To calculate bolt length, you need to determine the total thickness of all components being fastened, including any washers and the nut, plus an allowance for proper thread engagement and protrusion. The bolt's length itself is officially measured from the underside of its head to the end of the shank.
Understanding Bolt Length Measurement
When selecting or measuring a bolt, its length is defined as the distance from the underside of the bolt head to the end of the shank. This is a crucial distinction, as the head's thickness is generally not included in the stated bolt length for most standard bolt types (e.g., hex bolts, socket cap screws, carriage bolts). Bolt diameter, the other principal dimension, is measured across the widest part of the shank. Both dimensions can be measured in millimeters or inches.
Key Factors for Determining Required Bolt Length
Accurately determining the required bolt length involves summing several thicknesses to ensure a secure and effective connection.
- Total Grip Length: This is the combined thickness of all materials or workpieces being joined together.
- Washer Thickness(es): Account for any flat washers, lock washers, or other specialized washers used under the bolt head or under the nut.
- Nut Thickness: The height of the chosen nut. This can vary depending on the nut type (e.g., standard hex nut, thin nut, heavy hex nut).
- Thread Engagement: Sufficient thread engagement within the nut is essential for strength. Typically, full engagement of the nut's threads is desired.
- Protrusion Allowance: A small amount of thread extending beyond the nut (often 2-3 threads) is recommended for full thread engagement and to accommodate minor variations. This also provides visual confirmation of proper assembly.
The General Formula
The most common method for calculating the required bolt length can be expressed as:
Required Bolt Length = Total Grip Length + Washer Thickness(es) + Nut Thickness + Protrusion Allowance
Step-by-Step Calculation Guide
Follow these steps to accurately calculate the bolt length needed for your application:
- Measure Total Material Thickness: Sum the thicknesses of all parts you intend to fasten together. For example, if joining two 10mm thick plates, the total material thickness is 20mm.
- Add Washer Thickness: Measure and add the thickness of any washers. If you use a flat washer and a lock washer under the nut, add both their thicknesses.
- Add Nut Thickness: Determine and add the thickness of the nut you plan to use. Standard nut thicknesses are usually available in specification charts.
- Incorporate Protrusion Allowance: Add a small allowance for the bolt threads to extend beyond the nut. A common recommendation is to add 2 to 3 thread pitches or an equivalent fixed value (e.g., 5-10 mm or 1/4 - 3/8 inch, depending on bolt size).
- Calculate Theoretical Length: Sum all these individual measurements to get your theoretical required bolt length.
- Select Standard Length: Once you have your theoretical length, you must select the next standard bolt length available that is equal to or greater than your calculated value. Bolts are manufactured in specific standard lengths, so you'll rarely find a bolt that perfectly matches your exact calculated value.
Practical Example
Let's calculate the required length for fastening two steel plates, each 8mm thick, using one 2mm thick flat washer and a standard M10 hex nut (approx. 8mm thick), with a desired 5mm protrusion beyond the nut.
- Total Grip Length (Plates): 8mm + 8mm = 16mm
- Washer Thickness: 2mm
- Nut Thickness (M10): 8mm
- Protrusion Allowance: 5mm
- Theoretical Required Length: 16mm + 2mm + 8mm + 5mm = 31mm
Since 31mm is not a standard bolt length, you would likely select the next available standard length, which might be a 35mm or 40mm long M10 bolt, depending on manufacturer offerings.
Considerations for Specific Applications
- Tapped Holes: When fastening into a tapped (threaded) hole without a nut, the calculation simplifies. You sum the material thickness and then add the desired thread engagement depth into the tapped hole. There is no nut thickness or protrusion beyond a nut to consider.
- Countersunk Bolts: For countersunk bolts (where the head sits flush or below the surface), the measured length often includes the head. However, the calculation for effective grip still focuses on the length below the surface and the engagement with the mating component or nut.
- Locking Nuts: If using nylon insert (nyloc) or all-metal prevailing torque nuts, ensure the bolt is long enough for the locking feature to fully engage the bolt's threads.
Common Standard Bolt Lengths (Illustrative)
Bolts are produced in specific standard lengths, which vary by diameter and region (e.g., ISO, DIN, ANSI standards).
| Metric Diameter | Common Standard Lengths (mm) |
|---|---|
| M6 | 10, 12, 16, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80 |
| M8 | 12, 16, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100 |
| M10 | 16, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 120 |
Always confirm the available standard lengths with your fastener supplier or engineering standards to ensure you choose the most appropriate bolt.
