Voltage drop across a resistance

Work out the voltage drop from a resistance and a current. By Ohm's law, current through a resistance produces a drop across it.

Resistance

Current

SI prefixes accepted (4k7 / 1M / 10m / 220). Upper-case M = mega, lower-case m = milli

Voltage drop

Derived values

Resistor dissipation: 1 mW; I² × R
Recommended power rating: 2 mW; 50% derating

A 1/16W (1005 / 0402) chip resistor is enough.

History

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Formula

Voltage drop: V = R[Ω] × I[A]
With current in mA: V = R × I[mA] × 0.001

Design notes

Ohm's law, V = IR, is the foundation of circuit design. Current limiting resistors, wiring resistance, connector contact resistance — knowing the drop matters in all of them.

Where it comes up:
• Checking the drop along a long supply cable (say 2 A down a 1 m run)
• Checking the loss along a PCB trace carrying a heavy current
• Loss across the contact resistance of a connector

Rules of thumb for what is acceptable: ±5% on digital rails (0.25 V on a 5 V rail), and whatever your accuracy demands on analogue ones.

When you need this

The most basic check there is: the drop across a series resistance. Useful for damping resistors, current-sense shunts, series elements in a supply rail, and pull-ups — anywhere you need to know what inserting a resistor costs you in volts.

Where it comes up

Current-sense shunts. A larger sense voltage improves accuracy but takes voltage away from the load and dissipates more. 100mΩ at 1A is 100mV of drop and 100mW of loss.
Damping resistors. A 33Ω series resistor into a CMOS input passes almost no DC, so there is no static drop. It acts only on transient current.
Pull-up resistors. An I²C bus with 4.7kΩ sinks 3.3V / 4.7kΩ ≈ 0.7mA in the low state. Check that this satisfies the device's Vol specification.
Series filtering on a rail. Ferrite beads or resistors fitted for noise cost you their DC resistance. Bead DC resistance ranges from tens of milliohms to several ohms.

Related calculators

For the drop along a PCB trace, Trace resistance and parasitics is a better fit — it derives resistance from width, length and copper weight and reports heating at the same time.

To create a voltage by division, use Resistor divider, which also reports worst-case values including tolerance.

Frequently asked questions

What about the power dissipated in the resistor?

P = V × I — multiply the drop reported here by the current. 100mV at 1A is 100mW. Allow at least a factor of two over that when choosing the resistor rating.

I want the drop along a trace.

Use Trace resistance and parasitics. It derives resistance from width, length and copper weight and reports the drop and heating together. Remember to count both the outbound trace and the ground return.

Standards and references

IEC 60115-1 — Generic specification for fixed resistors.

Last updated: 2026-08-29