Electrical & RF Calculators
Power, impedance, antennas, and the dB family.
- dB ConverterConvert power between W, mW, dBm, and dBW in one shot.P(dBm) = 10 · log₁₀( P / 1 mW )
- Ohm's LawEnter any two of voltage, current, and resistance — get the third plus power.V = I · R P = V · I
- Frequency ⇄ WavelengthWavelength for any frequency, with velocity factor for cables.λ = (c · VF) / f
- Voltage DividerVout, loading current, and dissipation for the two-resistor divider.Vout = Vin · R2 / (R1 + R2)
- Parallel ResistorsEquivalent resistance of two or three resistors in parallel.1/Rp = 1/R1 + 1/R2 (+ 1/R3)
- Resistor Color CodeFour-band resistor value from the band colors.R = (10·D1 + D2) × 10^multiplier
- Op-Amp GainClosed-loop gain for inverting and non-inverting configurations.Non-inv: G = 1 + Rf/R1 Inv: G = −Rf/R1
- RC Low-Pass FilterCutoff frequency and time constant for the single-pole RC.fc = 1 / (2πRC) τ = RC
- RMS ⇄ Peak ⇄ Peak-to-PeakSine-wave conversions between RMS, peak, and peak-to-peak.Vpk = √2 · Vrms Vpp = 2 · Vpk (sine only)
- Shunt ResistorDrop, dissipation, and required rating for a current-sense shunt.V = I·R P = I²·R
- LED Series ResistorThe resistor between your supply and the LED, plus its dissipation.R = (Vs − Vf) / I
- Capacitor Energy & ChargeStored energy and charge at a given voltage.E = ½CV² Q = CV
- dB Voltage RatioGain in dB from two voltages — the 20·log flavor.dB = 20 · log₁₀(V2 / V1)
- VSWR / Return Loss / ΓEnter any one of VSWR, return loss, or reflection coefficient — get all of them plus mismatch loss.Γ = (VSWR−1)/(VSWR+1) RL = −20·log₁₀|Γ|
- Free-Space Path LossFSPL between isotropic antennas — the first line of every link budget.FSPL(dB) = 32.45 + 20·log₁₀ f(MHz) + 20·log₁₀ d(km)
- RF Link BudgetReceived power and margin from TX power, gains, distance, and losses.Pr = Pt + Gt + Gr − FSPL − L
- Thermal Noise FloorkTB noise power for a bandwidth — the −174 dBm/Hz line.P = kTB (−173.98 dBm/Hz at 290 K)
- Noise Figure ⇄ Noise TemperatureConvert between NF in dB and equivalent noise temperature in kelvin.Te = (F − 1) · 290 K
- Cascade Noise FigureTwo-stage Friis cascade — why the LNA wins.F = F1 + (F2 − 1) / G1
- Reactance (XL / XC)Inductive or capacitive reactance at frequency.XL = 2πfL XC = 1 / (2πfC)
- LC Resonant FrequencyResonance of an LC pair — the Thomson formula.f₀ = 1 / (2π√(LC))
- Q Factor ⇄ BandwidthLoaded Q from bandwidth, or bandwidth from Q, at a center frequency.Q = f₀ / BW
- Skin DepthHow deep current actually flows in a conductor at frequency.δ = 1 / √(π·f·µ₀·σ)
- PCB Trace WidthIPC-2221 trace width for a current and temperature rise.I = k · ΔT^0.44 · A^0.725 (IPC-2221; k = 0.048 ext / 0.024 int)
- Microstrip ImpedanceZ₀ and effective εr for a surface trace over a ground plane (Hammerstad).Hammerstad closed-form: Z₀ = f(w/h, εr)
- Pi AttenuatorResistor values for a Pi pad of given attenuation and impedance.R_shunt = Z(k+1)/(k−1) R_series = Z(k²−1)/2k, k = 10^(dB/20)
- T AttenuatorResistor values for a T pad of given attenuation and impedance.R_series = Z(k−1)/(k+1) R_shunt = 2Zk/(k²−1), k = 10^(dB/20)
- dBm ⇄ dBµV (50 Ω)Convert between the RF power and EMC voltage dialects.dBµV = dBm + 107 (in 50 Ω)
- Frequency ⇄ Periodf to T and back, with the angular frequency thrown in.T = 1/f ω = 2πf
- Dipole / Whip LengthCut lengths for a half-wave dipole and quarter-wave whip.L ≈ 0.95 · λ/2 (the ham's 468/f(MHz) in feet)
- Coax Loss & Power OutTotal cable loss from the per-100-ft spec, and what actually reaches the antenna.Loss = spec(dB/100 ft) × length / 100 ft
- Doppler ShiftFrequency shift for a moving target — one-way or radar (two-way).fd = 2·v·f/c (radar) fd = v·f/c (one-way)
- Radio / Radar HorizonLine-of-sight distance from antenna heights, with standard refraction.d(km) ≈ 4.12 (√h1 + √h2) (4/3-earth refraction)
- Gain from BeamwidthsApproximate directive gain from the two −3 dB beamwidths.D ≈ 41253 / (θaz · θel) (degrees)
- RF Power DensityFar-field power density from EIRP and distance — the exposure math.S = EIRP / 4πd² E = √(377·S)
- Transformer Turns RatioVoltage, current, and impedance transformation from the turns ratio.Vs = Vp·Ns/Np Zp = Zs·(Np/Ns)²
- RC Charge TimeTime for a capacitor to reach a target percentage of the supply.t = −RC · ln(1 − target)
- Inductor EnergyStored energy in a current-carrying inductance.E = ½LI²
- L-Network Impedance MatchTwo-component L-match between resistive source and load — Q, L, and C at your frequency.Q = √(R_hi/R_lo − 1) X_series = Q·R_lo X_shunt = R_hi/Q
- 555 Timer — AstableR1, R2, C → frequency, duty cycle, and high/low times for the classic astable hookup.f ≈ 1.44 / ((R1 + 2·R2)·C) t_hi = 0.693·(R1+R2)·C t_lo = 0.693·R2·C
- 555 Timer — MonostableOne-shot pulse width from R and C: t = 1.1·R·C.t = ln(3)·R·C ≈ 1.1·R·C
- Zener Shunt RegulatorSeries resistor window and worst-case zener dissipation for a simple shunt regulator.R ≤ (Vin_min − Vz)/(I_load + Iz_min) Pz_max = Vz·(Vin_max − Vz)/R
- Wire Gauge (AWG) PropertiesPick an AWG size — diameter, area, resistance, and rule-of-thumb ampacity for copper.d(mm) = 0.127 · 92^((36−AWG)/39) R = ρ/A, ρ_Cu = 1.724×10⁻⁸ Ω·m
- Wire Voltage DropRound-trip drop over a copper pair: AWG size, one-way length, and load current.V_drop = I · ρ/A · 2L (out and back; Cu at 20 °C)
- Power Sum in dBmAdd two incoherent powers expressed in dBm — the right way.P_total = 10·log₁₀(10^(P1/10) + 10^(P2/10))
- Fresnel Zone ClearanceFirst-zone radius at mid-path and the 60% clearance a microwave link actually needs.r₁ = 17.32·√(d/(4f)) at mid-path, d in km, f in GHz, r in m
- ERP ⇄ EIRPRadiated power against the dipole or isotropic reference — the 2.15 dB everyone flips.EIRP = ERP + 2.15 dB dBi = dBd + 2.15
- Coax Impedance from DimensionsZ₀ of a coaxial line from shield ID, center-conductor OD, and the dielectric.Z₀ = (138/√εr) · log₁₀(D/d)
- Series RLC Impedance|Z| and phase of a series R-L-C at frequency, plus where it resonates.|Z| = √(R² + (XL − XC)²) φ = atan((XL−XC)/R) f₀ = 1/(2π√LC)
- Battery Pack ConfigurationSeries × parallel cells → pack voltage, capacity, and watt-hours.V = S·V_cell Capacity = P·C_cell E = V × Ah
- Hold-Up Capacitor SizingBulk capacitance to ride through a dropout: C = 2·P·t / (V₁² − V₂²).C = 2·P·t / (V₁² − V₂²) (from ½CV₁² − ½CV₂² = P·t)
- Nearest E-Series ValueSnap a target resistance to the nearest E12 / E24 / E96 standard value.E-series: mantissas ≈ 10^(i/N), i = 0…N−1 (IEC 60063)
- LED Array / String DesignerSupply, Vf, and current → LEDs per string, string count, and the ballast resistor.n ≤ (Vs − V_headroom)/Vf R = (Vs − n·Vf)/I
- RC Snubber (Ring Killer)Size R and C from the measured ringing frequency and estimated parasitic capacitance.R = 1/(2π·f_ring·C_par) C_snub ≈ 4·C_par P_R = C_snub·V²·f_sw
- Inrush Limiter (NTC) SizingCold resistance to cap the surge into a capacitor bank, and the joules the NTC must swallow.R_cold ≥ V_peak / I_max E = ½·C·V² (energy the limiter absorbs at turn-on)