A bolt is designed to pass through an unthreaded hole and be tightened by turning a nut. A screw is designed to thread into a tapped hole, or cut its own thread, and be tightened by turning its head. That is the split in ASME B18.2.1, the US reference standard for these terms.
Appearance won't settle the bolt vs screw question. A hex-head fastener can be either one, depending on how it is installed. Here is the difference between a bolt and a screw in practice, how they compare on strength, and how to choose for a real joint.
Bolt vs Screw at a Glance
Bolt | Screw | |
|---|---|---|
Designed for | Insertion through holes in the parts | A tapped hole, or forming its own thread |
Tightened by | Turning the nut | Turning the head |
Mating part | Nut | Tapped hole or the material itself |
Access needed | Both sides of the joint | Head side only |
Examples | Hex bolt, carriage bolt | Hex cap screw, machine screw, tapping screw |
What Is the Difference Between a Bolt and a Screw?
ASME B18.2.1 draws the line at design intent, not appearance. Shape, size and thread length don't appear in its definitions, so one part can change category. A socket head cap screw threaded into a tapped block is a screw. Put the same part through a clearance hole with a nut and it is doing a bolt's job.
Usage also varies by trade and country. In British terminology, for example, a cap screw is a bolt threaded all the way to the head.
Three Practical Differences
1. Access: one side or two
Tightening a bolt means holding the head and turning the nut, so you need to reach both sides. A screw turns from the head side only. That is why screws are the default for blind holes and closed housings.
2. Threads and shear
Bolts are often made with a plain, unthreaded shank under the head. This matters when the load pushes sideways. Take an M10 × 1.5 fastener. The threaded section has a tensile stress area of 58 mm² (from π/4 × (d − 0.9382P)²). The plain shank has 78.5 mm². The threaded section has about 26% less cross-section.
For sideways loads, designers try to keep the unthreaded shank in the shear plane. Cap screws can be partially or fully threaded, so check the thread length before relying on one in shear.
3. Repeated assembly
A tapped hole in aluminum is often the weak link in a screwed joint, because aluminum has far lower shear strength than steel. Repeated assembly adds wear. In a bolted joint the nut takes that load, and a nut costs little to replace. If a screwed joint will be opened often, use a thread insert or switch to a bolt and nut.
Is a Bolt Stronger Than a Screw?
No. Strength comes from material, heat treatment and diameter, so bolts vs screws is not a strength contest. The grade marking tells you the result; the name doesn't.
A Grade 8 hex cap screw is a screw. Its 150 ksi minimum tensile strength is about twice the 74 ksi of a Grade 2 bolt, in sizes up to ¾ in.
Grade / class | Minimum tensile strength | Marking on head |
|---|---|---|
SAE Grade 2 | 74 ksi (¼–¾ in.) | None |
SAE Grade 5 | 120 ksi (¼–1 in.) | 3 radial lines |
SAE Grade 8 | 150 ksi (¼–1½ in.) | 6 radial lines |
ISO Class 8.8 | 800 MPa (up to 16 mm) | "8.8" |
ISO Class 10.9 | 1,040 MPa | "10.9" |
ISO Class 12.9 | 1,220 MPa | "12.9" |
A2-70 stainless | 700 MPa (about 101.5 ksi) | "A2-70" |
Minimums change with size. Grade 2 drops to 60 ksi above ¾ in. and Grade 5 to 105 ksi above 1 in. Class 8.8 goes the other way, rising to 830 MPa above 16 mm. Grade 2's 74 ksi applies to bolts up to 6 in. long.
Multiply by the stress area and the gap becomes concrete. An M10 × 1.5 fastener has a 58 mm² stress area. Its minimum tensile load is about 46 kN in Class 8.8, 60 kN in Class 10.9 and 71 kN in Class 12.9. Class 12.9 is rated for about 74% more tensile strength than A2-70 stainless of the same size.
Fasteners That Blur the Line
A few common fasteners are easy to misfile by name.
Fastener | Classed as | Note |
|---|---|---|
Screw | Looks like a hex bolt; made for a tapped hole | |
Machine screw | Screw | Small diameter; used in tapped holes or with a nut |
Bolt | Domed head and square neck stop it turning; takes a nut | |
Lag screw | Screw | Driven into wood with a wrench; often called a "lag bolt" |
A Typical Mix-Up: A Screw Where a Bolt Belongs
Say a design calls for an M10 screw into a 4 mm steel bracket, tapped to receive it. On paper it looks fine: the head is reachable from one side, and the screw is Class 8.8.
The problem is engagement. A common rule of thumb asks for about one diameter of thread engagement in steel, which is 10 mm for M10. A 4 mm plate gives 4 mm. At a 1.5 mm pitch that is fewer than three threads. The threads in the bracket become the weak link, not the 46 kN screw.
The fix is to change the joint, not the fastener class. Pass an M10 bolt through a clearance hole and tighten it with a nut, which brings its own thread depth. The same logic applies to any thin or soft base material.
Screw vs Bolt: How to Choose
Both sides accessible, joint will be reopened: bolt and nut.
One side only, or a blind hole: screw into a tapped hole, if the base part is thick enough to hold the thread.
Sideways (shear) load: bolt with the unthreaded shank in the shear plane.
Wood: lag screw driven into the wood, or a carriage bolt through it with a washer and nut.
Wet or outdoor: stainless or coated steel, then check the strength. A2-70 (700 MPa) sits below Class 8.8 (800 MPa). For Class 10.9 and 12.9, electroplating can cause hydrogen embrittlement; zinc flake coating avoids it.
When a drawing is unclear, ask how the joint is meant to be tightened. That settles the bolt or screw question faster than the part name does.
Browse the full range in LILY's screws and bolts category, or see all 12 types of screws and bolts. Need a non-standard fastener? LILY also offers customized fastener services.
FAQ
Can I use a hex bolt in a tapped hole?
Yes, if thread size and pitch match and enough thread engages to keep the internal threads from stripping. Only the threaded length engages, so check the thread length of a partially threaded bolt. If the drawing specifies a cap screw, confirm the grade before substituting.
What is the difference between a bolt and a stud?
A stud has no head and is threaded on both ends, or along its full length. One end goes into a tapped hole and the other takes a nut. See threaded rods and studs.
What is the difference between a bolt and a set screw?
A bolt clamps two parts together. A set screw is headless and locks a collar, gear or pulley onto a shaft by pressing its tip against the shaft. See set screws.
Can I replace a Grade 5 bolt with a Class 8.8 bolt?
The strength is close (120 ksi is about 827 MPa, against 800 MPa), but the thread systems differ. Inch and metric threads don't interchange, so replace the whole set: bolt, nut or tapped hole.
Sources: ASME B18.2.1; SAE J429; ISO 898-1; ISO 3506-1; ISO 10683.
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