Ohm's Law Calculator

Ohm’s Law Calculator works out voltage, current, resistance and power for a resistor or any other resistive load carrying a steady current. Fill in any two of the four — in volts, millivolts, amps, milliamps, ohms, kilohms, watts and more — and leave the other two empty; the page works them out straight away, shows them in the units you choose, and lists the formula and every step in base units so you can check the arithmetic. It also suggests a resistor power rating with the usual two-times safety margin, since a resistor run at its full rating gets hot. Typical uses: the resistor for an LED, the current a heater or a speaker coil draws, the power wasted in a wire, or the resistance a multimeter reading implies.

1. Type two values you know, for example voltage and resistance, and pick their units.
2. Leave the other two fields empty — they are worked out and highlighted.
3. Choose the unit you want each answer in; the answer changes to match.
4. Read the working under the answer to see which formula was used. Clear all starts again.

• Ohm’s law holds for resistive loads at a steady (DC) current, or for the RMS values of AC through a pure resistance. Motors, transformers, capacitors and coils on AC also have reactance, so voltage × current is not their real power.
• Some parts do not have a fixed resistance: an LED or a diode drops a nearly fixed voltage, and a filament lamp’s resistance rises as it heats up. For an LED, subtract its forward voltage from the supply first and use what is left across the resistor.
• Every value must be above zero. Directions and signs (current flowing the other way) are left to you.
• Results are shown to six significant figures; real resistors are usually accurate to 1% or 5%, so round to what your parts allow.
• The power-rating suggestion is a rule of thumb for small through-hole and surface-mount resistors in still air, not a thermal design.
• The calculation runs in your browser. Nothing you enter is sent or saved.

Subtract the LED’s forward voltage (about 2 V for red, 3 V for blue or white) from the supply, then divide by the current you want. For a 5 V supply, a 2 V LED and 20 mA: enter 3 V and 20 mA here and the page gives 150 Ω and 60 mW — a quarter-watt resistor is plenty.

P = V² ÷ R. Enter the voltage and resistance and leave power empty. 230 V across 52.9 Ω is 1,000 W, a typical kettle element.

A resistor at its full rated power runs hot — often over 100 °C — and drifts or fails sooner. Choosing a rating at least twice the power it dissipates keeps it cooler and lasts longer.

For purely resistive loads such as heaters and kettles, yes, with RMS voltage and current (230 V in India is RMS). For motors, fans or anything with a power factor below 1, real power is lower than voltage × current.