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Heat input equals arc energy per unit weld length. Multiply voltage, current, and arc efficiency, then divide by travel speed. Efficiency accounts for heat lost to spatter and radiation.
HI (kJ/mm) = (V × A × η × 60) / (1000 × Travel speed (mm/min))Typical arc efficiency (η): SMAW/stick 0.75–0.85, GMAW/MIG 0.80–0.85, SAW 0.95–1.0, GTAW/TIG 0.60–0.70. Use the value specified in your WPS or code when available.
η ≈ 0.8 for most manual and semi-auto processesTravel speed can be measured directly or estimated from wire feed rate and weld bead cross-section. Faster travel reduces heat input; slower travel increases HAZ and penetration.
Travel speed (mm/min) = Wire feed (mm/min) × Wire area / Bead areaUpdated: July 2026
At 24 V, 220 A, travel speed 350 mm/min, η=0.85: HI = (24×220×0.85×60)/(1000×350) ≈ 0.77 kJ/mm. Compare against WPS maximum of 1.0 kJ/mm for structural steel.
At 28 V, 130 A, travel speed 100 mm/min, η=0.80: HI ≈ 1.75 kJ/mm. High heat input increases HAZ width — may require PWHT or procedure revision for crack-sensitive steels.
At 12 V, 85 A, travel speed 80 mm/min, η=0.65: HI ≈ 0.49 kJ/mm. Low heat input minimizes distortion on thin stainless but risks lack of fusion if too low.
The standard formula uses mm/min. Convert from in/min (×25.4) or cm/min (×10) before calculating. Using inches per minute directly produces heat input values off by 25×.
Wire feed speed exceeds travel speed because metal is deposited as weld bead. Measure actual torch travel speed with a stopwatch or use weave-adjusted speed for accurate heat input.
Heat input controls weld metal properties, heat-affected zone (HAZ) size, and distortion. Codes and welding procedure specifications (WPS) often limit heat input for critical joints. This calculator computes heat input from arc parameters and travel speed.