RL Filter Calculator - Inductor Cutoff Frequency

Agarapu Ramesh — Editor and content reviewer
Editorially reviewed Reviewed by Agarapu Ramesh, science educator (chemistry). LinkedIn
Last reviewed: May 2026 | Standard electrical engineering formulas

An RL filter uses resistance and inductance to shape frequency response. The cutoff frequency is R / (2 x pi x L). Inductors are less tidy than capacitors because winding resistance and core behavior show up fast.

Formula at a glance

  • cutoff frequency: fc = R / (2 x pi x L)
  • inductive reactance: XL = 2 x pi x f x L
  • time constant: tau = L / R

Field note: A perfect inductor lives in textbooks. Real inductors get warm, saturate and have resistance.

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RL Filter Calculator

Resistor-Inductor filter calculator

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Result
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Formulas

ττ = L / R
fcfc = R / (2πL)
XLXL = 2πfL

RL vs RC

RL filters: Better for high current, power applications

RC filters: Smaller, cheaper, better for signal processing

Related

RC FilterOhm's Law

How to use the RL Filter Calculator

Use this as a bench check, then compare it with the part marking, tolerance and a meter reading when the circuit matters. Small components are cheap. Bad assumptions are not.

Worked example

Example: R = 100 ohm and L = 100 mH gives fc = 159 Hz.

Practical checks before you trust the number

  • Use henries for L and ohms for R.
  • The inductor winding resistance may need to be included.
  • Core saturation changes the effective inductance.

Common mistake

A perfect inductor lives in textbooks. Real inductors get warm, saturate and have resistance.

Sources and references

Related calculators

Frequently Asked Questions

f_c = R ÷ (2π × L). The cutoff is the -3 dB point. Example: R = 100 Ω, L = 10 mH → f_c = 100 ÷ (2π × 0.01) = 1591 Hz. Same idea as RC but with inductance instead of capacitance, and the role of L and R is swapped.

For an RL low-pass (L in series, R to ground): V_out = V_in × R ÷ √(R² + (2πfL)²). At f_c, V_out = 70.7% of V_in. Roll-off is 20 dB/decade above the cutoff.

For RL high-pass: R in series, L to ground. f_c = R ÷ (2π × L). Pick L and R based on the cutoff frequency and the impedance you want to present. Example: cut below 1 kHz at 50 Ω: L = 50 ÷ (2π × 1000) = 8 mH.

τ = L ÷ R, in seconds. Same role as RC's time constant — the time to reach 63.2% of final current when the inductor is energized. Example: 10 mH ÷ 1 kΩ = 10 µs. After 5τ, the current is essentially steady.

Inductance and cutoff are inversely related: f_c = R ÷ (2π L). Larger L lowers f_c; smaller L raises it. For a given R, picking L sets the cutoff. RL filters are common in power electronics for surge suppression and noise filtering on power lines.

Low-pass: L in series, R to ground. High-pass: R in series, L to ground. Both share f_c = R ÷ (2π L) but with mirrored topologies. Same conceptual difference as RC LP versus HP — pass low or pass high.

Yes. Useful for designing audio crossovers, EMI filters on power lines, and impedance matching networks. Inductors store energy in magnetic fields; they're better at handling DC current than capacitors are at handling DC voltage, so RL filters dominate in power applications.