Screw vs Bolt: Key Differences Explained Simply

The Core Difference

The fundamental distinction between screws and bolts lies in their fastening method: screws create their own threaded path and fasten objects directly, while bolts pass through pre-drilled holes and require a nut to secure materials together. This seemingly simple difference affects everything from installation technique to structural integrity.

A screw typically threads into the material itself—wood, plastic, or metal—cutting or forming threads as it turns. A bolt, conversely, works as part of a two-piece system where the threaded shaft passes completely through aligned holes in the materials being joined, then gets secured by a matching nut on the opposite end.

Thread Design and Coverage

Thread characteristics reveal another critical distinction between these fasteners.

Screw Thread Patterns

Screws feature threads that typically extend along most or all of the shaft length. Wood screws, for instance, have threads covering 60-70% of the shaft, with a smooth shank near the head. The thread pitch (distance between threads) varies based on application—coarse threads for soft materials like wood, fine threads for metal.

Self-tapping screws have particularly aggressive thread designs with sharp cutting edges that carve their own path. A typical #8 wood screw has approximately 15 threads per inch, while machine screws might have 32 threads per inch for precision applications.

Bolt Thread Characteristics

Bolts generally have threads that cover only a portion of the shaft, leaving a smooth section beneath the head. A standard M10 bolt might have only 30-40mm of threading on a 100mm shaft. This partial threading serves a purpose: the smooth shank provides more precise alignment and reduces stress concentration in critical applications.

Bolt threads follow standardized pitches—metric bolts use specifications like M8×1.25 (8mm diameter, 1.25mm thread pitch), while imperial bolts follow classifications like 1/2"-13 (1/2 inch diameter, 13 threads per inch).

Installation Methods

The installation process highlights the practical differences between these fasteners.

How Screws Are Installed

Screw installation requires rotational force applied to the head. The process typically involves:

• Positioning the screw at the desired location
• Applying rotational torque using a screwdriver or drill
• Driving the screw until the head seats against the material
• No need for access to the opposite side of the material

For a 3-inch wood screw in softwood, you might need 20-30 inch-pounds of torque. Pre-drilling pilot holes prevents splitting and reduces insertion torque by approximately 40%.

How Bolts Are Installed

Bolt installation demands a different approach:

• Drilling aligned holes through all materials being joined
• Inserting the bolt through the holes
• Threading a nut onto the opposite end
• Tightening with wrenches on both the bolt head and nut
• Requires access to both sides of the assembly

A Grade 8 half-inch bolt requires 130 foot-pounds of torque for proper clamping force. Engineers specify precise torque values because under-tightening reduces joint strength while over-tightening can strip threads or break the bolt.

Strength and Load Capacity

Load-bearing capacity varies significantly between screws and bolts due to design and material differences.

Bolts substantially outperform screws in strength applications. A Grade 8 bolt can handle 15-20 times the load of a comparable-sized wood screw. This difference stems from superior materials, heat treatment processes, and the distributed clamping force created by the nut-and-bolt combination.

Structural engineers specify bolts for critical connections in buildings, bridges, and machinery. A single 3/4-inch Grade 8 bolt can withstand over 19,000 pounds of tensile force—equivalent to the weight of four average cars.

Head Design and Drive Types

The head configuration affects both functionality and application suitability.

Screw Head Varieties

Screws offer diverse head styles designed for specific purposes:

• Flathead (countersunk): Sits flush with or below the surface, ideal for finished carpentry
• Pan head: Low profile with flat bearing surface, common in electronics
• Round head: Domed top, used where appearance matters less than strength
• Hex head: Six-sided head for wrench or socket drive

Drive types include slotted, Phillips, square (Robertson), Torx, and hex socket. Torx drives reduce cam-out by 90% compared to Phillips, making them preferred for high-torque applications.

Bolt Head Configurations

Bolts typically feature simpler head designs optimized for wrench engagement:

• Hex head: The most common type, available in standard and heavy-duty versions
• Square head: Traditional style, less common today but offers superior grip
• Socket head: Cylindrical head with internal hex drive for tight clearances
• Carriage bolt: Rounded head with square neck that prevents rotation

Hex heads dominate because they provide six engagement surfaces, distributing torque evenly and reducing the risk of rounding. A properly sized wrench on a hex bolt can transfer 95% of applied torque compared to 60-70% for slotted screws.

Practical Applications

Understanding when to use each fastener type prevents structural failures and ensures optimal performance.

When to Use Screws

Screws excel in scenarios requiring:

• One-sided access: Cabinet installation, drywall mounting, or anywhere you can't reach the back
• Wood joinery: Furniture assembly, deck building, framing (where permitted by code)
• Light to moderate loads: Shelf brackets, picture hangers, electrical boxes
• Frequent assembly/disassembly: Machine screws in threaded inserts or tapped holes
• Sheet metal work: Self-tapping screws create their own threads in thin gauge metal

For example, deck construction commonly uses #10 × 3" coated deck screws spaced 16 inches apart along joists, providing adequate holding power while simplifying installation.

When to Use Bolts

Bolts are essential for applications demanding:

• High structural loads: Building frames, heavy equipment, automotive chassis
• Critical safety connections: Bridge construction, crane assemblies, playground equipment
• Metal-to-metal joining: Steel beams, machinery frames, automotive components
• Vibration-prone assemblies: Engine mounts, suspension components (where lock nuts prevent loosening)
• Removable connections: Bolts can be loosened and retightened repeatedly without degrading the parent material

Building codes mandate bolts for earthquake-resistant connections. California seismic standards require 5/8-inch diameter anchor bolts spaced every 6 feet to secure wooden sill plates to concrete foundations.

Material and Finish Options

Both fasteners come in various materials and coatings suited to different environments.

Common Materials

Carbon steel dominates both screw and bolt production, offering good strength at low cost. However, uncoated carbon steel rusts rapidly—outdoor exposure can cause visible corrosion within weeks.

Stainless steel fasteners resist corrosion naturally. Grade 304 stainless works for most applications, while Grade 316 handles marine environments and chemical exposure. Stainless steel costs 3-5 times more than zinc-plated steel but lasts decades in harsh conditions.

Brass and bronze screws prevent galvanic corrosion when joining dissimilar metals and offer decorative appeal. Silicon bronze bolts are standard in boat building.

Protective Coatings

Surface treatments extend fastener life:

• Zinc plating: Provides 100-500 hours of salt spray resistance, suitable for indoor/dry outdoor use
• Hot-dip galvanizing: Thick zinc coating lasting 20-50 years outdoors
• Ceramic coating: Extreme corrosion resistance for chemical plants and offshore platforms
• Phosphate coating: Black finish providing oil retention and moderate rust prevention

Deck screws often feature specialized coatings. ACQ-rated coating protects against newer wood preservatives that accelerate corrosion of standard fasteners.

Cost Considerations

Price differences reflect manufacturing complexity, material costs, and strength requirements.

Standard wood screws cost $5-15 per pound depending on size and coating. A pound contains approximately 100-150 screws in common sizes. Specialty screws like stainless steel Torx-drive deck screws run $20-40 per pound.

Basic zinc-plated bolts cost $8-20 per pound, with a pound containing 20-50 bolts depending on size. Grade 8 bolts command premium prices—$25-50 per pound—due to alloy content and heat treatment. Stainless steel bolts range from $30-80 per pound.

For a typical deck project requiring 500 fasteners, screws might cost $40-60 total, while using bolts for the same connections could run $150-300. However, bolts eliminate material-specific concerns—they work equally well in treated lumber, composite decking, or metal framing, potentially simplifying inventory and reducing waste.

Common Misconceptions

Several myths persist about these fasteners that can lead to poor decisions.

Misconception: Lag screws are actually screws. Despite their name, lag screws (lag bolts) function more like bolts. They have coarse threads, hex heads designed for wrench application, and create high-strength connections. However, they don't require nuts, instead threading directly into pre-drilled holes in wood or masonry.

Misconception: Longer fasteners always mean stronger connections. Excessive length wastes money and can reduce holding power. In wood, effective thread engagement typically requires penetration of 1.5-2 times the screw diameter into the base material. Beyond that, additional length adds minimal strength.

Misconception: All bolts need washers. While washers distribute load and protect surfaces, they're not universal requirements. Bolts with large bearing surfaces or applications involving hardened materials often perform adequately without washers. However, using washers typically increases joint life by 30-50% by preventing surface crushing.

Misconception: Thread count doesn't matter much. Thread pitch critically affects performance. Fine threads provide greater tensile strength and work better in hard materials, while coarse threads resist stripping in soft materials and allow faster installation. Using the wrong thread pitch can reduce holding power by 40% or more.

Making the Right Choice

Selecting between screws and bolts requires evaluating several factors simultaneously.

Access requirements often dictate the choice immediately. If you can only reach one side of the assembly, screws become necessary regardless of other factors. Conversely, if both sides are accessible and loads are significant, bolts prove superior.

Load calculations determine minimum strength requirements. Divide the expected load by the fastener's rated capacity, then multiply by a safety factor (typically 3-5 for static loads, 5-10 for dynamic loads). If standard screws can't meet this threshold, upgrade to bolts or increase the number of fasteners.

Material compatibility influences success. Screws work beautifully in wood and soft plastics but struggle in hard metals. Bolts excel in metal-to-metal connections and situations where through-holes don't weaken the assembly.

Disassembly frequency matters for maintenance-intensive equipment. Machine screws into threaded inserts or bolts with removable nuts survive hundreds of removal/reinstallation cycles. Wood screws, by contrast, degrade their threads after 3-5 removals.

For critical applications—anything involving human safety, high values, or structural integrity—consulting engineering tables, building codes, or manufacturer specifications eliminates guesswork and potential liability.