Ohm's Law Calculator
Any two of V, I, R and P give the other two — with the working shown.
Enter any two values.
How it was calculated
The formulas appear here once two values are entered.
Resistor power rating
The twelve Ohm’s law formulas
Voltage V (V, volts)
- V = I × R
- V = P ÷ I
- V = √(P × R)
Current I (A, amperes)
- I = V ÷ R
- I = P ÷ V
- I = √(P ÷ R)
Resistance R (Ω, ohms)
- R = V ÷ I
- R = V² ÷ P
- R = P ÷ I²
Power P (W, watts)
- P = V × I
- P = V² ÷ R
- P = I² × R
The two formulas used for your values are highlighted.
About the Ohm's Law Calculator
Ohm's law says that the current through a resistor is proportional to the voltage across it: V = I × R. Combined with the power equation P = V × I, any two of voltage, current, resistance and power fix the other two.
Type any two values — with units from µ to M, or prefixes typed straight in (4.7k, 4k7, 20m) — and the calculator fills in the other two, shows the formula and the numbers it used, and suggests the resistor power rating to buy with a 2× safety margin. It also warns you when a value is so large or small that it is probably a prefix slip, such as kΩ typed as MΩ.
How to use it
- Type any two values you know — for example 5 in Voltage and 220 in Resistance. Choose the unit (µ, m, k or M) from the list next to each box, or type the prefix with the number:
4.7k,4k7,22m,500u. - Read the other two values in the boxes marked Calculated and in the result panel. How it was calculated shows each formula with your numbers in it.
- To change which values you know, type in a calculated box: the older of your two values becomes the calculated one. Clear all empties every box.
- Check Resistor power rating: the power the resistor turns into heat, doubled for margin, and the smallest common rating that covers it. Use Copy result to paste the working into notes or a report.
Examples
V = 5 V, R = 220 Ω
I = 22.73 mA · P = 113.6 mW · 2× margin = 227.3 mW → choose 1/4 W
V = 230 V, P = 1 kW
I = P ÷ V = 4.348 A · R = V² ÷ P = 52.9 Ω
R = 100 Ω, P = 0.25 W
V = √(P × R) = 5 V · I = √(P ÷ R) = 50 mA
That is the absolute maximum. With the 2× margin, design for 0.125 W: √(0.125 × 100) = 3.54 V.
The formulas
Ohm’s law, V = I × R, rearranges to I = V ÷ R and R = V ÷ I. Substituting it into P = V × I gives the power forms P = I² × R and P = V² ÷ R. Together they make the twelve formulas of the “Ohm’s law wheel”:
- Voltage: V = I × R, V = P ÷ I, V = √(P × R)
- Current: I = V ÷ R, I = P ÷ V, I = √(P ÷ R)
- Resistance: R = V ÷ I, R = V² ÷ P, R = P ÷ I²
- Power: P = V × I, P = V² ÷ R, P = I² × R
The units are the volt (V), ampere (A), ohm (Ω) and watt (W): 1 Ω = 1 V/A and 1 W = 1 V·A. The calculator works in these base units and rounds only what it displays (4 significant figures for calculated values).
Choosing a resistor wattage
A resistor turns P = I² × R into heat. Running it at its full rating makes it very hot, so a common design rule is to choose a rating of at least twice the power it dissipates. The calculator doubles the power and picks the next rating that resistor catalogues commonly stock: 1/16, 1/10, 1/8, 1/4 and 1/2 W for small film and chip resistors; 1, 2, 3 and 5 W for larger film and wirewound parts; and 10, 25, 50, 100, 150, 250 and 300 W for chassis-mount (aluminium-housed) resistors.
- Power ratings hold only up to the rated ambient temperature in the datasheet (often 70 °C) and fall above it — read the derating curve.
- Chassis-mount resistors reach their rating only when bolted to the heatsink the datasheet specifies; in free air they handle much less.
- Resistors also have a maximum working voltage. A high-value resistor can exceed it long before it gets hot.
- Short pulses (inrush, capacitor discharge) need a resistor with a pulse or surge rating, not just an average-power rating.
Ohm’s law and AC circuits
For alternating current, use RMS values (230 V mains is an RMS value). Ohm’s law and P = V × I then hold for resistive loads such as heaters and incandescent lamps. Motors, transformers, LED drivers and other loads with inductance or capacitance have an impedance instead of a plain resistance, and their real power is P = V × I × power factor — use the watts to amps calculator for those, or the three-phase power calculator for three-phase loads.
Prefixes and resistor codes
Each unit list runs from micro to mega: µ = 10⁻⁶, m = 10⁻³, k = 10³ and M = 10⁶. You can also type the prefix after the number (4.7k, 20m, 500u, 1meg) — once you leave the box it moves into the unit list. Upper-case M means mega and lower-case m milli, so 1 MΩ is a billion times 1 mΩ; K is accepted as kilo.
The RKM code of IEC 60062, printed on resistors and schematics, also works: the letter stands for the decimal point and the multiplier, so 4k7 is 4.7 kΩ, 2M2 is 2.2 MΩ, 4R7 is 4.7 Ω and R47 is 0.47 Ω.
Sources
- IEC 60050-131, International Electrotechnical Vocabulary — Part 131: Circuit theory (definitions of resistance and power; online at Electropedia)
- G. S. Ohm, Die galvanische Kette, mathematisch bearbeitet (Berlin, 1827) — the original statement of the law
- IEC 60062, Marking codes for resistors and capacitors — the RKM code (4k7, R47)
Limitations
- Only for components with a fixed resistance. Diodes and LEDs, lamp filaments (much lower resistance when cold), thermistors and varistors do not follow Ohm’s law with one value of R.
- For AC it assumes RMS values and a resistive load; reactive loads need impedance and power factor.
- The wattage suggestion is a rule of thumb. It does not replace the datasheet’s temperature derating, pulse rating or maximum working voltage.
- Values must be greater than zero. A value of 0 Ω (a short circuit) or 0 A has no finite answer.
Privacy
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Frequently asked questions
How do I calculate resistance from voltage and current?
Divide the voltage by the current: R = V ÷ I. For 12 V and 2 A, R = 12 ÷ 2 = 6 Ω. Watch the units — 12 V and 20 mA is 12 ÷ 0.02 = 600 Ω.
What wattage resistor do I need?
Work out the power it dissipates — P = I² × R or P = V² ÷ R — then choose a rating of at least twice that. A 220 Ω resistor with 5 V across it dissipates 25 ÷ 220 = 0.114 W, so a 1/4 W (0.25 W) resistor is the right choice.
Does Ohm’s law work for AC?
Yes for resistive loads, using RMS voltage and current. With motors, transformers or electronic power supplies the current and voltage are out of step, so use impedance and power factor instead — the watts to amps calculator handles power factor.
Why did one of my values change when I typed in another box?
Only two values can be chosen freely; the other two follow from them. When you type in a third box, the calculator keeps your two most recent entries and recalculates the older one. To start over, use Clear all.
What do 4k7 and R47 mean?
They are resistor codes from IEC 60062: the letter replaces the decimal point and gives the multiplier. 4k7 = 4.7 kΩ, 2M2 = 2.2 MΩ, 4R7 = 4.7 Ω and R47 = 0.47 Ω. You can type them directly into the Resistance box.
Why is a light bulb’s resistance different when I measure it?
A filament’s resistance rises sharply as it heats up, so a multimeter reading on a cold lamp is much lower than its resistance while lit. Ohm’s law gives the hot resistance from the rated voltage and power: a 60 W, 230 V lamp is 230² ÷ 60 ≈ 882 Ω when lit.