The screw size is determined by two factors: its diameter and its length. The diameter of a screw refers to the width of the screw shank, which is the threaded cylindrical portion.

This diameter is commonly expressed in millimeters or inches. On the other hand, the length of a screw is measured from under the head to the tip of the screw.

This measurement is also commonly expressed in millimeters or inches. A screw’s thread count and its diameter and length are important in determining its size. The thread count refers to the number of threads or ridges per inch or millimeter along the screw shank.

Screw size is determined by a combination of diameter, length, and thread count, resulting in a secure and tight fit for each project. Understanding these key factors is critical in determining the correct screw size for your specific requirements.

Source: Wikipedia

Jump immediately to the FAQ section

Quick Reference: Most Common Screw Sizes

For Quick Lookups:

• #6 screws (0.138″ / 3.5mm): Light furniture, drawer slides, small projects
• #8 screws (0.164″ / 4.17mm): General purpose, cabinets, most furniture – MOST VERSATILE
• #10 screws (0.190″ / 4.83mm): Heavy furniture, construction, structural work
• #12 screws (0.216″ / 5.49mm): Decking, outdoor projects, heavy-duty applications

How to Read Screw Sizes:

• Imperial: #8-32 x 1.5″ = #8 gauge (diameter), 32 TPI (threads per inch), 1.5″ long
• Metric: M6 x 1.0 x 40mm = 6mm diameter, 1.0mm pitch, 40mm long

Table of content

• Screw Size Chart and Thread Count Guide
• Screw Thread Count
• The Different Types of Screws and their sizes
  • Wood Screws sizing
• Understanding Screw Sizes
• How to measure screw size?
• Screw Types and Applications
• Screw Head Types
• Material specific recommendations
• Faq

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Screw Size Chart and Thread Count Guide

Related: different Bolt sizes explained

Screw Thread Count

What is the first thing that comes to mind when you hear the word fastener? It might be nails, bolts, nuts, washers, screws, or rivets.

All these are the different fastener categories. However, we will focus on screws. There are various types of screws available in the market.

The screws have different head types and head styles. Therefore, we present this comprehensive screw size chart and thread count guide. It has helpful information that will enable you to understand the difference and get the actual screw sizes.  

The Different Types of Screws

The following are the four major types of screws available in the market.

1.  Wood Screw sizing

The diameter of wood screws ranges from #0 to #24; the larger the number, the larger the size, and #0 is the smallest screw size.

Sizes ranging from #2 to #14 are widely available. The number size has no bearing on any measurement. Lengths range from 1/4″ to 6″; the longest typically found is 5″ or so, and larger screw sizes are available in the longer lengths.

Screws less than 1″ are measured in 1/8″ increments; screws 1″ to 3″ are measured in 1/4″ increments; and screws longer than 3″ are measured in 1/2″ increments.

Wood screws are measured the same way as other screws: “The length of a headed fastener is defined as the distance between the intersection of the largest diameter of the head and the bearing surface and the extreme.”

These are designed for connecting items such as metals to wooden bases. Generally, wood expands and shrinks depending on the surrounding temperature.

Therefore, you need a specialized screw that would be effective in a head wood screw drill. Wood screws have the capability of bending before snapping. Therefore, they are not used to drill holes in metals.

Wood Screw Dimensions, Sizes, Head Diameters & Threads per Inch

2. Sheet Metal Screws

These screws are typically sturdier and stronger than wooden screws because they are made of steel. Steel is the most common material used for the manufacture of screws.

These screws are used for applications in various materials but not on metals because they strictly stay in place once screwed on.  Note that tapped holes are required before a machine screw such as a hex head screw is inserted.

The sheet metal screws are self-tapping, meaning they do not need pre-drilled holes into the source before inserting them. They are best for application in heavy gauge sheet metal.

3. Phillips Head Screw

Its flat screw head cannot be used on sheet metal cross head screws. If you are always working with screws, get yourself at least one of these types of screwdrivers.

When using these screws, have a countersunk hole to ensure that the countersinking flat head screws are in place.

4. Drywall Screws

Most people have embraced the drywall screws as the standard fasteners to secure full or partial sheets of drywall on wood studs or on ceiling joists. Different types of such screws include coarse threads and finer thread drywall screws.

Also, there are differences between their pitch sizes. The coarse threads have a large thread pitch and the finer threads have a smaller thread pitch.

The drywall screws intended for construction always range from 1 inch to 8 inches long.This is because building materials vary in thickness. You can be working with dense materials, heavy materials, or even soft materials.

Drywall screws installed in homes can be ½ -inch thick, with the 5/8 thick drywall screws used in garages and walls adjacent to furnace rooms. The ¼ inch thick drywall screws are used as facing for walls, ceilings or to form curves. For kitchen and bathrooms, use the ½ -inch thick screws.

Understanding Screw Sizes

It is not enough for you to understand the acronyms and quotations on the packaging. You need to be conversant with the screw sizes.

This is because it sometimes gets tricky when it comes to sizing the screws, both metric sizes, and imperial sizes. Thread count also measures screw sizes. For example, a female ¼-20 socket screw has a diameter of ¼ and 20 female threads per inch.

Imperial and Metric Screw Sizes

Most of the companies combine both the imperial and metric sizes on the same box screws. However, when purchasing the screws online, you might find that most retailers do not do this.

Therefore, you are required to know the difference between the two sizes. Knowing the difference helps minimize the possibility of getting the wrong sizes.

Explanation on Imperial Screw Sizes

The screws used for wood application are available in two distinct sizes. The larger the number, the larger the diameter.

Therefore, a size 12 screw is larger than a size 4. The screws from gauge 6 and above have twice the diameter’s head.

Also, the given length for the screws is the length that is usually buried in the wood or in other materials. The head of the raised screw is usually not included. Therefore, the screw size is determined by the gauge and the length.

Explanation on Metric Screw Sizes

If you are unskilled, it would be difficult for you to understand the metric system. This is because you are not conversant with it or still working with imperial.

The metric and imperial system uses the diameter sizes measured in millimeters instead of a gauge table. Coincidentally, the gauge is approximately the size of the screw head in millimeters.

Therefore, a size 6 gauge screw has a head almost equal to 6 mm wide. If you are a professional, you understand how difficult it is to effectively correlate the diameter of the metric, the gauge (imperial), and the head sizes.

Therefore, it is important that you know no information can guarantee a perfect correlation. This might be why most professionals prefer getting the screws personally to ensure they get the right screw sizes.

The imperial diameter (in 16^th of an inch) of the screw head is usually twice the gauge (imperial). Use the formula below to estimate the screw head and gauge.

Gauge= (Head diameter in sixteenths of an inch X 2 ) – 2. E.g. 5/16 head times two equals 10, minus two equals 8. The Gauge is 8.

The above formula shows that the imperial gauge diameter in millimeters is almost half the gauge.  Most people do not know the said relationship. Therefore, having the formula in the screw size charts and thread count guide gives you an added advantage.

Screw Types and Applications

Wood Screws

Characteristics: Tapered shaft, coarse threads, wood-specific design

When to use:

• Furniture assembly
• Cabinet construction
• General woodworking
• Picture frames

Size selection:

• Light work: #6
• General purpose: #8 (most common)
• Heavy-duty: #10, #12

Drywall Screws

Coarse Thread (Wood Studs):

• 1-1/4″ for 1/2″ drywall
• 1-5/8″ for 5/8″ drywall

Fine Thread (Metal Studs):

• 1″ to 1-1/4″ for metal framing
• Self-tapping points

Installation: Drive until paper “dimples” (don’t break surface)

Deck Screws

Standard sizes:

• #8 x 2-1/2″ for 5/4 decking
• #10 x 3″ for hardwood decking

Requirements:

• ACQ-compatible coating (for treated lumber)
• Corrosion-resistant
• Sharp point, aggressive threads

Machine Screws

Characteristics: Uniform diameter, precise threading

Applications:

• Electronics
• Machinery
• Metal-to-metal fastening
• Used with nuts or tapped holes

Thread selection:

• UNC for general use
• UNF for vibration resistance

Imperial to Metric Conversion

Diameter Conversion

Length Conversion

• 1/2″ = 12-13mm
• 3/4″ = 19-20mm
• 1″ = 25mm
• 1-1/2″ = 38-40mm
• 2″ = 50mm
• 3″ = 75mm

WARNING: Even when measurements appear similar, imperial and metric screws are NOT interchangeable due to different thread

Screw Head Types

Flat/Countersunk

• Sits flush or below surface
• Requires countersinking
• Best for: finish work, cabinets, hinges

Pan Head

• Low-profile dome
• Sits on surface
• Best for: sheet metal, electrical boxes

Hex Head

• Six-sided, wrench-driven
• High torque capacity
• Best for: lag screws, structural connections

Socket Head (Allen)

• Internal hex socket
• Very high torque
• Best for: machinery, furniture, bicycles

Drive types: Phillips, Torx (best), Square, Slotted (avoid)

Material-Specific Recommendations

Softwood (Pine, Cedar, Fir)

Screws: #6 to #10 wood screws
Pilot holes: Optional for #6, recommended for #8+
Tips: Pre-drill near edges, avoid over-tightening

Hardwood (Oak, Maple, Walnut)

Screws: #8 to #12, high-quality screws
Pilot holes: ALWAYS required
Tips: Lubricate threads with wax, use sharp bits, slow drilling

Drywall

Into wood studs: 1-1/4″ coarse thread
Into metal studs: 1-1/4″ fine thread
Spacing: 12″ on center field, 8″ edges

Plywood

Screws: #6 to #10
Pilot holes: Recommended (prevents delamination)
Tips: Avoid edge screwing, don’t over-tighten

MDF/Particleboard

Screws: Coarse thread, confirmat screws
Tips: Pilot holes essential, don’t over-tighten (material crumbles), use reinforcement sleeves for heavy loads

Metal-to-Metal

Options:

• Machine screws with nuts
• Self-tapping metal screws
• Tek screws (self-drilling)

Requirements: Proper pilot holes, cutting oil, torque specifications

How to measure screw size?

Imperial & Metric Threads Sizes Chart

The thread sizes are given in nominal sizes, not actual measurements. For instance, a bolt size of 5.9 mm is referred to as a 6mm thread.

Often, thread sizes in inches are specified by nominal thread major diameter and threads per inch. For example, for a screw size 10-24, the first digit (10) refers to the nominal thread major diameter.

The second digit (24) refers to threads per inch. When the nominal thread diameter is smaller than ¼”, the value is indicated as #1or No. 1.

Screw size chart explained for wood screws (video)

1. Determine the type of screw.

2. Measure the diameter.

3. Measure the length.

4. Choose the right units for measurement.

5. Record the measurements for accuracy and to avoid future mistakes with size or type of screws.

More info on Wikihow

Frequently Asked Questions (FAQ)

What size in mm is a No 8 screw?

A No 8 screw has a diameter of 3.5mm and an overall length of 9.5mm. (mm to inches: divide the length value by 25,4)

What are screw standard sizes?

Screw standard sizes are standardized measurements to identify a given screw’s diameter, thread pitch, length, and head shape.

The size of machine screws is measured by numbers ranging from 0 to 14, with larger numbers indicating a bigger screw. Common threads per inch sizes are 4, 4-1/2, 5, 5-1/2, 6, 7, 8, 9, 10, 11, 12, 13, 14, 16, 18, 20, 24, 28, 32, 36, 40, 48, 56, 72, 80.

Different types of screws have other measures based on the application they will be used for. Standardized dimensions make it easy for users to choose the correct size when purchasing screws online or at a hardware store.

What size is a No 6 screw in mm?

A No 6 screw has a thread size of 3.5mm or 0.14 inch

Does M6 mean 6mm?

An M6 screw size refers to a metric 6 mm screw. The outside diameter of the threads is 6 mm.

How do you measure screw size?

Measure three dimensions:

1. Diameter: Use calipers on major diameter (outer thread edge)
2. Length: Top of head to tip (countersunk) or underside to tip (pan head)
3. Thread count: Count peaks in 1″ (imperial) or measure peak-to-peak in mm (metric)

What do screw numbers mean (e.g., #8-32 x 1.5″)?

• #8: Gauge size (0.164″ diameter)
• 32: Threads per inch
• 1.5″: Length in inches

For metric M6 x 1.0 x 40mm:

• M6: 6mm diameter
• 1.0: 1.0mm thread pitch
• 40mm: 40mm long

What size pilot hole for #8 screw?

• Softwood: 3/32″ (2.4mm)
• Hardwood: 7/64″ (2.8mm)
• Shank clearance: 11/64″ (4.4mm) through top piece

Can I use metric screws in imperial holes?

NO. Never force metric screws into imperial threaded holes or vice versa. Thread profiles are incompatible and will cross-thread, damaging both screw and hole.

What’s the difference between #8 and #10 screws?

• #8: 0.164″ diameter, lighter duty
• #10: 0.190″ diameter (16% thicker), 30-35% stronger
• #10 for: Heavy furniture, construction, structural work
• #8 for: General furniture, cabinets, most woodworking

Do I need pilot holes for self-tapping screws?

Depends on material:

• Metal: Yes, almost always
• Softwood: Usually no (modern screws)
• Hardwood: Yes, recommended
• Plastic: Yes, required
• Self-drilling (Tek) screws: No (that’s the point!)

What size screws for hanging cabinets?

Upper cabinets into studs:

• #8 x 2-1/2″ to 3″ wood or cabinet screws
• Minimum 2 screws per stud
• Penetrate 1-1/2″ minimum into stud

What size screws for deck boards?

Standard recommendation:

• #8 x 2-1/2″ for 5/4 decking (most common)
• #10 x 2-1/2″ for hardwood decking
• Must be ACQ-compatible coating
• 2 screws per joist crossing

Why is my screw stripping?

Common causes:

1. Wrong driver size/type
2. Pilot hole too small or missing
3. Too much speed, not enough pressure
4. Worn driver bit
5. Low-quality screws

Fix: Use exact-match driver, drill proper pilot, apply firm pressure, slow down speed

Why is wood splitting when driving screws?

Causes:

1. No pilot hole (most common)
2. Too close to edge (within 2-3″)
3. Screw too large for material
4. Driving too fast
5. Dry or brittle wood

Prevention: Always pilot hole, stay 3x screw diameter from edges, use correct size screws

How tight should screws be in wood?

Correct tightness:

• Head flush with surface
• Wood not crushed around head
• No gap between pieces
• Firm but not over-driven

Stop when:

• Gentle resistance felt
• Head reaches flush
• Drill clutch clicks (if set properly)

Over-tight signs: Wood crushing, screw sinking too deep, cracks forming

What’s the difference between UNC and UNF threads?

UNC (Coarse):

• Fewer threads per inch (20, 24, 32 TPI)
• Deeper threads
• Faster installation
• Best for: wood, general purpose, quick assembly

UNF (Fine):

• More threads per inch (28, 32, 36 TPI)
• Shallow threads
• Higher strength
• Best for: metal, precision, vibration resistance

What is the metric equivalent of #10 screw?

Closest match: M5

• #10 = 0.190″ = 4.83mm
• M5 = 5.0mm

BUT: Not interchangeable due to different thread profiles. Use for reference only.

How do I convert TPI to metric pitch?

Formula: Pitch (mm) = 25.4 ÷ TPI

Examples:

• 20 TPI = 25.4 ÷ 20 = 1.27mm pitch
• 32 TPI = 25.4 ÷ 32 = 0.79mm pitch (≈ 0.8mm standard)

What screws for metal to metal?

Best options:

1. Machine screws with nuts (strongest)
2. Machine screws into tapped holes
3. Self-tapping metal screws
4. Self-drilling (Tek) screws

Requirements: Proper pilot holes, UNC for aluminum, UNF for steel, thread locker for vibration

How to choose screw length?

General rule: Screw should penetrate 2/3 into base material

Calculation:

1. Top piece thickness
2. Add 2/3 of bottom piece depth
3. Round up to next standard size

Example: 3/4″ top + (2/3 × 1-1/2″ bottom) = 3/4″ + 1″ = 1-3/4″, use 2″ screw

What does coarse thread mean?

Coarse threads have:

• Wider spacing between threads
• Fewer threads per unit length
• Deeper V-shaped profile
• Faster installation
• Better for soft materials (wood, plastic)

When should I use fine thread screws?

Choose fine threads for:

• Hard materials (steel, hard alloys)
• High tensile strength needs
• Vibration-prone applications
• Thin-walled materials
• Precision adjustments
• Automotive engines, aircraft, machinery

What is thread pitch in metric screws?

Thread pitch = distance in millimeters between adjacent thread peaks

Examples:

• M6 x 1.0 = 1.0mm between threads (coarse)
• M6 x 0.75 = 0.75mm between threads (fine)

Smaller pitch = finer threads (closer together)
Larger pitch = coarser threads (farther apart)

Different pilot hole sizes for softwood vs hardwood?

Yes, hardwood needs larger pilots:

Softwood (50-60% of screw diameter):

• Less dense, compresses easily
• #8 pilot: 3/32″ (2.4mm)

Hardwood (70-90% of screw diameter):

• Very dense, high resistance
• #8 pilot: 7/64″ (2.8mm)

Hardwood pilots prevent screw breakage and head stripping.

What size screw for 2×4 lumber?

Edge to edge (wide board): #10-12 x 3″ to 3-1/2″
Face to edge (T-joint): #8-10 x 2-1/2″ to 3″
Face to face (laminating): #8 x 1-1/2″ to 2″

Always use pilot holes for structural connections.

How do I know if pilot hole is correct size?

Too small: Extreme resistance, wood starts cracking, screw gets hot

Just right: Smooth driving with some resistance, threads grip well, no splitting

Too large: Screw spins freely, no holding power, can pull out easily

Test method: Hold drill bit behind screw – should hide shaft but threads visible on sides

Why do screws work better than nails?

Screws advantages:

• Higher holding power (threads vs. friction)
• Removable/adjustable
• No hammer damage to materials
• Better for precise work
• Less vibration during installation

Nails advantages:

• Faster installation
• Better shear strength
• Lower cost
• Traditional appearance

What does self-tapping mean?

Self-tapping screws cut or form their own threads as driven. Sharp point and cutting edges create threads in material without pre-tapping.

Still need pilot holes for:

• Metal (almost always)
• Hardwood
• Near wood edges
• Plastic

Don’t need pilots for:

• Softwood (center of board)
• With modern wood screws

Difference between self-tapping and self-drilling?

Self-tapping: Cuts threads, requires pilot hole

Self-drilling (Tek screws): Has drill bit point, drills own pilot AND cuts threads in one operation

What’s the strongest screw for wood?

Factors for strength:

1. Size: Larger diameter = stronger (#10-12 vs. #6-8)
2. Material: Hardened steel vs. soft steel
3. Thread design: Modern aggressive threads
4. Length: Longer penetration = more grip

Strongest options:

• GRK structural screws
• FastenMaster HeadLOK
• Spax PowerLag
• TimberLOK

Can I reuse screw holes?

Wood: Difficult – threads are damaged

• Use larger screw
• Fill with glue + toothpicks, re-drill
• Move to new location

Metal (tapped holes): Sometimes possible if threads intact

Drywall: No – use new location or toggle bolt

What happens if I use wrong size pilot hole?

Too small:

• Wood splits
• Screw breaks
• Head strips
• Difficult driving

Too large:

• No holding power
• Screw spins
• Joint loosens
• Easy pull-out

How many screws per joist for decking?

Minimum: 2 screws per joist crossing

Placement:

• 3/4″ to 1″ from each edge
• Centered on joist width

Spacing: Every joist (typically 16″ on center)

Do I need washers with wood screws?

Usually not needed for:

• Standard wood applications
• Screws with bugle/flat heads

Use washers when:

• Distributing load over soft wood
• Preventing screw head pull-through
• Lag screws (always use washers)
• Outdoor applications (helps seal)

Conclusion on understanding a screw size chart

Finding the right screw sizes can be a tedious task.  There are a dozen screw head types that work in a variety of applications.

The screw size chart and thread count thread guide are the ultimate guides to choosing the correct size of screws you require.

We hope the guide was helpful!! Feel free to leave a comment.

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