Ohm's Law & Power Triangle Calculator
Enter any two of voltage (V), current (I), resistance (R), or power (P) and get all four values instantly with the formula shown. Type "3.3k" for 3300 Ω, "50m" for 50 mA. Nothing uploaded.
Enter any two values. Use prefixes: k, M, m, u (µ).
Resistors in series add together: Rtotal = R1 + R2 + … + Rn. Use prefixes like 2.2k, 47, 100u.
Resistors in parallel: 1/Rtotal = 1/R1 + 1/R2 + … + 1/Rn. The result is always less than the smallest resistor.
Click a scenario to pre-fill the Ohm's Law calculator with typical values.
LED Resistor
5 V supply, 20 mA forward current. Fills V and I so you can find the series resistor value and power.
Voltage Divider
1 mA through 10 kΩ total resistance. Fills I and R to find the voltage across the resistor.
Motor Stall Current
12 V supply, 6 Ω winding resistance. Fills V and R to find stall current and power dissipation.
Learn more: Ohm's law and electrical resistance
The relationship between voltage, current, and resistance
Ohm's law defines the relationship between three fundamental electrical properties: voltage (V, measured in volts - the electrical pressure), current (I, measured in amperes or amps - the flow of electrons), and resistance (R, measured in ohms - the opposition to flow). The law states that current equals voltage divided by resistance (I=V/R), or equivalently, voltage equals current times resistance (V=I×R). If you increase voltage while holding resistance constant, current increases proportionally. If you increase resistance while holding voltage constant, current decreases. This calculator rearranges Ohm's law to solve for any unknown when you know the other two values, making it essential for circuit design, troubleshooting, and understanding how electrical components interact.
Practical applications and power calculations
Ohm's law is the foundation for circuit analysis. Knowing voltage and current lets you calculate power (watts) using P=V×I. For example, a 12-volt circuit drawing 5 amps uses 60 watts. Resistors limit current to protect sensitive components - an LED needs a current-limiting resistor or it will burn out. If you want 20 mA through an LED with a 5-volt supply, Ohm's law tells you need a resistance of 250 ohms (5V ÷ 0.02A). The calculator handles these common scenarios, including series and parallel resistance combinations shown in the quick-reference presets.
FAQ
What does resistance do in a circuit?
Resistance opposes the flow of current. A higher resistance restricts flow (lower current), while lower resistance allows more current. Resistors are used intentionally to limit current for safety, to create voltage drops across components, and to control brightness in LEDs or other devices.
Why do I need to know Ohm's law?
Whether you're designing a circuit, choosing a resistor, troubleshooting a power supply, or understanding why a device draws a certain amount of power, Ohm's law explains the relationship. It's the single most important equation in electrical engineering.
What's the difference between series and parallel resistance?
In series, resistors add up (R_total = R1 + R2). In parallel, resistance decreases (1/R_total = 1/R1 + 1/R2). Series divides voltage; parallel divides current. The quick presets show examples of both for common values.