Resonant Frequency Calculator

How to use the Resonant Frequency Calculator

1. 1Enter the inductance and the capacitance with their units.
2. 2Pick the units or options from the menus if the resonant frequency tool offers them.
3. 3Read the resonant frequency in the large result box.
4. 4Look at the extra rows for related figures.
5. 5Press Copy result to keep the answer.

When you need the resonant frequency

Radio builders use it to tune coils and capacitors to a wanted frequency. Audio and power designers use it to avoid unwanted resonances, and hobbyists use it to pick parts for a metal detector or a crystal-set radio.

The same formula applies to a series circuit and a parallel circuit, but they behave differently at resonance: a series circuit has a very low impedance, a parallel circuit a very high one. Both are the basis of band-pass and band-stop filters.

How the resonant frequency is worked out

f = 1 ÷ (2π × √(L × C)). The characteristic impedance is √(L ÷ C). An inductor of 10 mH with a capacitor of 100 nF resonates at 5.033 kHz, with an impedance of 316 Ω. A tiny radio tuning pair of 1 µH and 100 pF resonates at 15.9 MHz.

Every answer on this page comes from that one rule, so you can check any result by hand with a calculator. The figures shown underneath, such as other units, are conversions of the same answer.

• Frequency, period and impedance
• pF to F and nH to H
• Works for series and parallel

Mistakes to avoid with resonant frequency

The formula ignores resistance. Real circuits have losses, which damp the resonance and lower its sharpness, described by the quality factor Q. Stray capacitance and inductance in wiring also move the real frequency, especially at high frequencies.

On phone and computer

On a phone, type into the boxes one after another and read the resonant frequency underneath; Copy result puts it on the clipboard for a message or a note. On a computer the inputs sit on the left and the answer on the right, so you can change one figure and watch the resonant frequency move. The resonant frequency calculator works the same on both. A resonant circuit needs a coil and a capacitor. Choose parts with the Inductor Calculator and the Capacitor Calculator, and check how a coil acts at that frequency with the Inductive Reactance Calculator.

Features

• Quadruple L for half the frequency
• Resistance damps the peak
• Stray parts shift the result
• Clear messages when the resonant frequency inputs make no sense
• Copy the result with one tap
• Private: nothing is uploaded

Frequently asked questions