BoltCore is an online bolted joint calculator built on the ECSS-E-ST-32-19C methodology used in the space industry. Enter the bolt, the clamped parts and the tightening conditions: it returns the tightening torque, the minimum and maximum preload and the margins of safety — in your browser, in seconds.
What the free calculator computes
- Tightening torque and preload — the relation between torque and bolt tension, including friction in the threads and under the head (Kellermann & Klein formula).
- Preload bracket FV,min / FV,max — the range of preload actually obtained once tightening-method and friction scatter are taken into account.
- Tightening margin — the Von Mises stress in the bolt at tightening (tension + torsion) compared with its yield strength.
- Assembly diagram — a to-scale view of the bolt and the clamped parts.
Free vs Pro
| Feature | Free | Pro |
|---|---|---|
| Bolt sizes | M6, M8 | M3 → M24 |
| Grades | 8.8, 10.9 | Steel + stainless A2/A4 |
| Tightening torque + preload FV | ✓ | ✓ |
| Tightening margin (Von Mises) | ✓ | ✓ |
| Custom friction coefficients | — | ✓ |
| Yield / ultimate, separation, slipping, thread stripping, bearing | — | ✓ (11 margins) |
| Thermal analysis (CTE mismatch) | — | ✓ |
| Clamped parts | 2 plates | Multi-plate, mixed materials, washer |
| PDF engineering report | — | ✓ |
See a complete Pro output in the sample report (M8 grade 8.8, PDF).
Calculator or spreadsheet?
Most bolted joint analyses are still done in in-house spreadsheets. They work, but they are hard to review: formulas hidden in cells, units mixed, diameters (d0, d3, duh) copied by hand, friction scatter forgotten. A dedicated calculator applies the same method every time and produces a traceable justification. The usual pitfalls are listed in common bolted joint justification mistakes.
Method and references
The calculation follows ECSS-E-ST-32-19C: geometry (d0, d3, duh), preload bracket, friction, then the margins of safety. The full method is explained in our ECSS-E-ST-32-19C guide. For a quick order of magnitude, the torque table by size and grade gives values computed with the same engine.
No account, no installation. The free version runs entirely in your browser.
Open the free calculator →Frequently asked questions
Is the calculator really free?
Yes. The free version runs in your browser with no account and no time limit. It covers M6 and M8 bolts in grades 8.8 and 10.9. The Pro version (7-day free trial) adds all sizes, stainless grades, the 11 margins of safety and the PDF report.
Which method does the calculator use?
The ECSS-E-ST-32-19C methodology used in the space industry: preload bracket from torque and friction scatter, then margins of safety on the bolt, the threads and the clamped parts.
Can I use it for stainless steel bolts (A2-70, A4-70)?
Stainless grades are available in the Pro version. Their lower proof strength changes the achievable preload significantly compared with 8.8 — see our grade comparison table.
Does it replace a finite element analysis?
No FE model is needed for the bolt justification itself: the ECSS method is analytical. FE is still useful upstream to obtain the loads applied to each bolt.