# Ohm's law explained: volts, amps, ohms and watts with worked examples

> Voltage, current, resistance and power are linked by two short formulas. Learn the triangle, the twelve-formula wheel, how to size an LED resistor, why a kettle draws 8.7 amps, and how series and parallel resistors change the numbers.

Published 2026-10-09 by Toolcloak. 4 minute read. Original: https://toolcloak.pages.dev/articles/ohms-law-explained-volts-amps-ohms-and-watts-with-worked-examples
Related free tool: [Ohm’s Law Calculator (Voltage, Current, Resistance, Power)](https://toolcloak.pages.dev/tools/ohms-law-calculator)

Every circuit, from a torch to a train, obeys the same short rule: the current through a conductor depends on the voltage pushing it and the resistance holding it back. **Ohm's law** puts that in one line, and with a second formula for power you can answer most everyday electrical questions.

> **In short**
> - **V = I × R**: voltage (volts) = current (amps) × resistance (ohms).
> - **P = V × I**: power (watts) = voltage × current. Combine the two and you get P = I²R and P = V²/R.
> - Know any two of the four quantities (V, I, R, P) and you can find the other two.
> - Resistors in **series** add up. In **parallel** the total is always smaller than the smallest one.

## The three-way triangle

Cover the quantity you want and the other two show how to get it. Voltage is on top; current and resistance sit below.

**One law, three forms**

1. **Find voltage.** V = I × R. A 0.5 A current through 10 Ω needs 5 V.
2. **Find current.** I = V ÷ R. A 12 V supply across 4 Ω drives 3 A.
3. **Find resistance.** R = V ÷ I. 9 V and 0.03 A means 300 Ω.

*Use base units: volts, amps and ohms. Convert milliamps by dividing by 1,000 (20 mA = 0.02 A) and kilo-ohms by multiplying by 1,000.*

Try it: [Ohm’s Law Calculator (Voltage, Current, Resistance, Power)](https://toolcloak.pages.dev/tools/ohms-law-calculator). Enter any two values to get the other two, with the formula used.

## Adding power

Power is how fast electrical energy is turned into heat, light or motion. Since V = IR, you can swap terms to write power in three useful ways, depending on which two numbers you know.

| You know | Power formula | Example |
| --- | --- | --- |
| Voltage and current | P = V × I | 230 V × 8.7 A ≈ 2,000 W |
| Current and resistance | P = I² × R | 3 A and 4 Ω → 9 × 4 = 36 W |
| Voltage and resistance | P = V² ÷ R | 12 V and 4 Ω → 144 ÷ 4 = 36 W |

*All three answers can agree. The 12 V, 4 Ω, 3 A circuit above uses 36 W by every route.*

## Worked example 1: an LED resistor

An LED needs a resistor in series, or it draws too much current and burns out. Suppose a 5 V supply, a red LED that drops about 2 V, and a target current of 20 mA.

**Sizing the resistor**

1. **Voltage left for the resistor.** 5 V − 2 V = **3 V**.
2. **Resistance.** R = V ÷ I = 3 ÷ 0.02 = **150 Ω**.
3. **Power in the resistor.** P = V × I = 3 × 0.02 = **0.06 W**.
4. **Pick a part.** A 150 Ω, quarter-watt (0.25 W) resistor has plenty of margin.

*If you cannot find 150 Ω, use the next higher standard value. A slightly higher resistance gives a slightly dimmer, cooler LED, which is safe.*

## Worked example 2: a 2,000 W kettle

A kettle is rated 2,000 W on a 230 V supply, the standard in India and much of the world. Rearranging P = V × I gives the current, and Ohm's law then gives the resistance of the heating element.

**Inside a 2,000 W, 230 V kettle**

|  | Value | Note |
| --- | --- | --- |
| Current drawn | 8.7 A | 2,000 ÷ 230 |
| Element resistance | 26.5 Ω | 230 ÷ 8.7 |
| Energy in one hour | 2.0 kWh | 2 kW × 1 h |
| Typical socket rating | 10 A | Small margin left |

*A 16 A socket is common for heavy appliances. Plug a kettle and a heater into the same extension board and the total can exceed what the cable allows.*

## Worked example 3: light bulbs

Higher wattage at the same voltage means more current and lower resistance. A thicker filament, not a thinner one, makes a brighter incandescent bulb.

| Bulb (230 V) | Current | Resistance when hot |
| --- | --- | --- |
| 60 W | 0.26 A | 882 Ω |
| 100 W | 0.43 A | 529 Ω |
| 1,500 W heater | 6.52 A | 35.3 Ω |

*Resistance of a filament rises as it heats, so a cold bulb measures much lower than these hot figures.*

**Current drawn at 230 V (amps)**

|  | Amps |
| --- | --- |
| 60 W bulb | 0.26 |
| 100 W bulb | 0.43 |
| 1,500 W heater | 6.52 |
| 2,000 W kettle | 8.70 |

*Current climbs in step with power when voltage is fixed. Heating appliances use by far the most.*

## Series and parallel resistors

Real circuits use more than one resistor. How you connect them decides how the values combine.

**100 Ω and 220 Ω resistors**

| In series (one after the other) | In parallel (side by side) |
| --- | --- |
| 320 Ω | 68.75 Ω |
| Totals add: 100 + 220 | 1/R = 1/100 + 1/220 |
| Same current through both | Smaller than the smallest resistor |
| Voltage splits between them | Same voltage across both |

*For two resistors in parallel, a handy shortcut is (R1 × R2) ÷ (R1 + R2) = 22,000 ÷ 320 = 68.75 Ω.*

A voltage divider is a series pair used to get a smaller voltage. With a 9 V battery across 100 Ω and 220 Ω, the current is 9 ÷ 320 = 0.028 A. The voltage across the 220 Ω resistor is 0.028 × 220 = 6.2 V, and across the 100 Ω resistor 2.8 V.

## Where the law stops being simple

- **Non-ohmic parts.** Diodes, LEDs and transistors do not have a fixed resistance. Their current does not rise in a straight line with voltage.
- **Temperature.** Metal resistance rises with heat, so a hot wire behaves differently from a cold one.
- **AC circuits.** With alternating current, coils and capacitors add reactance, and the law uses impedance instead of resistance.
- **Real batteries.** Voltage drops under load because of internal resistance.

> **Mains voltage is dangerous:** These formulas are for understanding and low-voltage projects. Never work on live mains wiring. If an appliance trips the breaker, get a qualified electrician to check it.

## Quick sanity checks

1. **Convert units first.** Milliamps and kilo-ohms are the usual slip.
2. **Check the power rating** of any resistor: P = I²R must be below its watt rating, with margin.
3. **Voltage drops add up** around a loop to equal the supply voltage.
4. **Parallel total below the smallest.** If yours is larger, recheck the calculation.
5. **Wire gauge matters.** Higher current needs thicker wire to avoid heating.

## Frequently asked questions

### What is Ohm's law?

It states that voltage equals current multiplied by resistance, V = I × R. It describes how much current flows through a resistor for a given voltage.

### How do I calculate current from power and voltage?

Divide power by voltage: I = P ÷ V. A 2,000 W kettle on 230 V draws about 8.7 amps.

### What is the formula for electrical power?

P = V × I. Using Ohm's law you can also write it as P = I² × R or P = V² ÷ R.

### How do I choose a resistor for an LED?

Subtract the LED's forward voltage from the supply voltage and divide by the desired current. For 5 V, a 2 V LED and 20 mA, the resistor is 150 ohms.

### How do resistors add in parallel?

The reciprocals add: 1/R = 1/R1 + 1/R2. The total is always smaller than the smallest resistor. For 100 and 220 ohms it is 68.75 ohms.

### Does Ohm's law work for all components?

No. It holds for ohmic parts such as ordinary resistors. LEDs, diodes and transistors have a current that does not rise in proportion to voltage.