Bolt Size Chart — Inch & Metric Dimensions with Tables

Measuring Tools - Calipers, Thread Gauge, Pitch Gauge

Accurate bolt dimension measurement requires a caliper (digital or vernier) for diameter and length, a thread gauge for pitch measurement in metric, and a thread pitch gauge for imperial TPI. For thread diameter, measure the major diameter across the thread crests at three points and average the readings. For pitch, use the thread gauge by aligning it with the thread profile until the teeth match perfectly. For length, measure from the bearing surface of the head to the end of the bolt, excluding any chamfer. Always verify measurements against the nominal dimension for the expected thread series.

Imperial vs Metric - How to Distinguish

Distinguishing between imperial and metric bolts is critical because threads from the two systems are not interchangeable. Imperial bolts have major diameters in fractional inches (1/4", 5/16", 3/8", etc.) and pitch expressed in threads per inch (TPI). Metric bolts have major diameters in whole millimeters with pitch in millimeters. A simple identification method: if the major diameter measures close to 6.35 mm, it is likely a 1/4-inch imperial bolt; if it measures 6.0 mm, it is M6 metric. Thread gauges provide definitive identification.

Imperial Bolt Size Chart (UNC / UNF)

Metric Bolt Size Chart (Coarse / Fine)

Bolt Length Standards and Measurement Methods

Bolt length is measured from the bearing surface under the head to the extreme end of the bolt. For hex bolts and cap screws, this means from the underside of the head to the tip. For countersunk head bolts, the length includes the head height. Standard length series for imperial bolts range from 1/4 inch to 12 inches in incrementally increasing steps, while metric standard lengths range from 6 mm to 300 mm. The length of threaded portion varies by standard - generally, the thread length increases with bolt diameter, and shorter bolts may be fully threaded while longer bolts have a partial thread with an unthreaded shank section.

Bolt Size Guide for Pipe Flanges

ASME B16.5 specifies standard bolt sizes for pipe flanges based on nominal pipe size and pressure class. For class 150 flanges, common bolt diameters range from 1/2 inch for 1/2-inch NPS to 1-1/4 inches for 24-inch NPS. Higher pressure classes require larger bolt diameters - class 900 typically needs 1-inch to 2-inch bolts, and class 2500 requires up to 3-inch or larger bolts. Bolt quantity increases with flange size, ranging from 4 bolts for small flanges to 32 bolts for 24-inch class 150 flanges. Accurate bolt sizing for flanges requires consulting the ASME B16.5 tables for the specific flange rating and size.

How to Select Bolts by Size - Procurement Checklist

When procuring bolts, follow this checklist: Confirm the thread system (UNC, UNF, or metric coarse/fine). Confirm the nominal diameter and thread pitch using the appropriate standard. Confirm the exact length required, including any allowances for washers and nut height. Determine whether fully threaded or partially threaded bolts are needed based on the grip length. Verify the grade and material requirements per the applicable ASTM, SAE, or ISO standard. Finally, specify the surface treatment required for the service environment.

ManufacturerPipe's Bolt Supply Range

ManufacturerPipe supplies imperial bolts from 1/4 inch to 4 inches and metric bolts from M6 to M100 across all standard length series. We maintain inventory of common sizes for rapid delivery while offering custom length and non-standard size manufacturing for specialized requirements. All size options are available in the full range of material grades and surface treatments. We accept orders from single pieces with no minimum order quantity requirements.

Need the Right Bolt Size for Your Project?

Contact ManufacturerPipe for free size confirmation and expert advice.