Welder Duty Cycle Calculator
Work out how long your welder can run at the current you need. Enter its rating, such as 150 A at 40 %, and get the duty cycle at another current, the highest current for the duty cycle you need, or a check of your own job, drawn as one 10-minute cycle.
How to Use
- Find the rating on the machine’s plate or in its manual, for example 150 A at 40 %, and enter the current and the duty cycle.
- Pick what to work out: the duty cycle at your current, the highest current for the duty cycle you need, or a check of a job.
- Enter your welding current, the duty cycle you need, or how many minutes you weld in each 10. The answer fills the highlighted field and the first readout.
- Read the bar: the coloured part is welding time and the grey part is cooling time in one 10-minute period. The curve shows the duty cycle at every current.
- If the machine’s manual has a duty-cycle chart, use the chart’s figure; this estimate is the fallback when you only know one rating.
Worked Example
A lower current. A MIG welder rated 150 A at 40 % is run at 130 A. By the square law, DC₂ = 40 % × (150 ÷ 130)² = 53.25 %: 5.33 minutes of welding and 4.67 minutes of cooling in every 10 minutes.
Continuous welding. The same machine at 100 %: I = 150 × √(40 ÷ 100) = 94.87 A. The Millermatic 211, which is rated 150 A at 40 % on 240 V, has a chart in its manual showing 100 % at 100 A, so the estimate is a little cautious there. On 120 V the same manual rates it 115 A at 20 % and 100 % at 90 A, where the square law would say only 32.65 % at 90 A: when the manual has a chart, use the chart.
The common mistake: treating the percentage as a share of the hour. 40 % does not mean 24 minutes of welding at a stretch and 36 minutes of rest. The rating is measured over 10 minutes, so it allows 4 minutes of welding and then 6 of cooling. Welding 24 minutes non-stop at the rated current is six times the allowed run and will trip the thermostat long before the end.
Show Work
Formulas
How Duty Cycle Is Rated
Miller’s owner’s manuals define duty cycle as the percentage of 10 minutes that a unit can weld at rated load without overheating, and Lincoln Electric’s manuals say the same: the rating is based on the amount of welding done in a 10-minute period. When a machine runs past it, a thermostat opens and the output stops while the fan keeps running. The Millermatic 211 manual (OM-265809F, section 4-6) tells the welder to wait fifteen minutes and then reduce the current or the duty cycle.
The test behind the number matters. Weldclass describes the European and Australian standard, EN/AS 60974-1 (IEC 60974-1): the machine is first warmed up until its overload trips at least twice, then tested in a chamber heated to 40 °C over a 10-minute period. Its FORCE 205MST welded at 200 A for 3 of the 10 minutes, so it is rated 200 A at 30 %. A cold machine, a lower room temperature or a 5-minute period would have produced a much higher figure, which is why ratings measured in different ways can’t be compared.
The square law in this calculator comes from how the heat is made: the resistive loss in copper and semiconductors is I² × R, so halving the current cuts the heat to a quarter. It is an estimate for when you know only one rating. Real machines also have fans, input-supply limits and electronics with their own limits, so the manufacturer’s chart can differ either way; Miller’s Millermatic 211 chart is more generous than the square law at low current. Sources: Miller Millermatic 211 owner’s manual OM-265809F; Lincoln Electric LN-25 PRO operator’s manual IM2071; Weldclass, “Welding machines: what is duty cycle and how is it calculated”.
About This Tool
This calculator takes one duty-cycle rating from your machine and estimates the duty cycle at another current, the highest current for the duty cycle you need, or whether a job with a set number of welding minutes fits. It draws one 10-minute period as a bar and the whole duty-cycle curve, with the rated point and yours marked. The figures are starting points: follow the duty-cycle chart in your machine’s manual where it has one, keep the vents clear, and wear a welding helmet with the right filter shade.
Everything runs in your browser; nothing you enter is sent anywhere.
Related tools: Welding Amperage Chart, Welding Cost Estimator, and Shielding Gas Cylinder Duration Calculator.
Frequently Asked Questions
What does a 30 % duty cycle mean?
It means the machine can weld at that current for 30 % of a 10-minute period, which is 3 minutes, and must then cool for the other 7. A welder rated 200 A at 30 % can run 200 A for 3 minutes in every 10 before its thermal cut-out trips.
How do I work out the duty cycle at a lower current?
The usual estimate scales with the square of the current: DC₂ = DC₁ × (I₁ ÷ I₂)². A machine rated 150 A at 40 % run at 130 A gives 40 × (150 ÷ 130)² = 53.25 %, about 5.3 minutes in every 10. If the manual has a duty-cycle chart, the chart is the better figure.
What current can I weld at continuously?
Set the duty cycle to 100 %: I = I₁ × √(DC₁ ÷ 100). For 150 A at 40 % that is 150 × √0.4 = 94.87 A. Miller’s chart for the Millermatic 211 on 240 V, which is rated 150 A at 40 %, lists 100 % at 100 A, so on that machine the estimate is about 5 % cautious.
What happens if I go over the duty cycle?
In the Millermatic 211 manual, the thermostat opens, the output stops and the fan keeps running; Miller says to wait fifteen minutes for the unit to cool and to reduce the current or the duty cycle before welding again. It also warns that exceeding the duty cycle can damage the unit and void the warranty.
Why are duty cycles rated at 40 °C?
The IEC/EN 60974-1 test is run in a chamber heated to 40 °C (104 °F) on a machine that has already been warmed up, over a 10-minute period. Weldclass notes that a 200 A at 30 % machine tested cold, at a lower temperature or over 5 minutes could be quoted at 50–60 %, so ratings from different tests do not compare directly.
How do I use the Welder Duty Cycle Calculator?
Just type your numbers. The answer shows up right away — there is no button to press. Change anything and it updates by itself.
Does it cost anything or need an account?
No. The tool is completely free, there is no account to create, and it keeps working offline after the page first loads.
Is anything I type uploaded?
No. The tool works entirely on your device, so the values you enter never leave your browser.
Common Use Cases
Choosing a machine
You need 60 % at 140 A for long fillet runs. A 200 A at 30 % machine gives 200 × √(30 ÷ 60) = 141.4 A at 60 %, just enough.
Stick welding all day
Running 1/8 in electrodes at 120 A on a 225 A at 20 % machine: 20 × (225 ÷ 120)² = 70.31 %, about 7 minutes of every 10.
Production cells
A robot cell welds 4 minutes in every 10 at 180 A on a 200 A at 30 % source. That allows only 3.70 minutes, so turn down to 173.2 A or shorten the runs.
Spray-transfer MIG
Spray transfer at 250 A on a 300 A at 60 % machine: 60 × (300 ÷ 250)² = 86.4 %, or 8.64 minutes in every 10.
Planning breaks
At 150 A on a 150 A at 40 % machine you get 4 minutes of welding and 6 of cooling in every 10, so plan fit-up and grinding into those 6 minutes.
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