Shunt Resistor Calculator

V = I x R

P = I² x R

P = V² / R

Enter Any Two Values — The Other Two Are Calculated

I Current to Measure Calculated

V Voltage Drop Calculated

R Shunt Resistance Calculated

P Power Dissipation Calculated

Enter any two values to begin

All Values

Current

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Voltage Drop

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Resistance

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Power

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Formulas used:

Shunt Resistor Circuit

A shunt resistor is a low-value resistor placed in series with the load. The small voltage drop across it is proportional to the current flowing through the circuit, allowing you to measure current indirectly.

R shunt = V drop / I — Measure the voltage across the shunt to determine current flow.

Shunt Resistor Calculator — How It Works

A shunt resistor is a low-resistance component placed in series with a load. When current flows through it, a small voltage drop develops across the shunt — proportional to the amount of current. Measuring that voltage drop lets you determine the current flowing without breaking the circuit. The calculator above uses Ohm’s law to compute shunt resistance, voltage drop, current, or power dissipation from any two known values.

Calculator Inputs and Formulas

Enter any two of the four values and the calculator computes the remaining two instantly. Here are the core relationships it uses:

V = I x R — voltage drop across the shunt
R = V / I — shunt resistance from voltage and current
P = V x I — power dissipation (heat) in the shunt
P = I² x R — shunt power from current and resistance

Shunt resistance values are typically in milliohms or microohms to keep the voltage drop small — usually in the mV range — and minimise power loss in the circuit. The calculator supports milliamps, microamps, millivolts, milliohms, microohms, and milliwatts so you can work in the units your data sheet specifies.

Current Shunt Resistance — Sizing the Shunt

Choosing the Right Resistance Value

The value of a shunt resistor depends on two things: the full-scale current you need to measure and the resulting voltage you want across the shunt. A standard shunt rated for 100 A might produce a 50 mV or 75 mV drop at full scale. The shunt resistance must be low enough that the voltage drop does not starve the load, yet high enough to give your meter or ADC a readable signal.

R_shunt = V_fullscale / I_fullscale
Example: 75 mV at 100 A = 0.075 / 100 = 0.00075 ohm (0.75 milliohm)

Power Rating of the Shunt

Every shunt resistor must dissipate the heat generated by the current passing through it. The power rating sets the maximum continuous current it can handle safely. Use P = I² x R to find the power dissipation at your required current. For the 0.75 milliohm shunt above at 100 A: P = 100² x 0.00075 = 7.5 W. Choose a shunt with a power rating well above this to account for ambient temperature, long-term stability, and transient surges. For detailed power analysis, use the Power Dissipation Calculator.

Ammeter Shunts — Extending Meter Range

How Ammeter Range Extension Works

An ammeter measures current by passing it through an internal coil with a known meter resistance. Both moving-coil and digital meters have a limit on internal full-scale current — often just a few milliamps. To measure larger DC current, an external shunt resistor is connected in parallel with the meter so that most of the current bypasses through the shunt while a small, proportional amount passes through the meter.

R_shunt = R_meter / (N – 1)
where N = I_total / I_meter (the multiplying factor)

Example: a meter has 1 kΩ internal resistance and a full-scale current of 1 mA. To measure up to 1 A (N = 1000), the shunt resistance must be R = 1000 / (1000 – 1) = approx 1.001 Ω. At full scale, 999 mA flows through the shunt and 1 mA through the meter.

Shunt Resistors Explained

Common Applications

4-20 mA Current Loops

Industrial 4-20 mA loops use a current shunt to convert a loop signal into a voltage that a PLC or controller can read. A 250 Ω shunt turns the 4-20 mA range into a 1-5 V signal. The voltage drop across the shunt scales linearly with current, giving accurate measurement without signal degradation over long cable runs.

Battery and Automotive Monitoring

High-current shunts (hundreds of amps) monitor charge and discharge in battery packs. The shunt sits on the return path so the full line current passes through it. A sense amplifier reads the mV-level voltage drop and feeds it to a microcontroller. Shunt resistance in these applications is often under 100 microohm to keep power loss below a watt.

Field and Bench Testing

During bench testing, a known DC current is forced through the shunt and the voltage drop is measured with a precision voltmeter. Comparing actual test data against the data sheet confirms calibration. For field and bench testing, portable current sources and calibrated digital meters are standard. The test setup should follow the current edition of IEC 60051 or ANSI C39.1 for accurate current sensing.

How to Use the Calculator

Enter any two of the four values — current, voltage drop, shunt resistance, or power dissipation — and the calculator returns the other two instantly with a step-by-step breakdown. Select the matching unit from the dropdown next to each field (the calculator supports mA, uA, mV, milliohm, microohm, and mW). The results update in real time as you type.

Important: Always verify that the shunt’s power rating exceeds the calculated power dissipation. A shunt that cannot dissipate the heat will drift in resistance value, affecting long-term stability and measurement accuracy.

Frequently Asked Questions

What is a shunt resistor?

A shunt resistor is a very low-value resistor placed in series with a circuit to measure current. The small voltage drop that develops across it is proportional to the current flowing, allowing you to determine the current by measuring voltage — without breaking the circuit.

How do I choose the right shunt resistance?

Divide your desired full-scale voltage drop by the maximum current you need to measure: R = V / I. For example, if you want 75 mV at 50 A, the shunt resistance should be 0.075 / 50 = 0.0015 ohm (1.5 milliohm). The resistance must be low enough to avoid starving the load but high enough to give a readable signal.

Why is shunt resistance so low?

A low resistance minimises power loss and voltage drop in the circuit. If the shunt resistance were too high, it would waste energy as heat and reduce the voltage available to the load. Milliohm and microohm values keep the impact on the circuit negligible while still producing a measurable voltage.

How do I calculate the power dissipation of a shunt?

Use P = I squared x R, where I is the current in amps and R is the shunt resistance in ohms. For example, 50 A through a 1.5 milliohm shunt dissipates 50 x 50 x 0.0015 = 3.75 W. Always choose a shunt rated above this value to ensure safe operation.

Can I use a shunt resistor to extend my ammeter’s range?

Yes. Connect the shunt in parallel with the meter. Use R_shunt = R_meter / (N – 1), where N is the ratio of the total current to the meter’s full-scale current. Most of the current flows through the low-resistance shunt while a proportional fraction passes through the meter.

What is a 4-20 mA current loop shunt?

In industrial automation, a 250 ohm shunt converts a 4-20 mA loop signal into a 1-5 V voltage that a PLC or controller can read. The linear relationship between current and voltage makes this a simple and reliable way to transmit sensor data over long distances.

Does temperature affect shunt accuracy?

Yes. All resistors have a temperature coefficient that causes their resistance to change with temperature. Precision shunts use alloys like manganin or constantan with very low temperature coefficients to minimise drift. Always check the data sheet for the temperature coefficient and derate accordingly.

What units does the calculator support?

The calculator supports amps, milliamps, and microamps for current; volts and millivolts for voltage drop; ohms, milliohms, and microohms for resistance; and watts and milliwatts for power dissipation. Select the matching unit from the dropdown next to each field.

Last updated: March 2026