Ohm's law is the most basic relationship in electronic circuits. If you know any two of voltage (V), current (I), resistance (R), and power (P), you can calculate the other two. Fill in just the two boxes you know among the four in the calculator above and the rest appear right away. Values with prefixes such as 4.7k, 100m, and 2.2u can be typed as they are.
The basic formulas
V = I × R (voltage = current × resistance) P = V × I (power = voltage × current) P = I² × R = V² / R
Every combination for finding the rest from two values comes from these three equations.
| Known values | Formulas to find the rest |
|---|---|
| V, I | R = V / I, P = V × I |
| V, R | I = V / R, P = V² / R |
| V, P | I = P / V, R = V² / P |
| I, R | V = I × R, P = I² × R |
| I, P | V = P / I, R = P / I² |
| R, P | V = √(P × R), I = √(P / R) |
Examples of use
- What if you connect a 1 kΩ resistor to a 5 V supply? Enter V=5 and R=1k and you get a current of 5 mA and a power of 25 mW. A 1/4 W (250 mW) resistor has plenty of room.
- What is the resistance of a load that draws 0.5 A at 12 V? Enter V=12 and I=0.5 and you get R=24 Ω and P=6 W. It has to be a large resistor or load that can take 6 W.
- What voltage can you apply to a 100 Ω resistor that can only handle 0.25 W? Enter R=100 and P=0.25 and you get V=5 V and I=50 mA.
Prefix input rules
| How to write | Meaning |
|---|---|
m | milli (10⁻³). 20m = 0.02 |
u | micro (10⁻⁶). 2.2u = 0.0000022 |
n / p | nano / pico |
k | kilo (10³). 4.7k = 4700 |
M | mega (10⁶). 1M = 1,000,000 |
Note that m (milli) and M (mega) are case-sensitive. A notation like 4k7, which puts the prefix in the decimal point position, can also be read.
To go on, read resistance values with the resistor color code calculator, and check LED circuits with the LED resistor calculator.