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Full changelogEngine 1.18.3

Ohm's Law & Power

Fundamentals

Enter any two of V, I, R and P to get the other two, and the resistor rating to fit.

Inputs

Accepts 3V3, 1k, 4k7, 10n, 1e-3
V
A
Ω
W
s

Results

Current I

Not judged: this is exact arithmetic on your values, so there is nothing to pass or fail.
22.73mA
Show working
  1. CurrentI = V / R = 5 V / 220 Ω = 22.73 mA
  2. DissipationP = V² / R = 5 V² / 220 Ω = 113.6 mW
Dissipation P
113.6mW
All results (2)
Voltage V
5V
Resistance R
220Ω

Use a resistor rated at least 2× its dissipation, and derate further above 70 °C ambient. Minimum here: 0.25 W.

Resistor selection
ItemValue
Dissipation113.6 mW
Minimum rating (2× derating)0.25 W → 1206
Voltage across5 V
Current through22.73 mA

Accuracy

Verified against

2 independent cases. See the working.

This is a design aid. The engineer remains responsible for the design and for checking the standard itself.

Parameter sweep

Vary one input over a range and see the answer and verdict at each step, as a table and a curve.Pro

Worst-case corners

Put a tolerance on each input and get the worst-case band around the answer.Pro

Circuit

The principle

Ohm's law is an empirical relation: in a linear conductor at constant temperature, current is proportional to voltage. Bias networks, dividers, shunts and terminations all follow from these equations.

Ohm’s law
V=I⋅RI=VRR=VIV = I \cdot R \qquad I = \frac{V}{R} \qquad R = \frac{V}{I}
Electrical power (all equivalent)
P=V⋅I=I2R=V2RP = V \cdot I = I^{2} R = \frac{V^{2}}{R}
Use I²R when the current is known (shunts), V²/R when the voltage is known (pull-ups, dividers).
Energy dissipated as heat
E=P⋅t[J]1 Wh = 3600 JE = P \cdot t \qquad \t{[J]} \quad \t{1 Wh = 3600 J}
  • VVvoltage across the element, volts
  • IIcurrent through the element, amperes
  • RRresistance, ohms
  • PPpower dissipated as heat, watts
  • tttime the current flows, seconds
More detail

Beyond the arithmetic

Chip resistor power ratings assume 70 °C ambient and fall linearly to zero at 155 °C for most thick film. Each size also has a working-voltage limit, typically 75 V for 0603 and 150 V for 0805, whatever P = V²/R allows.

Practical rule. Keep dissipation within ½ the rating at worst-case ambient, and voltage within ⅔ of the limit. For pulses, check the single-pulse curve, not the DC rating.
Reference: IEC 60115 / EIA-575 (fixed resistors), IPC-9592 for derating practice.

Engine version ⁨1.18.3⁩