Watts / Volts / Amps / Ohms Calculator
A DC Ohm's law calculator for power (W), voltage (V), current (A) and resistance (Ω). Enter exactly two of the four values, choose a unit for each row — from microamps and millivolts up to kilowatts and megaohms — and press Calculate to get the other two. Everything runs locally in your browser.
Fill in exactly two values
Type numbers into any two of the four rows — Resistance (R), Current (I), Voltage (V) and Power (P). The remaining two rows are what the calculator solves for; leaving fewer or entering more than two values shows an error.
Pick a unit for each row
Each row has its own unit menu: ohms / kiloohms / megaohms, microamps through megaamps, microvolts through megavolts, and microwatts through megawatts. Values are converted to base units (Ω, A, V, W) before the math.
Press Calculate, read the result
The two missing quantities appear in base units with at most 10 significant digits, alongside an echo of your inputs. Copy the result to your clipboard, or press Reset to clear all fields and units.
Ohm's law and DC power formulas
- How do you calculate current (amps)?
- Current I in amps (A) equals voltage V in volts (V) divided by resistance R in ohms (Ω): I = V / R. It also equals power P in watts (W) divided by voltage: I = P / V, and the square root of power divided by resistance: I = √(P / R). Example: 12 V across 100 Ω gives 12 / 100 = 0.12 A.
- How do you calculate voltage (volts)?
- Voltage V in volts (V) equals current I in amps (A) times resistance R in ohms (Ω): V = I × R. It also equals power divided by current: V = P / I, and the square root of power times resistance: V = √(P × R). Example: 2 A through a circuit dissipating 100 W has 100 / 2 = 50 V across it.
- How do you calculate resistance (ohms)?
- Resistance R in ohms (Ω) equals voltage divided by current: R = V / I. It also equals voltage squared divided by power: R = V² / P, and power divided by current squared: R = P / I². Example: 230 V delivering 1 kW gives 230² / 1000 = 52.9 Ω.
- How do you calculate power (watts)?
- Power P in watts (W) equals voltage times current: P = V × I. It also equals voltage squared over resistance: P = V² / R, and current squared times resistance: P = I² × R. Example: 12 V across 100 Ω dissipates 12 × 0.12 = 1.44 W.
- Why does the calculator reject some pairs, like zero resistance or zero current?
- This tool models a passive DC circuit: resistance must be greater than zero, and current, voltage and power cannot be negative. When the pair you entered would require dividing by zero — for example R = 0 Ω with a known voltage, or I = 0 A with a known voltage — the result is undefined and the calculator says so instead of printing an infinite value.
Privacy: every calculation runs in your browser tab — no value you type is sent anywhere. Copy result uses your clipboard only. Results are ideal steady-state DC values; verify real installations with a multimeter or power analyzer.