Case 1
- Ratio resistor R1
- 100Ω
- Ratio resistor R2
- 200Ω
- Reference resistor R3
- 150Ω
300.0000Ω
Open with these valuesWheatstone Bridge Calculator
300.0000Ω
Result: 300.0000 ΩWhen the galvanometer reads zero the bridge is balanced, and the unknown arm follows from the other three: Rx = R2 × R3 ÷ R1. With 100 Ω, 200 Ω and 150 Ω that is 300 Ω. Only the ratio matters — the supply voltage never enters the result.
The numbers at a glance
Held fixed: Ratio resistor R1 100.000 Ω, Ratio resistor R2 200.000 Ω.
| Reference resistor R3 (Ω) | Result (Ω) |
|---|---|
| 50.000 | 100.0000 |
| 100.000 | 200.0000 |
| 150.000Your value | 300.0000 |
| 200.000 | 400.0000 |
| 250.000 | 500.0000 |
| 300.000 | 600.0000 |
Worked examples
Case 2
- Ratio resistor R1
- 220Ω
- Ratio resistor R2
- 470Ω
- Reference resistor R3
- 1000Ω
2,136.3636Ω
Open with these valuesCase 3
- Ratio resistor R1
- 47Ω
- Ratio resistor R2
- 68Ω
- Reference resistor R3
- 33Ω
47.7447Ω
Open with these valuesHow it's calculated
Rx = R2 × R3 ÷ R1
- StepEnter R1 and R2, the two known resistors of the ratio arm.
- StepEnter R3, the reference resistor facing the unknown one.
- ResultAdjust until the meter reads zero, then read Rx in ohms.
Reference table
| R1, R2, R3 | What it is | Rx |
|---|---|---|
| 0.5, 2, 3 | A ratio arm below one ohm | 12 |
| 47, 68, 33 | Three E24 values off the reel | 47.7447 |
| 100, 200, 150 | The textbook bridge | 300 |
| 100, 330, 470 | A 1 : 3.3 ratio arm | 1551 |
| 220, 470, 1000 | A 1 kΩ reference against 220 Ω | 2136.3636 |
| 1000, 1000, 1000 | All three equal, so Rx equals them too | 1000 |
Questions
How do I calculate the unknown resistance in a Wheatstone bridge?
At balance, multiply R2 by R3 and divide by R1: Rx = R2 × R3 ÷ R1. For R1 = 100 Ω, R2 = 200 Ω and R3 = 150 Ω that is 300 Ω.
What does a balanced bridge mean?
It means no current flows through the galvanometer in the middle — it reads exactly zero. At that point both arms divide the voltage in the same ratio, which is what makes the simple formula apply.
Why is a bridge more precise than an ohmmeter?
Because it works by null detection rather than by measuring a value, and detecting zero can be done very sensitively. The answer then depends only on the ratios of precise reference resistors.
Does the supply voltage affect the result?
No. At balance only the ratios of the three known resistors matter. A higher voltage makes the meter more sensitive away from balance, but the balance point and therefore Rx stay the same.
Where are Wheatstone bridges used?
They are the core of strain gauges, load cells, thermistor and Pt100 temperature sensors and pressure transducers. A tiny change in one resistor unbalances the bridge by a measurable amount.
Sources and last check
Information, not professional advice.