A practical calculator for volts, amps, ohms and watts
This tool combines the most common Ohm’s law and electrical power calculations in one place. Select any two different known quantities, enter their values and the calculator determines the remaining voltage, current, resistance or impedance, and power figures.
The same form can be used for a small DC circuit, an appliance rating, a power supply, or a simplified single-phase AC load. It also accepts mA, A, W, kW, Ω and kΩ, so routine conversions do not require separate pages.
Quick start
- Choose DC or single-phase AC.
- Select the first known quantity and its unit.
- Enter the first positive value.
- Select a different second quantity and enter its value.
- For AC, enter the power factor stated for the load where available.
- Add an operating time to estimate energy use in kWh.
What the results include
Output from the calculation
| Result | Unit | What it represents |
|---|---|---|
| Voltage | V | The potential difference across the simplified load. |
| Current | A | The calculated steady-state current. |
| Resistance / impedance | Ω | The ratio between voltage and current in the selected model. |
| Active power | W | Power converted into useful work, motion, light or heat. |
| Apparent power | VA | Voltage multiplied by current, particularly useful for AC equipment. |
| Energy used | kWh | Active power multiplied by operating time. |
| Load level | Text guidance | A broad description of the power scale, not a circuit rating. |
The core Ohm’s law relationships
Common DC equations
| Known values | Calculated value | Equation |
|---|---|---|
| Voltage and resistance | Current | I = V ÷ R |
| Current and resistance | Voltage | V = I × R |
| Voltage and current | Resistance | R = V ÷ I |
| Voltage and current | Power | P = V × I |
| Voltage and power | Current | I = P ÷ V |
| Current and power | Voltage | V = P ÷ I |
| Resistance and power | Current | I = √(P ÷ R) |
When resistance and power are known, voltage can be found from V = √(P × R). When current and power are known, resistance follows from R = P ÷ I². These rearrangements are selected automatically.
How single-phase AC changes the power calculation
For a simplified single-phase AC load, active power is calculated as P = V × I × PF. Apparent power is S = V × I. A power factor below 1 means the current is higher than it would be for a purely resistive load delivering the same active power.
Single-phase AC rearrangements
| Available values | Required result | Equation used |
|---|---|---|
| V and I | P | P = V × I × PF |
| V and P | I | I = P ÷ (V × PF) |
| I and P | V | V = P ÷ (I × PF) |
| V and Z | I | I = V ÷ Z |
| I and Z | V | V = I × Z |
| P and Z | I | I = √(P ÷ (Z × PF)) |
Unit handling and automatic conversion
Accepted input units
| Quantity | Available units | Internal conversion |
|---|---|---|
| Current | mA or A | mA is divided by 1000 to obtain A. |
| Power | W or kW | kW is multiplied by 1000 to obtain W. |
| Resistance / impedance | Ω or kΩ | kΩ is multiplied by 1000 to obtain Ω. |
| Voltage | V | No scaling is required. |
The two selected quantities must be physically different. Current in mA and current in A are two units of the same quantity and cannot form a valid input pair.
Worked example: a low-voltage DC load
What are the resistance and power of a simplified 24 V DC load drawing 2 A?
Answer: Resistance: R = 24 ÷ 2 = 12 Ω. Power: P = 24 × 2 = 48 W.
Explanation: If the load runs for 6 hours, its calculated energy use is 0.288 kWh. Actual consumption may vary if the load cycles or changes during operation.
Worked example: watts to amps for AC
What current is estimated for a 900 W single-phase AC load at 200 V and a power factor of 0.9?
Answer: I = 900 ÷ (200 × 0.9) = 5 A. Apparent power is 1000 VA.
Explanation: These are example values only. Enter the actual rated voltage and power factor for the equipment being assessed.
One calculator for common electrical conversions
How to set up popular calculations
| Calculation | First selection | Second selection |
|---|---|---|
| Watts to amps | Active power — W or kW | Voltage — V |
| Amps to watts | Current — A or mA | Voltage — V |
| Volts to amps | Voltage — V | Resistance / impedance or active power |
| Ohms to amps | Resistance / impedance — Ω or kΩ | Voltage or active power |
| Volts and amps to ohms | Voltage — V | Current — A or mA |
| Power and resistance to voltage | Active power | Resistance / impedance |
Combining these paths avoids multiple near-duplicate tools. The selected pair tells the calculator which equation must be rearranged, while the result panel keeps all four main quantities together.
Understanding energy use
Energy is calculated from active power and time: kWh = W × hours ÷ 1000. The result assumes the entered power remains constant throughout the selected period.
What the load description does — and does not — tell you
The load description groups active power into broad bands such as very low, low, moderate, high and very high. It is intended to make the number easier to interpret, not to decide whether a socket, cable, breaker, inverter or generator is suitable.
Common input mistakes
Checks before relying on the result
| Mistake | Effect | Better approach |
|---|---|---|
| Selecting current twice in A and mA | There are not two independent quantities | Choose one current unit and pair it with V, Ω or W. |
| Entering a kW figure as W | The result is wrong by a factor of 1000 | Select the unit that matches the source data. |
| Using PF = 1 for every AC load | Current may be understated | Use the documented power factor when available. |
| Treating apparent power as energy | VA and kWh describe different things | Use active power and operating time for energy. |
| Using the result as a wiring design | Installation conditions are missing | Apply the relevant design method and local requirements. |
Scope and limitations
- DC and simplified single-phase AC are supported; three-phase calculations are not included.
- Values are treated as steady-state quantities.
- Cable losses, contact resistance and conversion losses are not added.
- Reactive components are not solved separately.
- Harmonics, distorted waveforms and non-linear load behaviour are outside the model.
- The tool does not select protective devices or conductor sizes.
- Measurements and equipment documentation take priority over a general-purpose estimate.
Frequently asked questions
Can I enter any two values?
You can enter any two different quantities from voltage, current, resistance or impedance, and active power. Both numeric values must be positive. Two units of the same quantity are not a valid pair.
Why is apparent power shown in VA?
VA represents voltage multiplied by current without applying power factor. It is useful when comparing the electrical loading of AC sources such as transformers, inverters and uninterruptible power supplies.
Is resistance the same as impedance?
For DC or a purely resistive AC load, the distinction may be small in a basic calculation. For inductive or capacitive AC loads, impedance includes reactive behaviour, so the displayed Ω value is only a simplified magnitude.
Does the calculator work for three-phase systems?
No. Three-phase power calculations require additional information and commonly use a √3 factor. A dedicated three-phase calculator should be used instead of adapting the single-phase result.
Can the result be used to choose a breaker or cable?
Not by itself. Conductor and protection selection also depends on installation method, length, temperature, grouping, insulation, voltage drop, fault protection, starting current and the rules applicable to the installation.
A single calculation path with clearer assumptions
Use the calculator when two electrical values are known and the remaining relationships need to be checked quickly. Select the correct units, distinguish DC from single-phase AC, enter a realistic power factor where relevant, and treat the output as an engineering estimate rather than an installation approval.