Welding Calculator uses HI = (V × A × 60 ÷ speed ÷ 1000) × efficiency to estimate net heat input, gross arc energy, arc power, travel time, and kJ/in or kJ/mm conversions from entered weld data.
Why Heat Input Is the Number That Matters
Most welders track amperage and voltage on the machine display, but neither number alone tells you what’s actually happening in the base metal. The arc voltage and current determine how much electrical power the arc generates — but the travel speed controls how long that energy dwells in one place.
A 200A, 24V arc moving at 6 ipm delivers twice the thermal energy per inch compared to the same arc moving at 12 ipm. Slow down, and you’re not just taking longer to finish the weld: you’re fundamentally changing the metallurgy of the heat-affected zone.
Heat input also isn’t a single number — standards distinguish between gross (arc energy) and net (effective) heat input, and the two differ by the thermal efficiency of the process. TIG’s tight, focused arc loses less energy to the surroundings than a stick electrode, which is why TIG carries a 60% efficiency factor while submerged arc runs at 95%. The calculator reflects these distinctions directly.
Formulas
All outputs can be reproduced from these expressions. proc is the decimal process efficiency (e.g., 0.80 for MIG/GMAW).
Arc Power
- Arc Power (W) = Voltage (V) × Current (A)
- Arc Power (kW) = Arc Power (W) ÷ 1,000
Travel Time
- Time per Unit Length (s/in or s/mm) = 60 ÷ Travel Speed (ipm or mm/min)
- Total Arc-On Time (s) = Time per Unit Length × Weld Length
- Total Arc-On Time (min) = Total Arc-On Time (s) ÷ 60
Heat Input
- Gross Heat Input (kJ/in or kJ/mm) = [Arc Power (W) × 60] ÷ [Travel Speed × 1,000]
- Net Heat Input (kJ/in or kJ/mm) = Gross Heat Input × proc
- Heat Loss Adjustment (kJ/in or kJ/mm) = Gross Heat Input − Net Heat Input
Total Energy
- Arc-On Energy (kJ) = Arc Power (kW) × Total Arc-On Time (s)
- Total Net Energy (kJ) = Net Heat Input × Weld Length
- Total Net Energy (BTU) = Total Net Energy (kJ) × 0.947817
Unit Conversions
- kJ/in → kJ/mm: value ÷ 25.4
- kJ/mm → kJ/in: value × 25.4
Process Efficiency Factors
- TIG / GTAW: 60% (proc = 0.60)
- MIG / GMAW: 80% (proc = 0.80)
- Stick / SMAW: 80% (proc = 0.80)
- Flux Core / FCAW: 85% (proc = 0.85)
- Sub Arc / SAW: 95% (proc = 0.95)
How the Calculation Works
The calculator starts from two values your power source controls directly — arc voltage and welding current — and multiplies them together to get arc power in watts. This is the rate at which the electrical circuit is delivering energy to the arc at any given instant.
Travel speed then converts that instantaneous power rate into a per-length quantity. Dividing arc power (in watts) by travel speed (in inches or millimeters per minute) — with a factor of 60 to reconcile seconds and minutes, and 1,000 to convert joules to kilojoules — produces the gross heat input. This represents the theoretical thermal energy delivered per unit of weld length, sometimes called arc energy.
The process efficiency factor is applied next. Because no welding process transfers 100% of arc energy into the base metal (some radiates as light, some is lost to spatter and fumes, some heats the torch or electrode), the efficiency factor reduces the gross figure to the net heat input — the value that actually drives the thermal cycle in the workpiece. Net heat input is the primary output displayed in the hero field.
The travel time cards work independently from the heat calculations. The calculator divides 60 by your travel speed to find how many seconds elapse per inch (or millimeter) of weld, then multiplies by your total weld length. This gives you both the arc-on time for the full pass and the time per unit length — useful for estimating preheat hold times and planning multi-pass sequences.
The Alternate Units card converts the primary net and gross heat values into the opposing unit system (US to metric, or metric to US) using the 25.4 mm/in factor, and also converts total net energy from kilojoules to BTU for reference in contexts where imperial thermal units are standard.
Switching between US Customary and Metric measurement systems resets the speed and length defaults to reasonable starting values for that system (12 ipm and 10 in for US; 300 mm/min and 250 mm for metric), preventing nonsensical outputs when a user forgets to update those fields after switching.
Where the Arc-On Energy and Total Net Energy Diverge
The Arc Power card shows two sub-values that are easy to conflate: Arc-On Energy and Total Net Energy (visible in the Alternate Units card as the BTU figure). They are calculated differently and mean different things.
Arc-On Energy is computed as arc power (kW) multiplied by total arc-on time in seconds: it represents the total electrical energy consumed by the circuit for the entire weld pass, before efficiency losses. Total Net Energy, by contrast, is net heat input per unit length multiplied by total weld length — which means it already incorporates the process efficiency factor. For a MIG weld, Total Net Energy is always 80% of Arc-On Energy. For a TIG weld, it drops to 60%.
This distinction matters when you’re sizing preheat requirements or calculating interpass temperature. The base metal only ever sees the net figure. A submerged arc process consuming 500 kJ of arc energy deposits roughly 475 kJ into the joint; a TIG process consuming the same electrical energy deposits only 300 kJ. Specifying preheat based on gross energy alone will consistently underestimate the thermal input for high-efficiency processes and overestimate it for low-efficiency ones.
Worked Example: Qualifying a Procedure for a Low-Alloy Pressure Vessel
A weld procedure specification calls for heat input between 30 and 55 kJ/in on 1-1/4 Cr-1/2 Mo (P91 adjacent) material using FCAW. The welder runs qualification passes at the following parameters:
- Measurement System: US Customary
- Welding Process: Flux Core / FCAW (85%)
- Arc Voltage: 28 V
- Welding Current: 230 A
- Travel Speed: 10 ipm
- Weld Length: 6 in
Arc Power: 28 × 230 = 6,440 W (6.44 kW)
Gross Heat Input: (6,440 × 60) ÷ (10 × 1,000) = 38.64 kJ/in
Net Heat Input (hero field): 38.64 × 0.85 = 32.84 kJ/in — within the 30–55 kJ/in window.
Heat Loss Adjustment (Card 1): 38.64 − 32.84 = 5.80 kJ/in
Arc-On Energy (Card 2): 6.44 kW × 36 s = 231.84 kJ
Travel Time (Card 3): 6.0 s/in × 6 in = 36.0 s total (0.60 min)
Alternate Units (Card 4): 32.84 kJ/in ÷ 25.4 = 1.293 kJ/mm net; total net energy = 32.84 × 6 = 197.04 kJ × 0.947817 ≈ 187 BTU
If the welder slows travel to 8 ipm to improve tie-in on a difficult position, recalculating gives a net heat input of 41.05 kJ/in — still within procedure limits, but worth documenting as a separate data point for the qualification record.
Frequently Asked Questions
MIG and Stick both show 80% efficiency — does that mean they behave identically in the calculation?
Mathematically, yes: both use proc = 0.80, so the net heat output for identical voltage, amperage, and travel speed will be the same number regardless of which option you select. The distinction between the two in the process dropdown is informational — it confirms which real-world process matches that efficiency class — not a trigger for separate logic. If you’re using SMAW parameters, select Stick for clarity in your records even though the computed result is numerically identical to MIG at the same inputs.
What happens if I enter zero or leave a field blank?
Any input that is zero, negative, or non-numeric triggers a validation check that clears all output fields and replaces the insight alert with a warning message stating “Data Required.” The calculation does not run with invalid inputs. The minimum accepted value for all four numeric fields is any positive number greater than zero.
If I switch from US Customary to Metric mid-session, do my old values carry over?
No. When you switch measurement systems, the speed and length inputs are reset to metric defaults (300 mm/min and 250 mm). Voltage and amperage are unit-neutral and are not reset. This is intentional: a travel speed of 12 entered as ipm is a very different physical condition from 12 entered as mm/min, and silently keeping the numeric value would produce a meaningless result. Always re-enter your travel speed and weld length after switching systems.
The Alternate Units card shows both kJ/mm and BTU — which one is the alternate unit?
The primary value displayed at the top of the card (the large number) is always the net heat input converted into the opposing length unit: kJ/mm if you’re in US mode, kJ/in if you’re in Metric mode. The BTU figure in the second sub-row is a separate conversion — it converts the total net energy for the full weld length (in kJ) to BTU using the factor 0.947817 kJ/BTU. These are two independent conversions shown in the same card for convenience.
Can I use this calculator to verify that my parameters stay within a qualified heat input range?
Yes, and that is one of its most common uses. Enter the upper and lower bounds of your procedure’s permitted parameters separately to find the corresponding heat input limits, then use those as your envelope. Note that the calculator computes a single-point result for one specific set of parameters — it does not plot a range. To bracket a heat input window, run the calculation at least twice: once at the low-heat condition (minimum amperage, minimum voltage, maximum speed) and once at the high-heat condition (maximum amperage, maximum voltage, minimum speed).