Basics calculator
Ohm's law & power
Voltage pushes, resistance holds back, current is what gets through.
Free. It runs in your browser: nothing to install.
The example, worked out
Given
- I know
- V & R
- Voltage V
- 9 V
- Resistance R
- 470 Ω
Working
- the current
- the heat
Results
Current19.1 mA
Power172 mW
Voltage9 V
Resistance470 Ω
The formulas
- V = I × Rvolts are amps times ohms
- P = V × I = I² R = V² / Rthe heat, three ways round
The usual traps
- A resistor rated at the power it dissipates runs hot: buy twice it.
- Mixing mA and A: 20 mA is 0.02 A.
Questions
How do I use Ohm's law to find a current or a resistor's power?
V = I × R: volts are amps times ohms. P = V × I = I² R = V² / R: the heat, three ways round.
What does it give for typical values?
9 V, 19.1 mA, 470 Ω, 172 mW. Current: 19.1 mA; Power: 172 mW.
What are the usual mistakes?
A resistor rated at the power it dissipates runs hot: buy twice it. Mixing mA and A: 20 mA is 0.02 A.
In the app
Your circuit, drawn live
Change a value: the schematic redraws with your numbers on the parts.
The part you can buy
Results rounded to standard values (E12, E24, E96) and power ratings.
The working, shown
Every step written out. Try it first by hand, then check.
Into your notebook
One click puts the circuit on a sheet of your project.
Open the calculatorOpens in the app on 1 December, with Circuit Notebook 1.0
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