Ohm's Law
Given any two of voltage U, current I, resistance R, solve the third and output power P=UI (DC).
Inputs
Formula
U = I·R; I = U/R; R = U/I; P = U·I
How to use
- Fill in Voltage U, Current I, Resistance R in the Inputs section (watch the unit on each field).
- Click Calculate; the tool evaluates the formula shown above.
- Read Voltage U, Current I, Resistance R, Power P in the results area.
Formula notesOhm's law U = I·R; hence I = U/R, R = U/I. Power P = U·I = I²·R = U²/R.
Formula · Worked Example · Knowledge
Formula
At constant temperature, voltage and current of a linear resistor are proportional — Ohm's law:
$$U=I\cdot R\quad\Rightarrow\quad I=\frac{U}{R},\quad R=\frac{U}{I}$$
Power can be written as:
$$P=U\cdot I=I^{2}R=\frac{U^{2}}{R}$$
Conductance $G=1/R$ in siemens (S).
$$U=I\cdot R\quad\Rightarrow\quad I=\frac{U}{R},\quad R=\frac{U}{I}$$
Power can be written as:
$$P=U\cdot I=I^{2}R=\frac{U^{2}}{R}$$
Conductance $G=1/R$ in siemens (S).
Worked Example
Given $I=2.5\ \text{A}$, $R=4\ \Omega$.
Step 1 — voltage: $U=I\times R=2.5\times4=10\ \text{V}$.
Step 2 — power: $P=U\times I=10\times2.5=25\ \text{W}$ (or $P=I^{2}R=2.5^{2}\times4=25\ \text{W}$).
Step 1 — voltage: $U=I\times R=2.5\times4=10\ \text{V}$.
Step 2 — power: $P=U\times I=10\times2.5=25\ \text{W}$ (or $P=I^{2}R=2.5^{2}\times4=25\ \text{W}$).
Key Points
- The $I$–$V$ curve is a line through the origin with slope $G$; heating raises resistivity (non-linear).
- Keep units consistent: V, A, Ω; common prefixes mA, μA, kΩ, MΩ.
- Normalize magnitudes first (mA→A, kΩ→Ω) before calculating.