Free Engineering Calculator
Bolt Size and Strength Calculator
Required stress area = load x safety factor / yield strength. A 20 kN tensile load with a safety factor of 2 needs M12 in class 8.8, M10 in 10.9 or M8 in 12.9. Enter your load below to size the bolt.
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Engineering Fastener Specifications
Selecting the correct bolt size and property class is a core requirement for mechanical joint reliability. An undersized fastener risks stripping threads or snapping under tension, while an oversized fastener adds unnecessary weight and cost. The estimation process requires a clear understanding of the applied load and the required clamping force to keep the assembly secure.
By analyzing the tensile stress area and the yield strength of different property classes (such as 8.8, 10.9, or 12.9), you can identify the most efficient hardware for your application. This tool helps you bridge the gap between theoretical load requirements and real-world hardware availability.
Preload Target
For most rigid joints, the target preload should be approximately 75% of the fastener’s proof strength to prevent loosening and joint separation.
Interactive Sizing & Grade Tool
Input your expected tension or shear load and select a safety factor to estimate the minimum bolt diameter and property class required for your structural assembly.
Bolt Size & Grade Estimator
Determine optimal fastener dimensions and property class for structural integrity.
Recommended Min. Bolt Size
Class 8.8 Hardware
Req. Stress Area ($A_s$)
0.00 mm²
Target Preload
0.00 kN
Using Class 8.8
--
Using Class 10.9
--
Using Class 12.9
--
Design Load (Load x Safety Factor)
0.00e+0 N
*Estimation based on ISO 898-1 yield strengths (8.8: 640MPa, 10.9: 940MPa, 12.9: 1100MPa; minimum yield for M16 and below).
Target preload = 75% of the proof load of the recommended size (proof stress 580, 830 and 970 MPa, ISO 898-1).
How to Use the Bolt Size Calculator
- Enter the working load in N or kN.
- Choose tension (load along the bolt) or shear (load across it).
- Enter a safety factor (1.5 to 2 is common for static loads, higher for shock or fatigue).
- Choose the property class and read the smallest metric coarse bolt whose stress area is large enough, plus the result for the other two classes.
Formula and Worked Examples
As = F x S / Rp0.2, using minimum yield strength from ISO 898-1: 640 MPa for 8.8, 940 MPa for 10.9 and 1100 MPa for 12.9 (M16 and below). For shear the tool uses 60% of the yield strength and assumes the shear plane crosses the threads. The target preload shown is 75% of the proof load of the recommended size (proof stress 580, 830 and 970 MPa).
| Load case | Class 8.8 | Class 10.9 | Class 12.9 |
|---|---|---|---|
| 20 kN tension, S = 2 (40 kN design) | M12 (62.5 mm² needed) | M10 (42.6 mm² needed) | M8 (36.4 mm² needed) |
| 10 kN shear, S = 2 (20 kN design) | M10 (52.1 mm² needed) | M8 (35.5 mm² needed) | M8 (30.3 mm² needed) |
| Target preload of the tension result | M12: 36.7 kN | M10: 36.1 kN | M8: 26.6 kN |
This is a first sizing check for a static load. It does not cover fatigue, joint stiffness, thread stripping in the tapped part or a shear plane on the plain shank. For a safety-critical joint, verify with VDI 2230 or ask our engineers.
Critical Selection Criteria
| Specification | Engineering Impact | Joint Performance Goal |
|---|---|---|
| Tensile Stress Area | The cross-sectional area that resists pull-apart forces. | Ensures load distribution. |
| Yield Strength | The threshold where the fastener begins to deform permanently. | Prevents joint failure. |
| Safety Factor | The multiplier used to account for unexpected loads or fatigue. | Ensures long-term reliability. |
| Property Class | A standardized numbering system defining the strength of metric fasteners. | Simplifies hardware sourcing. |
Frequently Asked Questions
What size bolt do I need for a 20 kN load?
With a safety factor of 2, M12 in class 8.8, M10 in class 10.9 or M8 in class 12.9, in tension.
What is the yield strength of an 8.8 bolt?
At least 640 MPa for M16 and below and 660 MPa above M16 (ISO 898-1). Tensile strength is 800 and 830 MPa respectively.
How is bolt shear strength calculated?
A common estimate is 60% of the tensile yield strength over the stress area when the threads are in the shear plane. With the plain shank in the shear plane, use the larger shank area.
Is a bigger bolt or a higher grade better?
A higher grade saves size and weight but needs a stronger tapped thread and is more sensitive to hydrogen embrittlement when plated. Class 12.9 should not be electroplated without baking.
Next Steps
- Bolt Grade Chart: strength of every inch and metric grade
- Torque and Preload Calculator: tightening torque for the size you chose
- Thread Engagement Calculator: how much thread the tapped part needs
- Request a quote for the fasteners in your joint.
Related Calculators
More free tools for the same job:
- Standard Torque and Preload Calculator: Calculate tightening torque and preload for hex and socket head screws.
- Thread Engagement Length Calculator: Find the minimum thread engagement for blind and through holes.
- Disc Spring Calculator: Size disc springs to DIN 2093.
- Hardness Converter: Convert between Brinell, Rockwell and Vickers hardness scales.
Developing a Safety-Critical Joint?
Our engineering team can help you perform advanced calculations to verify fastener performance under specific environmental and loading conditions.