Volts to Amps Calculator

Volts × amps = watts: 12 V at 2 A is 24 W

Power in watts (W) is voltage in volts (V) times current in amps (A). Divide watts by volts to get amps, and divide volts by amps to get resistance in ohms (Ω). The SI Brochure defines the watt as a joule per second, the volt as a watt per ampere and the ohm as a volt per ampere, so the four units are tied together: know any two of voltage, current, power and resistance and the other two follow.

AC needs one more step. Voltage times current in an AC circuit is apparent power in volt-amperes (VA); the real power that does the work, in watts, is that figure times the power factor. In a three-phase circuit, multiply line voltage by line current and then by √3. The converter above covers DC, single-phase AC and three-phase AC: enter any two of the four values and it fills in the other two along with the formulas it used.

When you need it

• Reading a charger or adapter label in watts: an output of 5 V 2 A is 5 × 2 = 10 W. USB-IF says USB Power Delivery can carry up to 100 W over a 20 V, 5 A cable, so a laptop charger marked 20 V 5 A is a 100 W charger.
• Finding the current of an appliance: a 1,500 W hair dryer on a 120 V circuit draws about 12.5 A. An appliance that uses the same 1,500 W at 240 V, such as a larger heating appliance, draws about 6.25 A. You need this number to compare against the rating of a plug, cord or breaker.
• Reading VA and kVA on a UPS or generator: VA is apparent power, not watts, so you multiply by the power factor to get the watts you can actually use. That is why the answer to "how many kW is 1 kVA" changes with the power factor.
• Sizing current for three-phase equipment: a 7.5 kW motor on 480 V three-phase at a power factor of 0.85 draws about 10.61 A per line. Leaving out √3 overstates the current by more than 70 %.
• Picking resistors or checking currents in electronics: 5 V across 220 Ω gives about 22.73 mA. The Ohm's law wheel shows which formula was used.
• Comparing voltages in different countries: the same 1,500 W draws 15 A at Japan's 100 V and 12.5 A at the 120 V used in US homes, and about 6.522 A at 230 V. The "same power, different voltage" bars show the difference.

How to use it

1. Pick the circuit type. Batteries and charger outputs are DC, a wall outlet in a home is single-phase AC, and motors and building power systems are usually three-phase AC. An AC load with a power factor of 1, such as a resistive heater, is calculated with the same formulas as DC.
2. Enter any two of the four values. Each field has a unit menu (mV, V, kV; μA, mA, A, kA; mW, W, kW, MW; mΩ, Ω, kΩ, MΩ; VA, kVA, MVA), and you can type thousands separators such as 1,234.5.
3. The two most recently edited fields are the inputs and the other two become results. Type a new value into a result field and it becomes an input, while the oldest input turns into a result.
4. For AC, enter a power factor. Leave it empty and 1 is used. The W field and the VA field are the same power linked by the power factor, so only the one you edited last counts as an input.
5. The two formula lines show which formula was used with your numbers filled in. The rows below give the energy used in one hour (Wh) and, for AC, reactive power and phase angle.
6. The circuit drawing, the Ohm's law wheel (DC), the power triangle (AC) and the voltage bars all show the same values. Tap a bar to see the current for your power at that voltage, or tap a common example to load it into the meter.

How the calculation works

1. DC: P = V × I and V = I × R give everything else. At 12 V and 2 A, P = 12 × 2 = 24 W and R = 12 ÷ 2 = 6 Ω. If you know only power and resistance, V = √(P × R) and I = √(P ÷ R).
2. Single-phase AC: apparent power S = V × I (VA), real power P = S × power factor (W) and reactive power Q = S × √(1 − PF²) (var). At 120 V, 10 A and PF 0.9, S = 1,200 VA, P = 1,080 W and Q is about 523.1 var.
3. Three-phase AC: S = √3 × line voltage × line current and P = S × PF. At 480 V with 10 kW real power and PF 0.85, S = 10,000 ÷ 0.85 ≈ 11,765 VA and the line current is about 14.15 A. For a wye connection the phase voltage is line voltage ÷ √3 ≈ 277.1 V. The 480 V here is a worked example, not a claim about any standard.
4. Unit prefixes: μ is one millionth, m is one thousandth, k is a thousand and M is a million. 2,000 mA = 2 A and 3.52 kW = 3,520 W. Result fields pick the unit that gives a number of 1 or more; W and VA stay as they are up to 9,999 and switch to kW and kVA from 10,000. If you pick a unit for a result field yourself, it keeps that unit.
5. Rounding: fields and formulas show 6 significant digits; the circuit drawing, bars and examples show 4. The formula lines and the common examples use "≈" instead of "=" when a value is rounded.

The 12 Ohm's law formulas

In DC there are 12 formulas for finding one value from two others. The Ohm's law wheel above is this table arranged in a circle, and it shades the formula you just used.

Current by power

Power factor: why watts and VA differ

The IEC's Electropedia defines apparent power as the product of the RMS voltage and RMS current (S = UI, unit VA) and active power as the mean of instantaneous power over one period (unit W). Power factor is the ratio of the absolute value of active power to apparent power. Reactive power is apparent power times the sine of the phase angle (Q = S sin φ); its unit is dimensionally the same as the VA, but it is given its own name, the var. The SI Brochure also lists the var as the special name for the unit of reactive power and equates it to one volt-ampere.

A load whose current and voltage are in phase, such as a heater or an incandescent lamp, has a power factor of 1, so watts and VA are equal. A motor draws current that lags the voltage, so its power factor is below 1 and it needs more current for the same work. On the power triangle above, the power factor chips show how the current or real power changes when only the power factor changes.

The same apparent power means the same current, so the 120 V column does not depend on the power factor. A Cummins technical note explains that ISO 8528-1 rates generator sets at a power factor of 0.8 lagging unless the manufacturer states otherwise. Schneider Electric's white paper "Watts and Volt-Amps: Powerful Confusion" says that, at the time it was written, a small UPS typically had a watt rating about 60 % of its VA rating, so a 1,000 VA UPS might not carry a 900 W load. For a UPS or a generator, check both the VA and the W rating.

Household voltage by country and the current at the same power

A Virginia Cooperative Extension publication (BSE-301) says most US appliances run on 120 V, while large appliances such as clothes dryers and electric cooktops run on 240 V. In the United Kingdom, regulation 27 of the Electricity Safety, Quality and Continuity Regulations 2002 sets the low-voltage supply at 230 V and 50 Hz.

Other countries in the table: Japan's Ordinance for Enforcement of the Electricity Business Act (art. 38) require 100 V to be kept at 101 V plus or minus 6 V and 200 V at 202 V plus or minus 20 V. Spain's low-voltage regulation (RD 842/2002, art. 4) gives 230 V between phase and neutral, 400 V between phases and 50 Hz. Indonesia's PLN standard SPLN T6.001:2013 lists low-voltage distribution at 230/400 V. Korea's rule sets the standard voltages at 110, 220 and 380 V with a frequency of 60 Hz, and the table uses its 220 V single-phase figure.

The lower the voltage, the more current the same power needs. More current heats wires and contacts more, so low-voltage systems use thicker wire and outlets with higher current ratings. If you take a device to a country with a different supply voltage, check the voltage range printed on the device first, separately from this calculation.

Common mistakes

• Entering mA as A: a charger marked 2000 mA is 2 A. Typing 2000 with the unit left on A gives a result 1,000 times too large, so choose mA in the unit menu or divide by 1,000 first.
• Treating V × A as watts for an AC appliance: 120 V at 10 A is 1,200 VA, and at a power factor of 0.9 the real power is 1,080 W.
• Dropping √3, or multiplying by 3, in three-phase: at 480 V line voltage, 10 A line current and PF 1, the real power is √3 × 480 × 10 ≈ 8,314 W. Multiplying 480 × 10 gives 4,800, which is too small, and 3 × 480 × 10 = 14,400 is too large. You multiply by 3 only when you use phase voltage and line current in a wye (Y) connection.
• Reading the kW on a motor nameplate as input power: the nameplate kW or horsepower is the mechanical output at the shaft at rated voltage, current and frequency. The electrical power taken from the supply is the rated output divided by the efficiency, so the real line current is larger than this converter shows.
• Treating a charger's maximum current as the current that always flows: an adapter marked 2 A can supply up to 2 A. The actual current depends on what the connected device draws.

Limits

• The AC calculation assumes sinusoidal voltage and current and, for three-phase, an equal load on all three phases. Equipment with strong harmonics, or loads that differ between phases, will not match.
• Wire size, breaker and outlet ratings, safety margins and voltage drop are not calculated. Follow the applicable electrical code and the advice of a qualified electrician for wiring and equipment capacity.
• The impedance |Z| for AC and the phase voltage for three-phase are reference values from V and I. Three-phase phase voltage and per-phase impedance assume a wye connection.
• Each field accepts values above 0 up to 1 billion (in the selected unit), and a result below 10⁻¹² or above 10¹⁵ is not calculated. Power factor runs from 0.01 to 1.
• Your numbers are calculated in this browser and are not sent anywhere. "Copy link" puts the input values in the address.

Basis: the formulas follow the SI definitions and IEC terminology, which do not change from year to year. The voltage references for each country and the manufacturer and standards documents were checked against the source texts on October 20, 2026.

Sources

• 01BIPM — The International System of Units (SI Brochure, 9th edition)Table 4: watt = J/s, volt = W/A, ohm = V/A, the ampere definition; var is the special name for the unit of reactive power (1 var = 1 V A)
• 02NIST SP 330 — Section 2The var is the special name for the unit of reactive power; in coherent SI units it equals the volt-ampere
• 03IEC Electropedia 131-11-41 apparent power · 131-11-42 active powerS = UI, unit VA / mean of instantaneous power over a period, unit W
• 04IEC Electropedia 131-11-46 power factor · 131-11-44 reactive powerλ = |P| ÷ S / Q = S sin φ, special name var
• 05Virginia Cooperative Extension — Estimating Appliance and Home Electronic Energy Use (BSE-301)Most US appliances use 120 V; clothes dryers and electric cooktops use 240 V; watts = amps × volts
• 06legislation.gov.uk — Electricity Safety, Quality and Continuity Regulations 2002, reg. 27Low-voltage supply of 230 V (phase to neutral) at 50 Hz
• 07e-Gov Law Search — Ordinance for Enforcement of the Electricity Business Act (Japan), art. 38100 V kept at 101 V ± 6 V, 200 V at 202 V ± 20 V (Japanese original)
• 08BOE — Real Decreto 842/2002 (Reglamento electrotécnico para baja tensión)Art. 4: 230 V (phase to neutral) and 400 V (between phases) at 50 Hz (Spanish original)
• 09PT PLN — SPLN T6.001:2013 Tegangan-tegangan standar (public copy)Table 1: low-voltage three-phase four-wire 230/400 V, 50 Hz (Indonesian original)
• 10Korea National Law Information Center — Enforcement Rule of the Electric Utility Act, art. 18 and annex 3Standard voltages 110, 220 and 380 V with their tolerances; standard frequency 60 Hz (Korean original)
• 11USB-IF — USB Charger (USB Power Delivery)100 W over a 20 V, 5 A cable; 240 W at a fixed 48 V
• 12Schneider Electric — Watts and Volt-Amps: Powerful Confusion (WP-15, public copy)W ÷ VA = power factor; small UPS watt rating about 60 % of the VA rating (industry practice at the time)
• 13Cummins Power Generation — Understanding ISO 8528-1 Generator Set Ratings (public copy)Unless stated otherwise, generator sets are rated at a power factor of 0.8 lagging
• 14CED Engineering — Understanding Motor Nameplate Information: NEMA vs. IEC StandardsNameplate kW or horsepower is shaft output at rated voltage, current and frequency

Frequently asked questions

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