), generally starting from 6mm, increasing by 2mm/5mm increments.
- American / Imperial: Unit in inches, e.g., #8×1/2" (length 0.5 inches).

II. Metric Screws (Most Common, Chinese National Standard)

1. Common Coarse Pitch Specifications (Standard pitch, unmarked)

Specification	Nominal Diameter (mm)	Standard Pitch (mm)	Common Lengths (mm)	Typical Applications
M1.6	1.6	0.35	3–10	Micro electronics
M2	2.0	0.4	4–12	Small electronics
M2.5	2.5	0.45	5–16	Electronics & precision parts
M3	3.0	0.5	6–30	Home appliances & furniture
M4	4.0	0.7	8–40	Machinery & automotive
M5	5.0	0.8	10–50	Machinery & equipment
M6	6.0	1.0	12–60	Construction & machinery
M8	8.0	1.25	16–80	Heavy machinery & structures
M10	10.0	1.5	20–100	Large equipment & steel structures
M12	12.0	1.75	25–120	Heavy structures & bridges

2. Common Fine Pitch Specifications (Common for anti-loosening & thin-walled parts)

M3×0.35, M4×0.5, M5×0.5, M6×0.75, M8×1.0, M10×1.25, M12×1.5, etc.

III. American Standard Screws (UNC/UNF, Commonly Used in North America)

1. Common Specifications (# Number + Thread Count)

Specification	Nominal Diameter (inch)	Coarse (UNC)	Fine (UNF)	Common Lengths
#2	0.086	56 TPI	64 TPI	1/8"–1/2"
#4	0.112	40 TPI	48 TPI	3/16"–3/4"
#6	0.138	32 TPI	40 TPI	1/4"–1"
#8	0.164	32 TPI	36 TPI	5/16"–1-1/4"
#10	0.190	24 TPI	32 TPI	3/8"–1-1/2"
1/4"	0.250	20 TPI	28 TPI	1/2"–2"

IV. Imperial Screws (Whitworth Thread, 55° Profile)

Common sizes: 1/8", 1/4", 5/16", 3/8", 1/2", 5/8", 3/4", 1", etc.
Thread count: e.g., 1/4"-20 BSW (coarse), 1/4"-26 BSF (fine).

V. Self-Tapping Screws (ST Series, No Nut Required)

Common metric sizes: ST2.9, ST3.5, ST3.9, ST4.2, ST4.8, ST5.5, ST6.3, etc.
Example: ST4.8×25 (diameter 4.8mm, length 25mm).

VI. Example Model Number Formats (Easy Interpretation)

Metric: M6×1.0×25 (M6 coarse pitch, length 25mm), M8×1.0×30 (M8 fine pitch, length 30mm).
American standard: #10-24×3/4" (#10 coarse pitch, length 3/4 inch).
Self-tapping: ST4.8×25 (self-tapping, diameter 4.8mm, length 25mm).

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