Wheatstone Bridge Calculator
Enter the three known resistors of a balanced bridge and read off the unknown resistance — the classic way to measure a resistor with precision.
Three known, one unknown
Give the calculator R1, R2, and R3 and it returns the unknown resistance Rx using the balance condition Rx = (R2 × R3) / R1.
Balance is assumed
The formula only holds when the bridge is balanced — the galvanometer reads zero. An unbalanced bridge needs a different, voltage-based analysis.
What is a Wheatstone bridge calculator?
Three resistors in, the unknown out
A Wheatstone bridge is a diamond-shaped circuit of four resistors with a sensitive galvanometer across the middle. Three of the resistors are known and the fourth, Rx, is the one you want to measure. You adjust one known resistor until the galvanometer reads exactly zero — the "balanced" state — and at that moment the unknown resistance is fixed entirely by the other three. This calculator does that final step: enter R1, R2, and R3 and it returns Rx. It is the standard textbook method for measuring an unknown resistance far more precisely than a basic ohmmeter, and the same bridge idea underlies strain gauges, thermistor sensors, and load cells.
Enter the three known resistances in ohms to get the unknown resistance Rx instantly.
At balance no current flows through the galvanometer, so the two arms divide the voltage in the same ratio. That gives the balance condition R1/R2 = R3/Rx, which rearranges to a single formula.
Rx = (R2 × R3) ÷ R1Only the ratios matter, not the absolute values: doubling every resistor leaves Rx unchanged. That is why a bridge can be built from a precise reference resistor and a calibrated ratio without needing a precise supply voltage.
Suppose R1 = 100 Ω, R2 = 200 Ω, and R3 = 150 Ω, with the bridge tuned to balance.
Multiply the opposite arm
R2 × R3 = 200 × 150 = 30,000 — the product of the two resistors that pair against R1.
Divide by R1
30,000 ÷ 100 = 300 — divide by the resistor in the same arm as Rx.
Read the unknown
Rx = 300 Ω — the unknown resistance that balances the bridge.
The result Rx is the resistance that makes the galvanometer read zero — nothing more, nothing less. With R1 = 100 Ω, R2 = 200 Ω, and R3 = 150 Ω the bridge balances at exactly 300 Ω, so any 300 Ω resistor dropped into the fourth arm would null the meter. The key insight is that the answer depends only on the ratios of the known resistors: the ratio R2/R1 acts as a multiplier on the reference R3. Here R2/R1 = 2, so Rx is simply twice R3. That is why a real bridge keeps R3 as a finely adjustable resistor — sweep it until the needle sits on zero, and read Rx straight off the dial. Because the balance point is detected as "no current" rather than measured as a voltage, the result is independent of the supply voltage and of small meter inaccuracies, which is exactly what makes the bridge so precise. The same principle scales up: in a strain gauge or thermistor sensor, a tiny change in one resistor unbalances the bridge by a measurable amount, turning resistance into a clean signal.
The formula is exact, but it only describes one specific operating point of the circuit.
Balanced bridges and ideal resistors only
Rx = (R2 × R3) / R1 holds only at balance, when the galvanometer current is zero. An unbalanced bridge produces an output voltage that this formula does not describe — that case needs a voltage-divider or Thévenin analysis instead. The result also assumes ideal, purely resistive components: real measurements are affected by wire and contact resistance, the tolerance of the reference resistors, temperature drift, and any reactance in the circuit. For low resistances especially, lead resistance can dominate the error, which is why precision work uses four-terminal (Kelvin) connections.