Carbon Equivalent Calculator
Enter the steel composition in weight percent from the mill certificate. The carbon equivalent estimates how prone the steel is to hardening and cold cracking at a weld, and is the number a welding engineer reads before deciding on preheat.
Carbon equivalent — CE (IIW)
CE (IIW) is the index for carbon and low-alloy steels of roughly 0.12 % carbon and above. The welding procedure decides preheat.
What this calculates
Carbon equivalent folds a steel's alloying elements into one figure that stands in for carbon when judging weldability. Carbon hardens the heat-affected zone more than any other element, but manganese, chromium, molybdenum, vanadium, nickel and copper all contribute, each at a lower weight. The higher the equivalent, the more likely a weld is to crack on cooling without preheat or controlled hydrogen.
Two formulas are in use, and the page runs both. CE (IIW), from the International Institute of Welding, is the general-purpose form written into most structural steel specifications. Pcm, the Ito–Bessyo parameter, is tuned for modern low-carbon, low-alloy steels below about 0.12 % carbon, where the IIW form overstates the risk.
Read the figure against the rule of thumb your specification gives: a CE (IIW) below about 0.40 % is generally weldable without preheat in moderate thicknesses, 0.40 to 0.60 % calls for preheat that rises with thickness and restraint, and above 0.60 % demands a qualified procedure. The welding procedure specification decides; the index informs it.
Show the formula and a worked example
The formula
- CE (IIW) = C + Mn ÷ 6 + (Cr + Mo + V) ÷ 5 + (Ni + Cu) ÷ 15 (wt.%)
- Pcm = C + Si ÷ 30 + (Mn + Cu + Cr) ÷ 20 + Ni ÷ 60 + Mo ÷ 15 + V ÷ 10 + 5 × B (wt.%)
Dimensions in millimetres, density ρ in kg/m³ — as the engine runs them.
CE (IIW) = C + Mn ÷ 6 + (Cr + Mo + V) ÷ 5 + (Ni + Cu) ÷ 15, all in weight percent. Manganese is the biggest contributor after carbon in plain carbon steels, which is why the formula divides it by only 6.
Pcm = C + Si ÷ 30 + (Mn + Cu + Cr) ÷ 20 + Ni ÷ 60 + Mo ÷ 15 + V ÷ 10 + 5 × B. Boron carries the highest weight of any element because even a few thousandths of a percent affect hardenability. Silicon appears here and not in the IIW form.
Blank fields count as zero, so a plain carbon steel needs only carbon and manganese. Negative entries are refused. The engine reports the figure at full precision and the page rounds it for display.
Worked example
| Calculation | Carbon equivalent — CE (IIW) |
|---|---|
| Carbon (C) | 0.18 wt.% |
| Manganese (Mn) | 1.2 wt.% |
| Chromium (Cr) | 0.1 wt.% |
| Molybdenum (Mo) | 0.05 wt.% |
| Nickel (Ni) | 0.1 wt.% |
| Copper (Cu) | 0.15 wt.% |
| Theoretical result | 0.43 % |
Computed by the calculation engine from these inputs at render time, rounded only for display.
Frequently asked
- 01Which formula should I use?
- Use the one your specification names. If it names neither, CE (IIW) for steels at or above about 0.12 % carbon and Pcm below that. Where a steel sits near the boundary, run both; a procedure written for the higher figure is the conservative choice.
- 02Do I use the specification maximums or the certificate?
- The certificate for the plate in front of you, because that is what will be welded. The specification maximums give the worst case a grade may reach, which is the right basis when writing a procedure for a grade rather than a heat.
- 03Does carbon equivalent apply to stainless steel?
- No. These formulas are for carbon and low-alloy steels. Stainless weldability is judged on other measures, such as ferrite number and sensitisation, and the PREN calculator on this site ranks corrosion resistance rather than weldability.
- 04What preheat does a given CE need?
- That depends on thickness, restraint, hydrogen level of the consumable and the standard in force, not on CE alone. The number tells the welding engineer where on the scale the steel sits; the procedure sets the temperature.