Factor of Safety Calculator
Compute FoS from strength or load, or work backward to the allowable load from a target FoS — complete with margin of safety and honest interpretation.
Calculate Factor of Safety
Faktor Keamanan (FoS)
Formula
FoS = failure strength ÷ working stress or FoS = failure load ÷ working load
Reversed for design: allowable load = failure load ÷ target FoS and allowable stress = failure strength ÷ target FoS. Margin of safety: MoS = FoS − 1.
Interpretasi jujur: FoS < 1 = FAIL (working load exceeds capacity), FoS = 1 = right at the threshold of failure with no margin, FoS > 1 = SAFE with margin of FoS−1.
Verified examples
Balok beton
Failure load 48 kN ÷ 16 kN
FoS = 3,0
→ SAFE, MoS 200%
Baut kelas 8.8
Yield 640 MPa ÷ 160 MPa
FoS = 4,0
→ SAFE, MoS 300%
Kabel baja
Failure 98.1 kN ÷ 5
Izin = 19,62 kN
→ FoS target 5
All figures above were recomputed from definitions (not from memory) and tested by the test suite.
Typical-value guide table
| Aplikasi | FoS tipikal | Note |
|---|---|---|
| Komponen statis, beban terukur pasti | 1,25–1,5 | material & beban terkontrol |
| Struktur bangunan, beban statis | 1,5–2,0 | desain konvensional |
| Structures with dynamic / vibration loads | 2,0–3,0 | fatigue & kejut |
| Bejana tekan | 3,5–4,0 | konsekuensi kegagalan fatal |
| Tali baja / alat angkat | 5–10 | beban tak terduga, aus |
| Sambungan baut kritis | 2–4 | pra-beban & relaksasi |
Why this calculator
- Two-way solver: compute FoS from strength/load, or work backward to the allowable load from a target FoS.
- Interpretasi jujur: FoS < 1 flagged FAIL, = 1 MARGINAL, > 1 SAFE — plus automatic margin of safety.
- Pembedaan ultimate vs yield: bahan getas memakai kekuatan ultimate, bahan ulet memakai kekuatan luluh.
- Unit conversion: metric ↔ imperial (N, kN, kgf, lbf, kip; Pa–MPa, psi, ksi) with no manual math.
- 100% client-side: no data is sent to the server.
Frequently Asked Questions
What is the factor of safety?
The factor of safety (FoS) is the ratio between a component’s failure capacity and its actual working load or stress: FoS = failure load ÷ working load. It states how many times stronger a design is than its minimum requirement — FoS 2 means the component only fails at twice the working load.
What is the factor of safety formula?
Two equivalent forms: FoS = failure strength ÷ working stress, or FoS = failure load ÷ working load. Verified example: a beam with failure load 48 kN ÷ 16 kN gives FoS = 3.0.
What is the difference between using ultimate vs yield strength?
For brittle materials like concrete or cast iron, ultimate strength (fracture point) is used. For ductile materials like structural steel, yield strength is used — because permanent plastic deformation already counts as functional failure, even though the material has not broken. This calculator lets you pick the appropriate strength type.
What is the margin of safety (MoS)?
The margin of safety is the reserve above requirements: MoS = FoS − 1. FoS 2.5 means MoS = 1.5 = 150% — the load may rise 150% above the working load before reaching failure capacity.
What FoS is considered safe?
It depends on the application and load uncertainty. Typical literature values: static components 1.25–1.5; building structures 1.5–2.0; dynamic loads 2.0–3.0; pressure vessels 3.5–4.0; wire rope/lifting gear 5–10. These are typical values, not mandatory code standards — always follow the standards applicable to your application.
Why does FoS below 1 mean failure?
Because FoS < 1 means the actual working load already exceeds the component’s theoretical failure capacity — such a design is predicted to fail. FoS = 1 means exactly at the failure threshold with no margin at all; a safe design needs FoS > 1.
How do I compute the allowable load from a target FoS?
Reversed: allowable load = failure load ÷ target FoS. Verified example: a steel cable with failure load 98.1 kN ÷ 5 gives an allowable load of 19.62 kN. The working load must not exceed this value.
Does FoS 2 mean twice as strong in every way?
Not exactly. FoS 2 means the failure load is twice the working load under linear-behavior assumptions — but it does not double resistance to every failure mode such as fatigue, buckling, corrosion, or shock loading. FoS is a single summary number, not a substitute for complete failure analysis.
Free tool by NRTechStudio