Electrical calculator

Watts to Volts Calculator

Find voltage from power and current (or resistance). Useful for DC and resistive AC checks with unit guidance. Runs locally in your browser.

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Instant result

Result

—

Enter values to calculate.

Inputs—

Mode—

Formula—

Trust summary Engine tested · Source checked · v1.3.1

Input interpretation: Enter values to calculate.
Result: —
Model: Voltage from watts and amps (DC/1φ/3φ) or from watts and ohms; watts from volts and amps.
Scope: RMS quantities for AC
Verification: Engine tested · Source checked · v1.3.1
Named expert review: Optional · Not performed
Sources: Watt’s law / power identity — Real power P = V × I × PF (1φ); P = √3 × V × I × PF (3φ L-L)
Resistive power identity — P = V²/R ⇒ V = √(P·R)
IEC 60050 — International Electrotechnical Vocabulary — Active power (IEV 131-11-42)
IEC 60050 — International Electrotechnical Vocabulary — Power factor (IEV 131-11-46)
IEEE Std 1459-2025 — Power factor in AC real-power calculations
BIPM SI Brochure (9th edition, version 4.01) — SI units for ampere, volt, ohm, and watt
NIST Guide to the SI (SP 811) — Volt, ampere, ohm, watt
Sources

Formulas

Core equations used by this calculator.

DC (with amps)V = P ÷ I

AC 1φV = P ÷ (I × PF)

AC 3φ (L-L)V = P ÷ (√3 × I × PF)

AC 3φ (L-N)V = P ÷ (3 × I × PF)

With ohms / impedanceV = √(P × R)

Volts → watts (DC)P = V × I

iFor DC or resistive AC, use single phase and PF = 1. Three-phase watts→volts returns line-to-line RMS. The ohms tab also covers RF-style V = √(P×Z) when Z is in ohms (e.g. 50 Ω).

How to use

Choose path

W → V (amps) for power and current; W → V (ohms) for power and resistance; V → W to reverse.

Enter values and circuit type

For AC with amps, set PF and single/three phase. For DC, PF = 1 and single phase.

Read volts or watts

Three-phase amps path returns line-to-line voltage.

Example calculations

Common configurations with formula and result.

DC · 20 W · 4 A

V = P ÷ I

20 ÷ 4

5 V

240 W · 2 A

Default DC check

240 ÷ 2

120 V

AC 1φ · 1300 W · 12 A · PF 0.9

With power factor

1300 ÷ (12 × 0.9)

120.37 V

From resistance

40 W · 10 Ω

√(40 × 10)

20 V

RF · 100 mW · 50 Ω

Enter 0.1 W

√(0.1 × 50)

2.236 V

Volts → watts

120 V · 2 A · PF 1

120 × 2

240 W

Quick V = P ÷ I (PF = 1)

Common values at a glance.

Power	Current	Voltage
20 W	4 A	5 V
60 W	5 A	12 V
120 W	10 A	12 V
240 W	2 A	120 V
1000 W	8.333 A	120 V
40 W · 10 Ω	—	20 V (√ path)

i For AC, divide by PF as well (and √3 for three-phase L-L). Ohms path: V = √(P×R).

Watts to Volts calculator specification

Version 1.3.1 · Engine tested

Calculation status

Engine tested 15 published cases
Named expert review Not performed
Calculation version 1.3.1

Review policy

Definition: You cannot convert watts to volts from power alone — you also need current (amps) or resistance/impedance (ohms). With current: DC V = P/I; AC 1φ V = P/(I×PF); AC 3φ L-L V = P/(√3×I×PF). With resistance: V = √(P×R). This tool also reverses volts × current → watts.
What it calculates: Voltage from watts and amps (DC/1φ/3φ) or from watts and ohms; watts from volts and amps.
Inputs: Power P (W) and current I (A), optional PF and phase → V
Power P (W) and resistance R (Ω) → V
Voltage V and current I, optional PF and phase → W
Outputs: Voltage in V, or power in W
Formula: V=P/(√3 (3φ only)·I·PF); V=√(P·R); P=√3 (3φ only)·V·I·PF
Assumptions: RMS quantities for AC
Balanced three-phase when √3 selected
Ohms path assumes real resistive (or |Z| for RF magnitude)
Units: W, A, PF → V
W, Ω → V
Boundary conditions: Amps path (phase-quotient): zero current (b) → 0 V — soft
Zero power (a) on amps path → 0 V
PF ≤ 0 on amps/reverse watts paths treated as PF = 1 (engine: pf > 0 ? pf : 1) — soft
Ohms path (sqrt-product): negative P×R clamped to 0 under √ — soft
Zero P or R on ohms path → 0 V
Engine does not hard-reject PF outside (0,1]; document PF ∈ (0,1] for physical AC use
Example: 240 W, 2 A, PF 1 → 120 V
Validation cases: 15 published on this page
20 W, 4 A, DC → 5 V
240 W, 2 A, PF 1, 1φ → 120 V
1300 W, 12 A, PF 0.9, 1φ → ≈120.37 V
40 W, 10 Ω → 20 V
0.1 W, 50 Ω → ≈2.236 V
120 V, 2 A, PF 1 → W → 240 W
1000 W, 10 A, PF 0.8, 3φ → ≈72.17 V
480 W, 2 A, PF 1 (2× P) → 240 V (linear in P)
240 W, 1 A, PF 1 (half I) → 240 V (inverse in I)
240 W, 0 A, PF 1 → 0 V (soft: zero current → 0)
0 W, 2 A, PF 1 → 0 V
240 W, 2 A, PF 0 → 120 V (soft: PF≤0 treated as 1)
-10 W, 10 Ω → 0 V (soft: negative P×R → √0)
0 W, 10 Ω → 0 V
240 W → 120 V → W round-trip at PF 1 → 240 W
Sources: Watt’s law / power identity — Real power P = V × I × PF (1φ); P = √3 × V × I × PF (3φ L-L)
Supports: V = P/(I·PF); P = V·I·PF; 3φ with √3
Resistive power identity — P = V²/R ⇒ V = √(P·R)
Supports: W → V (ohms) / RF |Z| magnitude path
IEC 60050 — International Electrotechnical Vocabulary — Active power (IEV 131-11-42) · accessed 2026-09-05
Supports: P is active power; unit watt (W). Watts-path voltage uses P, not apparent power S.
IEC 60050 — International Electrotechnical Vocabulary — Power factor (IEV 131-11-46) · accessed 2026-09-05
Supports: λ = |P| / S; AC watts path V = P / (I·λ) (1φ) or V = P / (√3·I·λ) (3φ L-L)
IEEE Std 1459-2025 — Power factor in AC real-power calculations · 2025-05-16 · errata checked · accessed 2026-09-05
Supports: PF scaling on amps and reverse watts paths
BIPM SI Brochure (9th edition, version 4.01) — SI units for ampere, volt, ohm, and watt · 2026-06 · DOI · accessed 2026-09-05
Supports: DC V = P/I; V = √(P·R) from P = V²/R
NIST Guide to the SI (SP 811) — Volt, ampere, ohm, watt
Supports: SI unit relationships for V, A, Ω, W
Calculation version: 1.3.1

Background

Interpretation and common distinctions.

Convert watts to volts using current or resistance. Power alone is not enough.

Default (amps path): 240 W ÷ 2 A = 120 V.

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Frequently asked questions

Key distinctions behind the calculation.

How do I convert watts to volts?

Divide power by current: V = P ÷ I for DC. For AC single-phase use V = P ÷ (I × PF). For three-phase line-to-line use V = P ÷ (√3 × I × PF). Or if you know resistance: V = √(P × R).

Can I convert watts to volts without amps?

Not with the amps formula. You need either current or resistance/impedance. Watts alone are not enough.

How do I handle AC power factor?

Use the W → V (amps) tab, set PF, and choose single or three phase. Resistive loads: PF = 1.

What about three-phase?

Select three phase on the amps tab. The result is line-to-line RMS voltage. Line-to-neutral: V = P ÷ (3 × I × PF).

When do I use the ohms formula?

When you know power and resistance (or characteristic impedance Z). V = √(P × R). Common for resistive heaters and RF systems (e.g. 50 Ω).

How do I convert volts back to watts?

Use V → W: P = V × I for DC; multiply by PF (and √3 for 3φ) for AC real power.

Is milliwatt RF supported?

Yes on the ohms tab — convert mW to watts first (100 mW = 0.1 W). Example: √(0.1 × 50) ≈ 2.236 V.