How Ohm's law works
Ohm's law connects voltage, current, and resistance in simple resistive circuits.
Ohm's Law Calculator
Solve voltage, current, resistance, and power using Ohm's Law.
Circuit values
Choose a value to solve for, then enter the two known values needed for that calculation.
Solved value
0.12 A
Solving for current using I = V ÷ R.
Formula used
I = V ÷ R
Known values used
Voltage and resistance
Voltage
12 V
Current
0.12 A
Resistance
100 Ω
Power
1.44 W
Unit conversions
All calculations use volts, amps, ohms, and watts internally.
Circuit note
No component note added.
Copy result
Copy the solved value, all derived values, formula, and component note.
Formula note
Ohm's law calculations use ideal mathematical relationships.
Unit conversion note
Inputs are converted to volts, amps, ohms, and watts before calculating.
Power rating note
Power dissipation can create heat, so check component ratings and margins.
Safety disclaimer
High-voltage, mains, automotive, and battery systems can be dangerous.
Local calculation
Inputs are processed in your browser. No external API is required.
Dynamic electronics insights
Circuit check
Power dissipation may create heat. Choose components with an appropriate power rating and safety margin.
Power planning
For a resistor, choose a power rating above 1.44 W with suitable margin.
Component guidance
Follow datasheets, component ratings, and qualified guidance for high-voltage or mains work.
Electrical values breakdown
Voltage in volts
12 V
Current in amps
0.12 A
Resistance in ohms
100 Ω
Power in watts
1.44 W
Formula used
I = V ÷ R
Known inputs
Voltage and resistance
Solved output
0.12 A
Component note
Not specified
Ohm's law formula guide
V = I × R
Find voltage
Use current in amps and resistance in ohms
I = V ÷ R
Find current
Prevent divide-by-zero by using a resistance above zero
R = V ÷ I
Find resistance
Current must be above zero
P = V × I
Find power
Power in watts from volts and amps
P = I² × R
Power from current and resistance
Useful for resistor heating checks
P = V² ÷ R
Power from voltage and resistance
Resistance must be above zero
Common electronics ranges
Small LED current
5 to 20 mA
Depends on LED type and brightness target
USB voltage
5 V
Common USB supply voltage
Microcontroller voltage
3.3 V or 5 V
Check board and component datasheets
Common battery voltages
1.5 V, 3.7 V, 9 V, 12 V
Voltage depends on chemistry and charge state
Resistor power ratings
0.125 W, 0.25 W, 0.5 W, 1 W
Choose a rating above calculated dissipation
Unit conversion guide
1000 mV
1 V
Millivolts to volts
1000 mA
1 A
Milliamps to amps
1000 Ω
1 kΩ
Ohms to kiloohms
1000 kΩ
1 MΩ
Kiloohms to megaohms
1000 mW
1 W
Milliwatts to watts
Resistor planning guide
Calculated resistance
Use the formula result as the target value
Use the formula result as the target value
Nearest standard value
Choose a nearby standard resistor value when exact value is unavailable
Choose a nearby standard resistor value when exact value is unavailable
Tolerance
Real resistors can vary by their tolerance rating
Real resistors can vary by their tolerance rating
Power rating
Select a resistor rated above the calculated wattage
Select a resistor rated above the calculated wattage
Heat margin
Extra margin helps reduce heat stress and improve reliability
Extra margin helps reduce heat stress and improve reliability
Ohm's law guide
These notes explain circuit calculation concepts without repeating the calculator result.
Voltage, current, and resistance
Voltage pushes charge, current is the flow of charge, and resistance limits that flow.
How power relates to Ohm's law
Power describes how quickly electrical energy is used or dissipated, often as heat in resistors.
Choosing resistor values
A calculated resistance is a target. Real projects often use the nearest standard resistor value.
Why power rating matters
A resistor or component can overheat if it dissipates more power than it is rated to handle.
Common calculation mistakes
Common issues include mixing milliamps with amps, kiloohms with ohms, and ignoring power dissipation.
When Ohm's law is limited
Reactive AC circuits, semiconductors, motors, LEDs, and changing loads may need more than simple resistance math.
Electrical safety reminder
Use proper equipment, component ratings, datasheets, and qualified guidance for high-voltage or mains work.
Formula
V = I × R
I = V ÷ R
R = V ÷ I
P = V × I
P = I² × R
P = V² ÷ R
Variables
- V = voltage in volts.
- I = current in amps.
- R = resistance in ohms.
- P = power in watts.
Worked example
For a 12 V circuit with 100 Ω resistance, I = 12 ÷ 100 = 0.12 A. Power is P = 12 × 0.12 = 1.44 W.
Assumptions
The calculator assumes ideal values and simple resistive circuit behavior.
Limitations
Real components have tolerance, temperature effects, maximum voltage, maximum current, and power limits.
Frequently asked questions
What is Ohm's law?
Ohm's law describes the relationship between voltage, current, and resistance in a simple resistive circuit.
How do I calculate current from voltage and resistance?
Use I = V ÷ R. For example, 12 volts across 100 ohms gives 0.12 amps.
How do I calculate resistance for an LED?
Subtract the LED forward voltage from the supply voltage, then divide by the desired LED current. Also check resistor power rating.
Why does power rating matter?
Power dissipation creates heat. Components should be rated above the calculated wattage with a suitable safety margin.
Can Ohm's law calculate watts?
Yes. Power can be calculated with P = V × I, P = I² × R, or P = V² ÷ R when the needed values are known.
What happens if resistance is too low?
Low resistance can allow high current, which may overload components, wires, batteries, or power supplies.
How accurate is an Ohm's law calculator?
The math is exact for ideal values, but real components have tolerance, heat effects, voltage limits, and current limits.
Does Ohm's law work for AC circuits?
It can apply to simple resistive AC loads, but reactive components need impedance and AC analysis.
Can I use this calculator for household wiring?
Use it only for learning and basic estimates. Household wiring and mains work should follow local codes and qualified guidance.
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