Spark Plug Gap Calculator
Convert a spark plug gap between millimetres, inches and thousandths, and see how far you need to open or close the gap to reach spec.
How it works
Inches = mm ÷ 25.4 · Thousandths = inches × 1000 · Adjustment = measured − target
The gap between a spark plug’s electrodes sets the size of the spark, and manufacturers specify it tightly — too small and the spark is weak, too large and the coil may not jump it, causing misfires. Since specs and feeler gauges come in both millimetres and thousandths of an inch, converting cleanly matters, and comparing your measured gap to spec tells you which way to bend the ground electrode. Adjust in small steps and re-measure, as electrodes also widen naturally with wear over the plug’s life.
Worked example
A 1.0 mm spec gap is 1.0 ÷ 25.4 = 0.039 in, or 39 thou. If the plug measures 1.1 mm, it is 0.1 mm too wide, so gently close the electrode by 0.1 mm and re-check with the gauge.
Frequently asked questions
What is the correct spark plug gap?
It is engine-specific — commonly 0.7 to 1.1 mm (about 28–44 thou). Always use the figure from your vehicle’s specification or the plug maker’s chart rather than assuming a universal value.
How do I convert mm to thousandths of an inch?
Divide the millimetre value by 25.4 to get inches, then multiply by 1,000 for thousandths. So 0.9 mm ÷ 25.4 × 1,000 ≈ 35 thou. This calculator does it both ways.
What happens if the gap is wrong?
Too narrow a gap gives a weak spark and can foul; too wide and the coil may fail to fire it under load, causing misfires, rough idle and poor economy. Setting it to spec restores a strong, reliable spark.
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This calculator is for educational purposes. Double-check important results before acting on them.