Quick start
- Choose the pair of values you know: V and I, V and R, I and R, V and P, I and P, or R and P.
- Enter amps as amps and resistance as ohms. Convert 500 mA to 0.5 A or 2 kOhm to 2,000 ohms before you calculate.
- Calculate to get the remaining voltage, current, resistance, and power values for the same simple circuit.
Best uses
Use this guide when you are checking simple positive resistor math, electronics homework, LED current checks, breadboard examples, battery-load estimates, or quick power-rating sanity checks.
- Find resistance from voltage and current.
- Find current from voltage and resistance.
- Find voltage from current and resistance.
- Find current and resistance from voltage and power.
What this calculator is solving
The Ohm's Law Calculator solves basic resistor and power relationships. Enter two known voltage, current, resistance, or power values and it fills in the rest.
Match each input label on the calculator to the exact pair you know: voltage and current, voltage and resistance, current and resistance, voltage and power, current and power, or resistance and power.
The formula in plain language
In plain language: The calculator uses V = I x R and P = V x I. Depending on the pair you choose, it can rearrange those relationships as I = V / R, R = V / I, P = V x I, and V = sqrt(P x R). The examples on the page are there so you can compare your inputs with a worked example before copying the answer.
The core relationship is V = I x R. Power adds P = V x I, plus the rearranged checks P = I^2 x R and P = V^2 / R when power is one of the known values.
How to read the answer
Read voltage, current, resistance, and power as one matched set for a simple DC or purely resistive circuit. The power line is especially useful for checking whether a resistor, LED setup, battery load, or test circuit might need a higher wattage rating.
- Voltage is electrical potential difference, shown in volts.
- Current is flow, shown in amps.
- Resistance is opposition to current, shown in ohms.
- Power is energy rate, shown in watts, and it matters for heat and component ratings.
Common mistakes to avoid
Most Ohm's law mistakes come from mixing milliamps with amps, kilohms with ohms, using a power rating as if it were a measured value, or applying simple resistor math to motors, speakers, capacitors, inductors, or other reactive AC loads.
- Do not use simple DC resistor math for every AC or reactive circuit.
- Do not ignore component power ratings and heat.
- Do not mix mA, A, kOhm, and ohm values without converting first.
- Never test live circuits without proper training and equipment.
Quick 12 V example
Say you know a simple load has 12 V across it and draws 2 A. Choose V and I, enter 12 and 2, and the calculator returns 6 ohms because R = V / I.
The same result also gives 24 W because P = V x I. That wattage is not just trivia: it tells you the load is turning energy into heat, light, motion, or another output at a 24-watt rate.
Solving from power values
If you know 12 V and 24 W, choose V and P. The calculator divides 24 W by 12 V to get 2 A, then divides 12 V by 2 A to get 6 ohms.
If you know 6 ohms and 24 W, choose R and P. The calculator uses I = sqrt(P / R), so sqrt(24 / 6) is 2 A, and then V = I x R gives 12 V.
- Use V and P when a supply voltage and wattage rating are known.
- Use I and P when current and wattage are known but voltage is not.
- Use R and P when resistance and wattage are known and you need the matching voltage and current.
When this guide is not enough
Ohm's law is a clean model for simple DC or purely resistive examples. Real electrical work can involve AC impedance, motors, speakers, capacitors, inductors, startup current, temperature, tolerance, duty cycle, code rules, and shock or fire hazards.
Use the guide to understand the math. Use manufacturer data, a multimeter when safe and appropriate, and qualified electrical help for live circuits, mains wiring, batteries that can deliver dangerous current, or any final design decision.
Research and references
These references help check the measurements, units, limits, or safety notes used in this guide.
Worked examples for Ohm's Law Calculator
6 ohms and 24 W
12 V and 24 W
3 A and 27 W
2 A and 6 ohms
12 V and 6 ohms
12 V and 2 A
FAQ in plain language
When should I use the Ohm's Law Calculator?
Use it when your task matches one of these common needs: Find resistance from voltage and current. Find current from voltage and resistance. It works best when you already know two positive values from the same simple DC or resistive circuit.
What is the Ohm's Law Calculator doing with my inputs?
In plain language: The calculator uses V = I x R and P = V x I. Depending on the pair you choose, it can rearrange those relationships as I = V / R, R = V / I, P = V x I, and V = sqrt(P x R). The examples on the page are there so you can compare your inputs with a worked example before copying the answer.
What do the main Ohm's Law Calculator inputs mean?
Voltage V: electrical potential difference, measured in volts. Current A: electrical flow through the circuit, measured in amps. Resistance ohms: how much the component or circuit resists current flow. Power W: energy rate in watts, equal to voltage times current for this simple calculation.
How should I read the Ohm's Law Calculator answer?
Read the headline answer, then check the supporting lines and examples to understand how the calculator got there. If one input changes, rerun the tool and compare the new answer instead of guessing.
What should I double-check before trusting the answer?
This is simple DC or resistive-circuit math. It is not a safety approval for live wiring, AC impedance, component heat, fuse sizing, battery limits, or code compliance. Also check that all values come from the same circuit, the mode matches the pair you know, and any real component can handle the calculated current, voltage, and power.
Why do I only enter two values?
Ohm's law connects voltage, current, and resistance, and power connects voltage and current. If you know any supported positive pair, the calculator can solve the remaining values.
Can I solve from power and one other value?
Yes. The visible modes include voltage and power, current and power, and resistance and power. For example, 12 V and 24 W gives 2 A and 6 ohms.
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