Wheatstone Bridge Calculator
A Wheatstone bridge finds an unknown resistance by comparison with three known resistors. The circuit has four resistors: R1, R2, R3, and the unknown Rx, arranged in two parallel branches (R1-R2 and R3-Rx). A galvanometer connects the midpoints. At balance, no current flows through the galvanometer, and the balance condition Rx = R2 * R3 / R1 gives the unknown resistance. This null-balance method is highly accurate because the result depends only on the ratios of known resistors, not on the supply voltage. Enter R1, R2, and R3 in ohms to calculate Rx.
Wheatstone bridge formula
At balance: R1/R2 = R3/Rx
Therefore: Rx = R2 * R3 / R1
The circuit has two voltage dividers sharing the same supply. The balance condition sets the output voltage of both dividers equal. Rearranging the balance equation gives Rx directly from three known values.
Wheatstone bridge applications
- Laboratory resistance measurement: precision ratio boxes (known R1, R2) and a calibrated standard (R3) allow measurement of Rx to parts-per-million accuracy.
- Strain gauges: the gauge replaces Rx; mechanical strain changes its resistance, producing an output voltage proportional to force or deformation.
- Temperature sensing: RTDs (resistance temperature detectors) in a bridge circuit produce a voltage output proportional to temperature change.
- Fault location in cables: bridge methods determine the distance to a cable fault by comparing resistances of the damaged and intact conductor loops.
Frequently asked questions
What is a Wheatstone bridge?
A Wheatstone bridge is a circuit with four resistors arranged in a diamond pattern, powered by a voltage source. At balance (when the galvanometer reads zero), the ratio of resistors in one branch equals the ratio in the other. This allows precise measurement of an unknown resistance.
What is the Wheatstone bridge balance condition?
The bridge is balanced when R1/R2 = R3/Rx, or equivalently Rx = R2 * R3 / R1. At balance, no current flows through the galvanometer between the midpoints, indicating the voltage at both midpoints is equal.
How accurate is the Wheatstone bridge method?
A Wheatstone bridge can measure resistance to very high accuracy (often better than 0.1%) when the known resistors are precision components. It is more accurate than a simple ohmmeter because it is a null-balance method independent of supply voltage.
What is a strain gauge bridge?
A strain gauge replaces one or more resistors in a Wheatstone bridge. When the gauge is mechanically strained, its resistance changes slightly, unbalancing the bridge and producing a voltage output proportional to the applied strain.
Can the Wheatstone bridge measure very low resistances?
For very low resistances (below about 1 ohm), the Kelvin double bridge (Thomson bridge) is used instead, which accounts for the resistance of connecting wires that would otherwise introduce significant error.
Official sources
- NIST: NIST Resistance Calibrations.
- IEEE: IEEE Standards Association.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.