Ohm's Law Calculator
Solve for voltage, current, resistance, and DC power from any two known values (V, I, R, P). Combines Ohm's law (V = I × R) with P = V × I = I² × R = V² ÷ R. Educational notes cover the same DC and AC real-power relationships as our Amps/Watts/Volts calculators. Runs entirely in your browser.
Uses positive DC magnitudes. For AC real power with power factor and three-phase line-to-line voltage, use our dedicated electrical calculators below.
Volts across the resistive load.
Amperes through the load.
DC resistive model: results assume V, I, R, and P obey Ohm’s law and Joule’s law together. Not a full AC impedance solver.
Table of Contents
Voltage, current, resistance, and power together
Ohm’s law relates voltage V, current I, and resistance R: V = I × R. Once you know any two of V, I, and R, the third follows.
Electrical power in watts for a resistive DC load is P = V × I. Combining with Ohm’s law gives P = I² × R and P = V² ÷ R — the same combinations used across our Amps to Watts, Watts to Amps, Volts to Amps, Amps to Volts, Volts to Watts, and Watts to Volts tools when the underlying model is DC or purely resistive.
This page lets you start from any two of V, I, R, and P and instantly see all four, with the exact rearrangements (e.g. R = V ÷ I, I = P ÷ V, V = √(P × R)).
For AC, real power also depends on power factor and whether voltage is line-to-line in three-phase systems. The reference section below matches the formulas in our AC-specific calculators so you can stay consistent.
Formulas & AC reference
Ohm's law
V (V) = I (A) × R (Ω) ⇔ I = V ÷ R ⇔ R = V ÷ I
Use consistent units: volts, amperes, ohms.
Power (Joule’s law, resistive DC)
Three equivalent forms:
- P = V × I
- P = I² × R
- P = V² ÷ R
From V = I×R: substitute into P = V×I to get P = I²R and P = V²/R. These are the same relationships behind our wattage and “volts + ohms” modes elsewhere.
AC real power (reference — same as our other tools)
When you model real power with RMS volts and amps, our dedicated calculators use:
DC
P = V × I
No power factor.
AC single-phase
P = VRMS × IRMS × PF
PF between 0 and 1.
AC three-phase (balanced, line-to-line)
P = √3 × VL-L × I × PF
√3 ≈ 1.732. Voltage is line-to-line RMS.
Where each combination appears
V + I → P is Amps to Watts / Volts to Watts (current path). V + R → I is Volts to Amps (Ohm mode). I + R → V is Amps to Volts (Ohm mode). P + V → I is Watts to Amps. P + I → V is Amps to Volts (power mode). P + V → R via V²/P is Watts to Volts (resistance path). This hub solves all six “two knowns” cases in one place for DC.
Quick Reference Table
| Known | V | I | R | P | How |
|---|---|---|---|---|---|
| V, I | 12 V | 2 A | 6 Ω | 24 W | R = V÷I, P = V×I |
| V, R | 24 V | 4 A | 6 Ω | 96 W | I = V÷R, P = V²÷R |
| V, P | 120 V | 10 A | 12 Ω | 1,200 W | I = P÷V, R = V²÷P |
| I, R | 10 V | 2 A | 5 Ω | 20 W | V = I×R, P = I²R |
| I, P | 60 V | 3 A | 20 Ω | 180 W | V = P÷I, R = P÷I² |
| R, P | 20 V | 2 A | 10 Ω | 40 W | V = √(P×R), I = √(P÷R) |
FAQ
For DC (or resistive) V–I–R–P, yes — any two values determine the other two. The other tools add AC workflow (power factor, three-phase line-to-line) and field order tailored to one target quantity.
With P and R, V = √(P × R) and I = √(P ÷ R) so that V = I×R and P = V×I stay consistent (positive magnitudes).
Not as a full model. Reactive AC loads need impedance and phase; use RMS and power factor with our Watts to Amps / Volts to Watts tools when real power is the goal.
You need two independent quantities among V, I, R, and P to fix the other two. Enter a second value to see results.