By Paul Dohrman
Updated Mar 24, 2022

Inductors are often hand‑wound by hobbyists or engineers, and when no inductance label is available, the value must be measured experimentally. The most accurate approach is to use a dedicated inductance bridge or meter, but if those tools are not at hand, a sine‑wave oscilloscope combined with a known resistor provides a reliable workaround.

Step 1 – Build the Test Circuit

Connect a precision resistor (whose resistance is known to within 1 % or better) in series with the coil you wish to test. Attach the oscilloscope probe to the junction between the resistor and the coil, and the second probe across the coil.

Step 2 – Measure Voltage Drops

Use two digital multimeters or the oscilloscope’s probe‑in‑parallel mode to capture the voltage drop across the coil and across the resistor simultaneously. This yields two time‑aligned waveforms.

Step 3 – Tune the Frequency

Adjust the oscilloscope’s sine‑wave generator until the peak voltages measured across the resistor and the coil are equal. At this frequency, the resistor’s DC resistance and the coil’s reactance are numerically identical, i.e. R = X_L.

Step 4 – Calculate the Inductance

With R (ohms) known and the frequency f (hertz) set in the previous step, the inductance L can be calculated from the inductive reactance formula:

L = R ÷ (2πf)

Because the resistor’s value does not change with frequency, the only variable is the oscillation frequency, making the calculation straightforward.

Things You’ll Need

• Sine‑wave oscilloscope (or function generator + oscilloscope)
• Two digital multimeters or a dual‑probe oscilloscope setup
• Precision resistor (close tolerance, e.g., 1 % or better)

Follow these steps and you’ll obtain an accurate inductance measurement for any hand‑wound coil without the need for specialized equipment.