Ohm's Law and Power Calculator with Series/Parallel and LED Resistor
Enter any two of voltage (V), current (I), resistance (R) and power (P) and the other two are calculated. The same page finds the combined resistance of resistors in series or parallel, and the current-limiting resistor for an LED, with the next standard E24 value. Values accept SI prefixes such as 4.7k (4,700) or 20m (0.02).
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Ohm's law and power (enter two, get the rest)
Fill in exactly two of the four fields and leave the others empty. Use base units (V, A, ohm, W); prefixes like k and m work too.
Series and parallel resistance
Separate two or more values with commas, spaces or new lines (example: 100, 4.7k, 220).
LED current-limiting resistor
The resistor takes up the difference between the supply voltage and the LED forward voltage. Write milliamps with m (20 mA is 20m).
How to use
- Under "Ohm's law and power", fill in only two of "Voltage V (volts)", "Current I (amperes)", "Resistance R (ohms)" and "Power P (watts)".
- The other two values appear automatically. Press "Clear" to start over.
- To combine several resistors, enter their values separated by commas in "Resistor values (ohms)" under "Series and parallel resistance".
- For an LED resistor, enter "Supply voltage Vs (V)", "LED forward voltage Vf (V)" and "LED current If (A, 20 mA is 20m)".
Notes
- Enter values in base units (V, A, ohm, W) with a prefix where needed: 20m for 20 mA, 4.7k for 4.7 kilohms. A lowercase m means milli and an uppercase M means mega.
- This covers DC circuits and purely resistive loads on AC (RMS values). Impedance and power factor with coils or capacitors are not handled.
- If any resistor in a parallel group is 0 ohms, the parallel total is 0 ohms (a short circuit).
- LED forward voltage depends on color and part: roughly 2 V for red and around 3 V for white or blue. Use the datasheet value.
- Rounding up to an E24 value lowers the current slightly. Real resistors have tolerance (for example 5%), so leave some margin in current and power.
- Pick a resistor rated for about twice the dissipated power or more. Results are a design guide, not a guarantee of safety.
Glossary
Ohm's law
The relation V = I x R between voltage, current and resistance. With power P = V x I, any two quantities give you the other two.
Equivalent resistance
The single resistance that can replace a group of resistors. In series the values add; in parallel the reciprocals add (1/R = 1/R1 + 1/R2 + ...).
Forward voltage (Vf)
The voltage across an LED while it is lit. It depends on the color and material, and the current-limiting resistor takes up the rest of the supply voltage.
E24 series
A set of standard resistor values with 24 steps per decade (1.0, 1.1, 1.2, 1.3, 1.5 and so on), widely stocked as 5% parts.
FAQ
Which two values do I need for Ohm's law?
Any two of voltage, current, resistance and power are enough to find the other two. The formulas are V = I x R and P = V x I; for example, from resistance and power the voltage is V = sqrt(P x R).
How do I enter mA or kilohms?
Add a prefix after the number: 20m for 20 mA, 4.7k for 4.7 kilohms, 1M for 1 megohm. Lowercase m is milli and uppercase M is mega. Unit symbols such as 5V or 100Ω are accepted too.
Why is parallel resistance smaller than the smallest resistor?
Each added branch gives current another path, so the whole group conducts more easily. The total comes from 1/R = 1/R1 + 1/R2 + ... and is always below every individual value. Two equal resistors in parallel give half.
Why is the LED resistor not exactly the calculated value?
Stocked resistors come in standard steps such as E24. The tool shows the nearest E24 value at or above the calculated one and the real current with it. Rounding up lowers the current slightly, which errs on the side of protecting the LED.
Does it work for AC or circuits with coils and capacitors?
It is meant for DC circuits and purely resistive AC loads using RMS values. Coils and capacitors bring in impedance and power factor, so these formulas do not apply to them.
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