Resonant Frequency Calculator
Calculate the resonant frequency of an LC circuit from its inductance and capacitance, or find the inductor or capacitor you need to resonate at a target frequency. You also get ω₀, reactance and characteristic impedance.
How to use the Resonant Frequency Calculator
- Choose what to solve for: frequency, inductance or capacitance.
- Enter the two known values and pick their units.
- Read the result with automatic unit prefixes.
- Check the extra values: ω₀, period, reactance and impedance.
- Use the E6 table to pick a real capacitor close to your target.
LC resonance formulas
| Solve for | Formula | Example |
|---|---|---|
| Frequency | f = 1 ÷ (2π√(LC)) | 10 µH, 100 pF → 5.033 MHz |
| Capacitance | C = 1 ÷ ((2πf)² L) | 1 MHz, 250 µH → 101.3 pF |
| Inductance | L = 1 ÷ ((2πf)² C) | 100 MHz, 22 pF → 115.1 nH |
| Angular frequency | ω₀ = 1 ÷ √(LC) = 2πf | 5.033 MHz → 3.162 × 10⁷ rad/s |
| Reactance at resonance | X = 2πfL = 1 ÷ (2πfC) | 10 µH at 5.033 MHz → 316.2 Ω |
| Characteristic impedance | Z₀ = √(L ÷ C) | 10 µH, 100 pF → 316.2 Ω |
Series vs parallel resonance
Both use the same frequency formula. A series LC circuit has minimum impedance at resonance and passes that frequency (band-pass, or a short-circuit trap). A parallel LC "tank" has maximum impedance at resonance, which is why it is used to select one station in a radio or to set the frequency of an oscillator. The quality factor Q (for series RLC, Q = X ÷ R) controls how narrow the resonance is: bandwidth ≈ f ÷ Q.
Frequently asked questions
What is the formula for resonant frequency?
For an ideal LC circuit, f = 1 ÷ (2π√(LC)), with f in hertz, L in henries and C in farads. In angular form, ω₀ = 1 ÷ √(LC) radians per second.
What happens at resonance?
At the resonant frequency the inductive reactance X_L = 2πfL equals the capacitive reactance X_C = 1 ÷ (2πfC). In a series LC circuit the impedance drops to the resistance only; in a parallel (tank) circuit the impedance becomes very high.
How do I find the capacitor for a target frequency?
Rearrange the formula: C = 1 ÷ ((2πf)² × L). For 1 MHz with a 250 µH coil, C ≈ 101.3 pF. The inductance is found the same way: L = 1 ÷ ((2πf)² × C).
Does resistance change the resonant frequency?
For a series RLC circuit, resistance does not change the resonant frequency; it sets how sharp the peak is (the Q factor). In a parallel circuit with a lossy inductor the peak shifts slightly, but for coils with a high Q the ideal formula is very close.
Why does doubling the capacitance not halve the frequency?
Frequency depends on the square root of L × C. Doubling C lowers f by a factor of √2 (about 29%). To halve the frequency you need four times the capacitance or inductance.
Where are LC resonant circuits used?
In radio tuners, oscillators, band-pass and notch filters, wireless charging coils, RFID tags and induction heaters. Real circuits also have stray capacitance and inductance, so the actual frequency can differ by a few percent.