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 the power relationships
Solving from any two values
Ohm's law links voltage (V), current (I) and resistance (R) as V = I x R, and power is P = V x I. Give the tool any two of the four and it works out the other two. A circuit at 12 V drawing 2 A has R = 12 / 2 = 6 ohms and P = 12 x 2 = 24 W.
You rarely have all four values measured. A resistor's printed value gives R and a multimeter across it gives V. From those two the tool derives I = V / R and P = V squared / R, so no second measurement is needed.
Power ratings
SparkFun's resistor power rating page says standard through-hole resistors usually have 1/4 W or 1/2 W ratings. It gives P = I squared x R and P = V squared / R, and warns that too much power in a resistor means heat, smoke and fire.
For a 100 ohm resistor at 5 V, the current is 50 mA and the power is 0.25 W, exactly the limit of a 1/4 W part. At 12 V the same resistor carries 120 mA and dissipates 12 squared / 100 = 1.44 W, almost six times the rating, though neither the voltage nor the current looks alarming on its own.
The tool adds a 50% margin and rounds up to a standard size. At 0.25 W it suggests 0.5 W or more, and at 1.44 W it suggests 5 W. The badge says Safe only when a 1/4 W part covers the margin, which is up to about 0.17 W. Between that and 5 W it shows Warm with the rating you need, and from 5 W up it says Hot and suggests a heatsink or a higher-wattage part.
Resistors in series and parallel
The tool's series and parallel helper adds series resistors directly and combines parallel ones as the reciprocal of the sum of reciprocals. Two 100 ohm resistors give 200 ohms in series and 50 ohms in parallel. Ohm's law then applies to that single equivalent value.
FAQ
Which two values do I need?
Any pair: voltage and current, voltage and resistance, voltage and power, current and resistance, current and power, or resistance and power. The tool solves for the other two.
Why does the tool show mA or kΩ instead of raw numbers?
Electronics values cover a huge range, from microamps to amps and from ohms to megaohms. The tool picks a prefix, up to four significant figures, so 0.0025 A reads as 2.5 mA. You can also type values with prefixes such as 4.7k or 10m.
Does it work for circuits with many resistors?
Reduce the network to one equivalent resistance with the series and parallel rules first, then enter that value. Ohm's law applies to each resistive element and to the equivalent for the whole network.