Welding Heat Input Calculator
Use this calculator to estimate welding heat input in kJ/mm and kJ/in.
What is heat input in welding?
Heat input describes how much thermal energy is delivered into the workpiece per unit length of weld. It is one of the most important variables in welding procedure development because it strongly affects penetration, fusion, cooling rate, microstructure, and mechanical properties.
If heat input is too low, you may get lack of fusion, incomplete penetration, or a brittle weld profile. If heat input is too high, you can produce excessive distortion, a large heat-affected zone (HAZ), grain coarsening, or reduced toughness. Finding the right range is critical for code compliance and performance.
Heat input formula (standard engineering form)
Primary equation
For most welding calculations, heat input is estimated using:
- HI (kJ/mm) = (V × I × 60 × η) / (1000 × S)
Where:
- V = arc voltage (volts)
- I = current (amps)
- η (eta) = process efficiency (decimal form, e.g., 0.80)
- S = travel speed (mm/min)
The factor 60 converts seconds to minutes, and 1000 converts joules to kilojoules.
Why efficiency matters
Not all electrical energy turns into useful heat in the weld joint. Some energy is lost through radiation, convection, and other pathways. That is why arc efficiency is included in the practical formula. If your procedure qualification record (PQR), code, or customer specification defines a required method, always follow that method exactly.
Quick example
Suppose you have:
- Voltage = 24 V
- Current = 220 A
- Travel speed = 300 mm/min
- Efficiency = 80% (0.80)
HI = (24 × 220 × 60 × 0.80) / (1000 × 300) = 0.845 kJ/mm (approx).
This gives you a useful baseline for checking if your setup is inside your approved heat-input window.
How to control heat input in real production
1) Adjust amperage and voltage carefully
Higher voltage and current generally increase heat input. Make small controlled changes, and document every adjustment during procedure development.
2) Control travel speed
Travel speed is often the fastest way to change heat input. Slower travel means more heat per unit length; faster travel means less heat per unit length.
3) Stay within qualified ranges
In regulated industries, your WPS may define minimum and maximum heat input. Exceeding limits can invalidate the procedure and risk quality issues.
Typical impacts of heat input on weld quality
- High heat input: wider bead, larger HAZ, more distortion, slower cooling.
- Low heat input: narrower bead, faster cooling, potential lack of fusion if too low.
- Balanced heat input: improved fusion, acceptable HAZ, better dimensional control.
FAQ
Is this calculator only for arc welding?
This tool is designed for common arc welding heat-input estimation (SMAW, GMAW, FCAW, GTAW). Other processes may use different models.
Should I use kJ/mm or kJ/in?
Use whichever your code, customer, or WPS specifies. This calculator gives both for convenience.
Can I use this for procedure qualification?
Use it for estimation and planning. For formal qualification, always follow your governing code and recorded essential variables.
Final note
A good heat input target is not “high” or “low”—it is appropriate for your material, thickness, welding process, and required mechanical properties. Use the calculator above to make informed adjustments and keep your welding parameters consistent.