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Wheatstone Bridge Calculator

Find the unknown resistor in a balanced bridge.

How it is calculated

The tool gives the unknown resistance of a bridge that is already balanced, meaning the galvanometer between the two midpoints reads zero. Under that condition the two voltage-divider legs have equal ratios, so Rx = R2 × R3 / R1, with all resistances in ohms (Ω). The result is rounded to two decimals and shown in ohms. The engine needs R1 to be greater than zero, because it is the divisor, and rejects negative R2 or R3; an R2 or R3 of 0 is accepted and gives 0 Ω. It does not calculate the galvanometer current of an unbalanced bridge.

Worked example

You balance a bridge with R1 = 1000 Ω, R2 = 2200 Ω and R3 = 470 Ω. Multiply R2 by R3: 2200 × 470 = 1,034,000. Divide by R1: 1,034,000 / 1000 = 1,034. The unknown resistor is 1,034.00 Ω, about 1.03 kΩ.

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Common mistakes

  • Putting the resistors in the wrong positions. The formula divides by R1, so swapping R1 with R2 or R3 gives a different answer; follow the same labelling as your bridge diagram.
  • Mixing units, for example R1 in kΩ and R2 in Ω. All three values must be in ohms, since the result is shown in Ω.
  • Using the tool before the bridge is truly balanced. If the galvanometer still shows a deflection, the formula does not hold and the answer is only an approximation.
  • Entering 0 for R1 (a short circuit in that arm). It is the divisor, so the tool returns no result.

About this calculator

A Wheatstone bridge is a circuit of four resistors arranged in a diamond, used to precisely measure an unknown resistance by comparing it against three known ones. When the bridge is balanced — meaning no current flows through the galvanometer connecting the two midpoints — the unknown resistor Rx satisfies the ratio Rx = (R2 × R3) / R1, derived from the two voltage-divider legs being equal.

Electrical engineers and metrologists use the Wheatstone bridge configuration in strain gauges, load cells, and precision resistance-based sensors (including thermistors and RTDs for temperature measurement), because comparing against known references gives far higher accuracy than a direct ohmmeter reading.

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Comments (1)

  • Editorial team

    Think of R2 ÷ R1 as the ratio arm and Rx as R3 times that ratio. With the defaults the ratio is 2, so 150 Ω becomes 300 Ω. R2 and R3 can be swapped without changing the answer, but R1 cannot, so label your diagram first. The tool assumes a balanced bridge. What range of Rx are you measuring?

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