ELECTRONICS TOOLBOX

Resistor calculator

Preferred resistor values from E-series.

Shared by all calculator cards. Press Calculate to apply.

Voltage divider

Find the closest resistor pair below and above your target voltage.

V
V
Ω
Ω

SI prefixes: p, n, u / µ, m, k, M, G, T. Examples: 500m, 4.7k, 1M. m = milli; M = mega. R1/R2 fields are used only when locked. With both locked, Vout target is ignored.

Vin connected through R1 to Vout, with R2 from Vout to ground VinR1VoutR2
Vout = Vin × R2 / (R1 + R2)

Ideal, unloaded divider. I(x) = Vin / ((R1 + R2) × 10ˣ). Multiplying both resistors changes the current, while preserving Vout.

Serial/Parallel

Combine two resistors to match a target resistance.

Ω
Connection
Ω

Engineering notation supported, e.g. 4.7k, 4,7k, 1M, 500m. Lock R1 to keep the entered value; otherwise both resistors use the shared E-series.

Serial: R = R1 + R2
Parallel: R = R1 × R2 / (R1 + R2)

With R1 locked, only R2 is selected from the E-series. A series total is always greater than either resistor; a parallel total is always smaller than either resistor.

2-tap voltage divider

Three resistors in series, two output voltages and a target total resistance.

V
V
V
Ω

Vin > Vout1 > Vout2 > 0. Engineering notation supported, e.g. 3.3, 500m, 30k. Rtot = R1 + R2 + R3. Set Rtot = 0 to optimize only the two output voltages.

Vin, R1, upper tap Vout1, R2, lower tap Vout2, R3, ground. All voltages relative to ground. VinR1Vout1R2Vout2R3
Vout1 = Vin × (R2 + R3) / Rtot
Vout2 = Vin × R3 / Rtot

Both outputs are relative to ground. The search minimizes the sum of squared relative errors for Vout1, Vout2 and Rtot, with equal weight. With Rtot = 0, only the two voltage errors are minimized. E-series values may require a compromise. Assumes unloaded outputs.