Ohm's law

Voltage is the electrical pressure between two points, current is the amount of charge flowing through the path, and resistance is the opposition to that flow. In a simple resistive load, these values are locked together: increase voltage and current rises; increase resistance and current falls.

The calculator uses the ideal DC relationship. That makes it excellent for resistors, shunts, pull-ups, voltage drops across known resistances, and first-pass checks on simple loads. It is less complete for parts whose resistance changes with voltage, temperature, speed, light, or semiconductor junction behaviour.

Power is the consequence that matters when you choose physical components. A calculated resistance can be electrically correct but still be a poor design if the part has to dissipate more heat than its package, enclosure, or airflow can handle.

V is voltage in volts. It may be the supply voltage, the voltage across one resistor, or the voltage drop measured across a known sense resistor. Always be clear about which two points the voltage is measured between.

I is current in amps. Most small electronics values are easier to think about in milliamps, but equations use amps internally. A 20 mA LED current is 0.02 A, and that unit conversion is one of the most common sources of wrong answers.

R is resistance in ohms. For a resistor it is the nominal value printed by the code or measured with a meter. For a load it may only be an equivalent resistance at one operating point.

P is power in watts. It tells you how fast electrical energy is being converted to heat, light, motion, or another form of work. For resistors and shunts, this is the value that drives wattage selection and temperature rise.

Choose Voltage when you know the current through a resistance and want the voltage drop. This is useful for shunt measurements and checking how much of a supply is being lost across a resistor.

Choose Current when you know the voltage across a resistor and want the current. This is the standard check for pull-ups, current-limit resistors, and measured voltage drops across sense resistors.

Choose Resistance when you know the desired current from a voltage source. This is the common starting point for LEDs, bias resistors, simple loads, and current-limited test circuits.

Choose Power when you already know the voltage and current, or when you know voltage/current with resistance and need to check heat. Power is the value to compare against a component wattage rating.

Resistors are usually rated at a maximum temperature rise in ideal conditions. A part rated for 0.25 W can be hot at 0.25 W, especially inside an enclosure or near other warm components.

A useful first pass is to choose a part rated for at least twice the calculated dissipation. For products, review the datasheet derating curve and the actual ambient temperature.

Small SMD resistors often have much lower power ratings than through-hole parts. A 0603 resistor may be fine for signal pull-ups and dividers, but it can become unsuitable quickly when used as a current limiter or shunt.

For an LED indicator on a 5 V rail, suppose the LED drops 2 V and you want 10 mA. The resistor sees the remaining 3 V. Resistance is 3 V divided by 0.01 A, so a 300 Ω resistor is the ideal value before choosing a nearby standard value.

For a pull-up resistor, a 10 kΩ pull-up on 3.3 V draws 0.33 mA when pulled low. The power is about 1.1 mW, which is negligible for most resistor packages but may matter in ultra-low-power battery products.

For a current-sense resistor, 100 mV across 0.2 Ω means 0.5 A is flowing. The resistor power is 0.05 W, so a 0.125 W part may work thermally in open air but still needs margin in a compact enclosure.

Use it to check LED current resistors, voltage dividers, pull-up currents, sensor loads, transistor base resistors, shunt resistors, and small heating elements.

It is also useful when debugging: a measured voltage drop across a known resistor immediately tells you the current through that part of the circuit.

In production design, Ohm’s law is often the first filter before a more detailed simulation or datasheet review. It will not replace a regulator efficiency curve, LED IV curve, motor stall-current check, or thermal model, but it quickly catches values that are plainly impossible.

Make sure current is entered in the expected unit. 20 mA is 0.02 A, not 20 A.

Ohm's law describes resistive behaviour. LEDs, motors, speakers, batteries, and regulators may not behave like fixed resistors across their operating range.

Be careful with where voltage is measured. The voltage across one resistor in a circuit is not always the same as the supply voltage, and using the wrong voltage can oversize or undersize the result by a large amount.

Resistance tolerance also matters. A 5% resistor can move current by roughly 5% before supply variation, temperature coefficient, and load changes are considered.