How to Measure Bolt Size: Custom Bolt Supplier Singapore
Choosing the right bolt is essential for safe assemblies, accurate fits, and high-quality fabrication work. Whether you’re working on CNC parts, metal structures, jigs, or custom machinery, you need to know how to measure bolt size correctly. A small measurement mistake can cause misalignment, thread failure, or costly rework.
This guide explains how to determine bolt size in a simple, accurate, and industry-standard way, covering bolt diameter, bolt head size, length, and thread pitch. It also includes real examples based on our daily production work at The Monster Builder.
Why Accurate Bolt Measurement Matters
In Singapore’s manufacturing and engineering environment, precision is everything. Correct bolt sizing helps you:
- Ensure fastener compatibility with nuts, tapped holes, and washers
- Improve joint strength and load reliability
- Reduce assembly mistakes and project delays
- Prepare proper BOMs (Bill of Materials) for machining and fabrication
- Maintain ISO-level quality standards in your production workflow
What Makes Up a Bolt Size?
When you identify or check bolt sizing, you typically measure four main attributes:
- Bolt Diameter – the major diameter of the threaded section
- Bolt Head Size – measured across the flats
- Bolt Length – from under the head to the tip (or full head for countersunk bolts)
- Thread Pitch – the spacing between threads
Understanding these four elements makes it easy to determine any bolt’s exact size.
Tools You Need to Measure Bolt Size
To measure a bolt accurately, prepare these tools:
- Vernier caliper – for diameter, head size, and length
- Thread pitch gauge – for identifying thread pitch
- Ruler or measuring tape – for quick checks
- Fastener size gauge (optional) – for fast verification
These tools are commonly used in fabrication workshops, CNC environments, and engineering labs.
How to Determine Bolt Size
Bolt diameter is the most important part of identifying the correct specification.
Steps:
Place the caliper across the outermost threads (major diameter).
Record the measurement in millimeters for metric bolts (e.g., 6 mm = M6).
For partially threaded bolts, you may also compare the shank diameter.
For example, if the major diameter reads 6.00 mm, the bolt size is M6. Never measure only the head and assume it represents bolt diameter.
Head dimension varies by bolt type.
For projects requiring pipe clamping or curved surfaces 316 stainless steel U-bolts provide a secure and reliable solution.
How to Measure Bolt Head Size Across Flats
To measure bolt head size (across flats), place a caliper or ruler on the two flat, parallel sides of the bolt head and record the distance between them. This measurement is called the “across-flats” (AF) size and tells you the correct wrench or socket size to use.
Steps:
- Position the bolt upright on a flat surface.
- Place a vernier caliper across the two parallel flats of the bolt head.
- Read the measurement in millimetres (e.g., 10 mm, 13 mm, 17 mm).
- Match the AF number with the corresponding wrench or socket size.
Do not measure corner-to-corner; this gives an incorrect value. Always measure flat-to-flat for accurate bolt head sizing.
For example, a bolt head that measures 13 mm across flats requires a 13 mm wrench.
How to Measure Bolt Length
To measure bolt length, place a ruler or caliper along the bolt and measure from the point where the head meets the shank to the tip. This gives you the usable length of the bolt.
How it works:
- Standard bolts (hex, pan, button head):
Measure from the underside of the bolt head to the tip. - Countersunk bolts (flat head):
- Measure from the top of the head to the tip, because the head sits flush with the surface.
- Keep the bolt straight and ensure the measuring tool lines up perfectly along the bolt’s axis.
For example, a hex bolt that measures 40 mm from under the head to the tip is written as M8 × 40.
Do not include the head height for standard bolts. Always measure along the full centre line for accuracy.
How to Measure Thread Pitch
To measure thread pitch, use a thread pitch gauge and match its teeth against the bolt’s threads until you find the exact fit. The number shown on the matching gauge blade is your pitch (e.g., 1.25 mm for metric or a Threads-Per-Inch value for imperial).
Steps:
- Hold the bolt upright for easy alignment.
- Select a blade from the thread pitch gauge.
- Place the gauge teeth against the bolt threads.
- Test different blades until one fits perfectly with no gaps or light showing.
- Read the number on the blade. Metric: shown in millimetres (e.g., 1.0, 1.25, 1.5). Imperial: shown as TPI (e.g., 20 TPI, 28 TPI).
For example, if a metric gauge labeled 1.25 mm fits perfectly, the bolt’s thread pitch is 1.25.
When combining bolts with metal frameworks, understanding angle bar fabrication ensures proper alignment and structural strength. Fastening angle bars can be done using various methods, including welding or 316 stainless steel bolts for angle bars to achieve a precise and secure connection.
Examples
Here’s how our team at The Monster Builder measures a bolt used in a metal fabrication bracket project:
- Diameter: Caliper reads 8.02 mm → M8
- Head Size: Across flats measures 13 mm → fits a 13 mm socket
- Length: 40 mm from under the head → M8 × 40
- Pitch: Gauge shows 1.25 mm → M8 × 1.25 × 40
This is the full specification we include in our machining drawings and BOM.
Common Mistakes When Measuring Bolt Size
Avoid these errors to ensure accurate identification:
- Measuring the bolt head instead of the threaded diameter
- Measuring the head diagonally
- Ignoring thread pitch, causing mismatched nuts
- Using uncalibrated or zero-error calipers
- Recording measurements in the wrong unit (mm vs inch)
Custom Bolts Manufacturer
Choosing the right bolt is essential for strong, reliable assemblies, whether you’re measuring bolt head size, diameter, length, or thread pitch. Accurate measurements reduce errors and ensure your projects perform as intended.
At The Monster Builder, we are a trusted custom bolt supplier and custom fasteners manufacturer in Singapore. We specialize in made-to-order bolts, bespoke fasteners, and precision bolts made to spec, including stainless steel and CNC-fabricated bolts. Our team also supports custom metal fabrication, laser cutting, and 3D printing, providing high-quality solutions tailored to your unique project requirements.
Whether you need custom bolts for machinery, special-size fasteners, or precision threaded components, we deliver reliable, high-performance parts that meet your specifications.
Contact us today to discuss your custom bolt requirements or request a fast quotation.
