Direct current
U = √(P × R)
Alternating single‑phase
U₀ = P / (I * cosφ)
U₀ = Z * I
U₀ = (R * I) / cosφ
U₀ = √((P * Z) / cosφ)
U₀ = √((P * R) / cosφ)
U₀ = S / I
U₀ = √(S * Z)
U₀ = √((S * R) / cosφ)
U₀ = Q / (I * sinφ)
U₀ = √((Q * Z) / sinφ)
U₀ = √((Q * R) / (sinφ * cosφ))
Alternating two‑phase
U = P / (I * cosφ)
U = Z * I
U = (R * I) / cosφ
U = √((P * Z) / cosφ)
U = √((P * R) / cosφ)
U = S / I
U = √((S * R) / cosφ)
U = Q / (I * sinφ)
U = √((Q * Z) / sinφ)
U = √((Q * R) / (sinφ * cosφ))
Alternating three‑phase
U = P / (√3 * I * cosφ)
U = Z * I
U = (R * I) / cosφ
U = √( (P * Z) / (√3 * cosφ) )
U = (P * R) / √(3 * cosφ²)
U = S / (√3 * I)
U = √((S * Z) / √3)
U = √((S * R) / (√3 * cosφ))
U = Q / (√3 * I * sinφ)
U = √((Q * Z) / (√3 * sinφ))
U = √((Q * R) / (√3 * sinφ * cosφ))
Where:
U₀ = Voltage [L‑N] (V)
U = Voltage [L‑L] (V)
P = Active power (W)
S = Apparent power (VA)
Q = Reactive power (VAr)
I = Current (A)
R = Resistance (Ω)
Z = Impedance (Ω)
cos φ = Power factor
Calculate DC and AC voltage from current, resistance, power, impedance, or power factor. Supports Ohm’s Law, real/reactive power, and RMS values for engineers, technicians, and students.
| Known Parameters | Formula | Circuit Type | Notes |
|---|---|---|---|
| Current (I), Resistance (R) | V = I × R | DC / AC resistive | Ohm’s Law; valid for heaters, incandescent lamps |
| Active Power (P), Current (I) | V = P / I | DC / AC (with real power) | Ensure P is active power (W), not apparent (VA) |
| Active Power (P), Resistance (R) | V = √(P × R) | DC / AC resistive | Derived from P = V²/R |
| Current (I), Impedance (Z) | V = I × Z | AC (inductive/capacitive) | Z includes resistance + reactance; use RMS values |
| P, I, Power Factor (PF) | V = P / (I × PF) | General AC | PF = cosφ; critical for motors, transformers |
Your choice depends on what you can measure or know:
Working with batteries or simple electronics? Use Ohm’s Law (I and R).
Reading a wattmeter or nameplate? Use V = P / I (ensure it’s active power in watts).
Dealing with motors, SMPS, or fluorescent lighting? You must account for power factor or impedance—use the AC-specific rows.
Voltage Drop in Wiring
Excessive drop (>3%) causes motors to overheat and lights to dim. Calculate using cable resistance and load current.
Power Supply Validation
A 12 V solar charge controller delivering only 10.5 V may fail to charge a lead-acid battery properly.
Equipment Compatibility
Applying 240 V to a 230 V-rated appliance may shorten its lifespan due to increased core losses.
DC Circuits: Voltage is constant. Only resistance matters.
AC Circuits: Voltage oscillates. What matters is the RMS value (e.g., 230 V AC means 230 V RMS, ~325 V peak). Reactive components shift current relative to voltage, requiring impedance (Z) and power factor (PF) for accurate calculation.
Using apparent power (S in VA) instead of active power (P in W) in V = P / I
Assuming PF = 1 for non-resistive loads
Ignoring that multimeters report RMS for AC by default (true for sine waves only)
Applying DC formulas to variable-frequency drives or LED drivers
Note: For three-phase systems, this calculator handles per-phase voltage. Total system analysis requires additional considerations (e.g., line vs. phase voltage).
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