Voltage
12 VEntered
Ohm’s Law Electrical Calculator
Enter any two values. Get the other two instantly.
Known inputs: Voltage, Current. Use as input releases the oldest known quantity when two are already entered.
Advanced: component power rating
Blank omits the comparison. If entered, rating must be positive. No automatic derating rule.
Power rating comparison is a mathematical check only. Real component selection can depend on temperature, enclosure, tolerance, transient loads, manufacturer derating, ventilation, and other operating conditions.
SOLVED CIRCUIT VALUES
Current
2 AEntered
Resistance
6 ΩCalculated
Power
24 WCalculated
This calculator uses positive scalar values for a simple resistive DC model. It does not model reactance, phase angle, power factor, nonlinear devices or AC system design.
Electrical note: This calculator performs equation and unit-conversion math. It does not determine whether a circuit, component, conductor, breaker, or installation is safe or code-compliant.
How we calculated it
All substitutions use normalized V, A, Ω and W; display rounding is applied last.
Resistance
R = V / I
R = 12 / 2
R = 6 Ω
Power
P = V × I
P = 12 × 2
P = 24 W
Component power-rating check
No rating supplied. Enter a component / resistor power rating in Advanced for a mathematical comparison.
Power rating comparison is a mathematical check only. Real component selection can depend on temperature, enclosure, tolerance, transient loads, manufacturer derating, ventilation, and other operating conditions.
Ohm’s Law / power reference
V = I × R
P = V × I
Show all formulas
Voltage
- V = I × R
- V = P / I
- V = √(P × R)
Current
- I = V / R
- I = P / V
- I = √(P / R)
Resistance
- R = V / I
- R = V² / P
- R = P / I²
Power
- P = V × I
- P = I² × R
- P = V² / R
V in volts, I in amperes, R in ohms, P in watts. Positive scalar DC/resistive model.
Electrical note: This calculator performs equation and unit-conversion math. It does not determine whether a circuit, component, conductor, breaker, or installation is safe or code-compliant.
How to use
Enter any two values. Solve voltage, current, resistance, and power instantly.
Choose Ohm’s Law, Resistors or Formula Reference. Start with two positive known quantities, each with a numeric value and unit. Calculated fields are read-only: choose Use as input to promote one and release the oldest known input. Release an input or Clear known inputs to start again. Advanced accepts an optional component rating. Resistors supports 1–50 rows with Series or Parallel. Copy Results exports the active valid calculation; Reset restores both default fixtures.
Ohm’s Law
V = I × R. P = V × I.
Voltage, current, resistance and power are connected in a simple resistive DC circuit. Two independent positive quantities determine the other two under these relationships. OpenStax’s Ohm’s Law chapter (opens in a new tab) describes the ohmic relationship. This tool does not calculate reactive or nonlinear circuits.
Voltage
Voltage is electric potential difference. Enter it in mV, V or kV; all calculations normalize to V. Given current and resistance, V = I × R. Other supported pairs give V = P / I or V = √(P × R).
Current
Current is the rate of electric charge flow. Units are µA, mA and A; uA is accepted by the conversion engine as a spelling of µA. Calculations normalize to A. Use I = V / R, I = P / V or I = √(P / R).
Resistance
Resistance describes the proportional relationship between voltage and current in this model. Units are Ω, kΩ and MΩ; normalize to Ω. Use R = V / I, R = V² / P or R = P / I². Core resistance inputs must be positive; network zero policy is separate.
Electrical power
Electrical power is energy transferred per unit time. This tool accepts mW, W and kW and normalizes to W. P = V × I; for the supported resistor model, P = I² × R and P = V² / R. OpenStax’s electric-power chapter (opens in a new tab) explains these relationships.
With a separately verified power assumption, you can estimate battery energy for a continuous DC load over an entered duration. Check the storage model’s efficiency assumptions separately; this ideal resistor calculation does not approve a battery, protection device or installation.
Enter any two values
Supported pairs: Voltage + Current; Voltage + Resistance; Voltage + Power; Current + Resistance; Current + Power; Resistance + Power. Input ownership is explicit and ordered: editing a known value does not move its age; promotion releases the oldest known quantity when two already exist.
With fewer than two known values, the circuit is incomplete. Invalid or zero known inputs hide calculated results. Three or four independently entered values are never reconciled into an ambiguous circuit.
Unit conversions
1 mV = 0.001 V; 1 kV = 1,000 V. 1 µA = 0.000001 A; 1 mA = 0.001 A. 1 kΩ = 1,000 Ω; 1 MΩ = 1,000,000 Ω. 1 mW = 0.001 W; 1 kW = 1,000 W.
NIST’s SI prefix reference (opens in a new tab) documents milli, micro, kilo and mega. Unit selectors preserve the unrounded physical quantity. Input units stay selected while typing; derived values use convenient engineering units unless you select another output unit. Results retain a normalized-unit or compact secondary representation where useful.
For example, 0.2 A = 200 mA. Decimal comma and dot are accepted, as is bounded e notation. Prefix capitalization matters: m is milli, M is mega.
Calculation formulas
Primary equations: V = I × R; P = V × I. All derived formulas are shown as text below:
Voltage
- V = I × R
- V = P / I
- V = √(P × R)
Current
- I = V / R
- I = P / V
- I = √(P / R)
Resistance
- R = V / I
- R = V² / P
- R = P / I²
Power
- P = V × I
- P = I² × R
- P = V² / R
How we calculated it is generated by the same engine that solves the active pair. Substitutions use normalized inputs. Formatting happens only after raw calculation; no rounded result feeds another formula.
Series resistors
Rtotal = R1 + R2 + … + Rn. Convert every row to Ω before adding. The default 100 + 220 + 330 = 650 Ω. Zero is a valid contribution in this ideal series model.
Parallel resistors
1 / Rtotal = 1/R1 + 1/R2 + … + 1/Rn. Equivalently, Rtotal = 1 / sum(1/Ri). The 100 and 200 Ω example gives 66.67 Ω; raw precision is retained.
If any ideal branch is exactly zero ohms, total parallel resistance is zero. The engine checks this before taking reciprocals and uses a scaled equivalent for finite positive rows. OpenStax’s series and parallel chapter (opens in a new tab) explains equivalent resistance. This tool does not solve mixed, bridge or nested networks.
Component power-rating check
Load percentage = calculated power / explicitly entered rated power × 100. A rating must be positive and may use mW, W or kW.
Calculated power at or below the entered rating gives WITHIN ENTERED RATING; above it gives CALCULATED POWER EXCEEDS ENTERED RATING and the excess in W. Equality is within the entered rating. These are mathematical comparisons, not component-selection recommendations.
Power rating comparison is a mathematical check only. Real component selection can depend on temperature, enclosure, tolerance, transient loads, manufacturer derating, ventilation, and other operating conditions. No fixed percentage derating or automatic wattage multiplier is applied.
For a specified wire run, check conductor voltage drop separately using that model’s conductor and circuit assumptions. Neither calculation determines ampacity, protection-device suitability, code compliance or installation approval; conductor selection requires appropriate professional review.
Worked examples
1. Voltage + Current
12 V and 2 A give 6 Ω and 24 W. R = V / I; P = V × I.
2. Voltage + Resistance
24 V and 120 Ω give 0.2 A (200 mA) and 4.8 W.
3. Current + Power
2 A and 24 W give 12 V and 6 Ω.
4. Resistance + Power
6 Ω and 24 W give 2 A and 12 V. Positive square-root magnitudes are used.
5. Series resistors
100 Ω + 220 Ω + 330 Ω gives 650 Ω.
6. Parallel resistors
100 Ω and 200 Ω in parallel gives 66.67 Ω.
7. Entered power rating
0.72 W calculated power / 1 W entered rating gives 72% load: WITHIN ENTERED RATING. Power rating comparison is a mathematical check only. Real component selection can depend on temperature, enclosure, tolerance, transient loads, manufacturer derating, ventilation, and other operating conditions.
Assumptions and limitations
This calculator uses positive scalar values for a simple resistive DC model. It does not model reactance, phase angle, power factor, nonlinear devices or AC system design. Core known values must be greater than zero; 0 V and 0 A do not uniquely determine resistance. No polarity/sign conventions or special open/short-circuit core model is implemented.
Software sanity limits after normalization: voltage/current up to 1e12 V/A, resistance/power up to 1e18 Ω/W. Known and derived values must remain positive and finite within bounds; network rows allow zero and network total is bounded at 1e18 Ω. Numeric strings are limited to 64 characters. These bounds are not operating recommendations. Six significant digits are used for primary engineering displays; network results such as the parallel example use two decimal places. Raw full JavaScript precision remains in the engine.
Electrical note: This calculator performs equation and unit-conversion math. It does not determine whether a circuit, component, conductor, breaker, or installation is safe or code-compliant. No wire, breaker, fuse or conductor sizing, installation recommendation, AC RMS system design or compliance assessment is provided. Calculations are local; no saved circuits, accounts or input persistence.
FAQ
What is Ohm’s Law?
Ohm’s Law relates voltage, current and resistance: V = I × R. This calculator uses positive scalar values for a simple resistive DC model.
How do I calculate voltage?
Use V = I × R, V = P / I, or V = √(P × R), depending on your two known quantities. The calculation normalizes inputs to V, A, Ω and W.
How do I calculate current?
Use I = V / R, I = P / V, or I = √(P / R). The square root gives the positive magnitude in this model.
How do I calculate resistance?
Use R = V / I, R = V² / P, or R = P / I². Core known values must be strictly positive.
How do I calculate electrical power?
Use P = V × I, P = I² × R, or P = V² / R for the supported resistive model.
Can I enter any two values in this calculator?
Yes. All six unique pairs of voltage, current, resistance and power are supported. Use as input promotes a calculated quantity and releases the oldest known input, so three conflicting user values are never reconciled.
How do I convert mA to A or kΩ to Ω?
Divide milliamperes by 1,000 to get amperes; multiply kilohms by 1,000 to get ohms. Changing a selector preserves physical quantity rather than reinterpreting the same numeric text.
How do I calculate resistors in series?
Add the individual normalized resistances: Rtotal = R1 + R2 + … + Rn. An ideal zero-ohm resistor contributes zero.
How do I calculate resistors in parallel?
Use Rtotal = 1 / sum(1/Ri). A zero-ohm ideal branch makes the total zero; the engine handles that case without dividing by zero. Only pure parallel networks are supported.
Does the component power check mean a circuit is safe?
No. It compares calculated power with the rating you explicitly entered. It does not determine component suitability or installation compliance, and applies no automatic derating rule.
Sources
Reviewed October 4, 2026. NIST supports SI prefixes; OpenStax educational references support Ohm’s Law, resistive power and equivalent resistance. No source endorsement is claimed.