JVSNUTBOLT
How to Choose the Right Nut Bolt Size
Choosing the wrong nut and bolt size causes stripped threads, loose joints and wasted orders. This is especially costly in bulk purchases for construction, machinery or solar structures. This guide shows how to measure and select the right size in three steps, with a metric size chart you can bookmark.
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A bolt size is written as M12 × 1.75 × 60:
Part | Meaning | Example |
M12 | Nominal diameter in mm | 12 mm |
1.75 | Thread pitch (distance between threads) | 1.75 mm |
60 | Length in mm | 60 mm |
You also need the grade (strength), such as 8.8 or 10.9, and the head type (hex, Allen, flange).
Step 1: Measure the diameter
The diameter is the outside width of the threaded part, measured across the thread crests with a vernier caliper.- A measurement of 11.8 mm means M12.
- Do not measure the head. The head size (across flats) is a separate spanner size.
- For nuts, measure the inner thread diameter, or match the nut to a known bolt.
Step 2: Identify the thread pitch
Pitch is the distance between two adjacent threads, in mm. You can measure it with a thread pitch gauge: hold the blades against the thread until one fits with no gaps. Most buyers need coarse pitch, which is the standard for general use. Fine pitch has more threads per length. It resists loosening under vibration and allows finer adjustment, but it is more prone to cross-threading.Metric Coarse Pitch Chart
Standard thread pitch, spanner size and clearance hole for common bolt sizes (all values in mm).
| Size | Coarse pitch (mm) | Common fine pitch (mm) | Hex head spanner size (mm)* | Medium clearance hole (mm) |
|---|---|---|---|---|
| M6 | 1.0 | — | 10 | 6.6 |
| M8 | 1.25 | 1.0 | 13 | 9 |
| M10 | 1.5 | 1.25 | 16 | 11 |
| M12 | 1.75 | 1.5 | 18 | 13.5 |
| M16 | 2.0 | 1.5 | 24 | 17.5 |
| M20 | 2.5 | 1.5 | 30 | 22 |
| M24 | 3.0 | 2.0 | 36 | 26 |
*Across-flats sizes can vary slightly between standards (ISO, DIN, IS). Always confirm against the standard you are ordering to.
Get a quote for your sizeStep 3: Calculate the right length
For standard hex bolts, length is measured from under the head to the tip. For countersunk bolts, it is measured over the full length, including the head. Formula: Bolt length = Thickness of materials being joined + washer thickness + nut thickness + 2 to 3 thread pitches protruding Worked example: you are joining two 12 mm plates with an M12 bolt, one plain washer (about 2.5 mm) and a hex nut (about 10 mm). 12 + 12 + 2.5 + 10 = 36.5 mm, plus about 4 mm for thread protrusion = 40.5 mm. The nearest standard length is M12 × 45. Tips for length:- Choose the next standard length up, not down.
- Make sure the threaded portion does not run through the joint plane. This is usually better for shear strength.
- For structural joints, follow your engineer’s specification.
Step 4: Match the grade to the load
Size alone doesn’t decide strength. The grade does.| Grade | Typical use |
| 4.6 | Light, non-critical fastening |
| 8.8 | General industrial, construction, machinery |
| 10.9 | Heavy machinery, automotive, high-load joints |
| 12.9 | Very high strength; Allen (socket head) bolts |
Step 5: Choose the head type and finish
- Hex bolt: general-purpose, tightened with a spanner.
- Allen bolt: compact, flush fitting, tightened with a hex key. See Allen bolts.
- Flange bolt/nut: built-in washer spreads the load.
- Finish: plain/black for indoor use, zinc plating for moderate protection, hot-dip galvanizing for outdoor, solar or marine environments.
Step 1: Measure the diameter
The diameter is the outside width of the threaded part, measured across the thread crests with a vernier caliper.
- A measurement of 11.8 mm means M12.
- Do not measure the head. The head size (across flats) is a separate spanner size.
- For nuts, measure the inner thread diameter, or match the nut to a known bolt.
Step 2: Identify the thread pitch
Pitch is the distance between two adjacent threads, in mm. You can measure it with a thread pitch gauge: hold the blades against the thread until one fits with no gaps.
Most buyers need coarse pitch, which is the standard for general use. Fine pitch has more threads per length. It resists loosening under vibration and allows finer adjustment, but it is more prone to cross-threading.
Buying nut bolts in Ludhiana and across India
Ludhiana is one of India’s major fastener and engineering manufacturing hubs, so buyers here can source factory-direct, in bulk, without middlemen. A manufacturer in the same industrial ecosystem can also supply custom sizes quickly. Whether you are in Ludhiana, Delhi NCR, Haryana, Gujarat, Maharashtra, Tamil Nadu, Karnataka, Telangana, West Bengal or Odisha, the sizing rules above stay the same. What changes is delivery time and freight, so confirm both when you order. Local buying tips:- Ask for mill or test certificates for grade verification on bulk orders.
- Request samples before large orders.
- Confirm the standard (IS, ISO or DIN) your project requires.
- For coastal regions (Gujarat, Tamil Nadu, Odisha, Maharashtra), consider hot-dip galvanized fasteners for corrosion resistance.
FAQs
It means a metric thread with a 10 mm nominal diameter.
Measure the thread’s outside diameter with a caliper, check the pitch with a thread gauge, and measure length from under the head to the tip.
1.75 mm for coarse thread. Fine pitch options are typically 1.5 mm or 1.25 mm.
Slightly longer is acceptable, but excess length wastes material and can cause interference. Choose the nearest standard length.
Based on load: 8.8 for general industrial use, 10.9 for high-load or vibration-prone applications, and 12.9 for very high strength.
Yes. Share your drawing or specification, quantity and delivery location.
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