Use the calculator to estimate required air clearance or breakdown voltage (Air Clearance tab), and to compute magnetic air-gap reluctance, MMF, inductance, or required gap length (Magnetic Air Gap tab).

Air Gap Clearance & Magnetic Reluctance Calculator

Pick a tab, enter values, then Calculate.
Air Clearance
Magnetic Air Gap

Estimate breakdown voltage or required clearance in air with altitude and waveform options.

Assumes baseline breakdown strength of dry air near sea level of about 3 kV/mm in a uniform field. For non-uniform fields a lower effective strength is used. Altitude correction uses a simple barometric-pressure scaling (~exp(-h/8150)) so clearance increases with altitude. Results are approximations for guidance only.

Related Calculators

Air Gap “Resistance” (Magnetic Reluctance) Formula

In magnetic circuit analysis (transformers, inductors, and electromagnets), the “resistance” of an air gap usually refers to magnetic reluctance (not electrical/ohmic resistance). The following formula is commonly used for a simple, uniform air gap (ignoring fringing):

\mathcal{R}_g = \frac{l_g}{\mu_0 \mu_r A}

Variables:

  • ℛg is the air-gap reluctance (units: A/Wb, equivalent to 1/H)
  • lg is the gap length (m)
  • μ0 is the permeability of free space (4π × 10−7 H/m)
  • μr is the relative permeability of the gap material (≈ 1 for air)
  • A is the effective cross-sectional area of the gap (m²)

To calculate the air-gap reluctance, divide the gap length by (μ0·μr·A). In many designs the air gap dominates the total reluctance, but fringing can increase the effective area and reduce the true reluctance.

What is Air Gap Resistance?

“Air gap resistance” is an ambiguous phrase. In most transformer/inductor contexts, it refers to the magnetic reluctance introduced by an intentional air gap in a magnetic core. This reluctance reduces inductance and increases the magnetizing force (MMF) required to produce a given flux, helping prevent core saturation.

Electrically, air is normally an insulator (not a simple ohmic conductor), so its behavior under high voltage is usually described in terms of dielectric strength and breakdown/flashover voltage rather than a fixed resistivity.

How to Calculate Air Gap Resistance?

The following steps outline how to calculate the air-gap “resistance” in the magnetic sense (reluctance).

  1. Measure the air-gap length lg (m).
  2. Determine the effective cross-sectional area A of the gap (m²).
  3. Use μr ≈ 1 for air (unless your gap is filled with another material).
  4. Compute reluctance using ℛg = lg / (μ0 μr A).
  5. Optionally, use the calculator above to estimate MMF for a target flux, inductance for a coil with N turns, or the gap needed for a target inductance.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

Gap length (lg) = 1 mm (0.001 m)

Area (A) = 100 mm² (1.0 × 10−4 m²)

Relative permeability (μr) = 1 (air)