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Reference chartLow code sensitivityLast reviewed June 1, 2026

Electrical reference chart

Ohm's Law Formula Chart

Use this Ohm's Law formula chart after the calculator result to document the solved variable, formula path, unit basis, and power cross-check.

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Quick reference table

Ohm's Law connects voltage, current, and resistance with V = I x R. Power relationships add P = V x I, P = I^2 x R, and P = V^2 / R. Use this chart as the formula-selection worksheet, then use the calculator result when values, units, and notes need a repeatable record.

Ohm's Law formula selection

Ohm's Law formula selection
Solve forUse this formulaKnown values needed
VoltageV = I x RCurrent and resistance
CurrentI = V / RVoltage and resistance
ResistanceR = V / IVoltage and current
Power from voltage and currentP = V x IVoltage and current
Power from current and resistanceP = I^2 x RCurrent and resistance
Power from voltage and resistanceP = V^2 / RVoltage and resistance

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Calculator result cross-checks

Calculator result cross-checks
Result showsCheck nextWhy it matters
Current solvedPower should also match P = V x IConfirms the current value was not entered with the wrong unit
Resistance solvedConfirm the value is ohms, not kilohms or milliohmsPrefix mistakes can change the answer by 1,000x
Power solvedConfirm whether the load is DC, resistive AC, or an AC load with power factorSimple Ohm formulas are not a full AC power model
Very small or very large resultRecheck decimal placement and source unitsMagnitude checks catch most formula-entry errors

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Ohm's Law search intent map

Ohm's Law search intent map
If the search asks forUse this chart forOpen the calculator when
Ohm's Law formulaChoosing the voltage, current, resistance, or power equationActual values need to be entered and checked with units
Voltage current resistance formulaMatching the known values to the unknown variableThe result should be saved with a project note or example
Watts volts amps relationshipConfirming whether the power relationship is P = V x I, I^2 x R, or V^2 / RPower factor, phase, or AC equipment behavior changes the model
Resistance from watts and voltsSelecting the derived resistance formula before calculationThe load is not a simple resistive load or the result looks unusual

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Formula basis

V = I x R, I = V / R, R = V / I, P = V x I, P = I^2 x R, and P = V^2 / R.

  • V is voltage in volts.
  • I is current in amperes.
  • R is resistance in ohms.
  • P is real power in watts for the selected circuit condition.

Worked examples

Find current from voltage and resistanceFor 120 V across 24 ohms, I = 120 / 24 = 5 A. The matching power check is P = 120 x 5 = 600 W.
Find resistance from power and voltageFor a 240 V resistive load at 4,800 W, R = V^2 / P = 240^2 / 4,800 = 12 ohms before checking whether the load model really is resistive.
Assumptions. Review the assumptions listed for this chart before applying its values to your case.
  • The formulas assume a simple DC or resistive AC relationship unless the calculator documents a different load model.
  • Power factor, phase, impedance, duty, and equipment behavior must be handled separately for AC project work.
  • This chart is for formula selection and result checking, not conductor, breaker, fuse, or equipment sizing.
Code and standard notes. Planning limits that should be checked before final equipment selection.
  • This chart is a formula reference and does not size conductors, breakers, fuses, or equipment.

How to use this chart

1Choose the unknown variableStart by identifying whether the result needs voltage, current, resistance, or power before selecting a formula.
2Match known inputsUse the formula row where the known voltage, current, resistance, or power values match the data you actually have.
3Cross-check the resultAfter the calculator result, use an alternate power relationship or magnitude check to catch unit and decimal-place errors.
Worksheet checklist. Record source basis, review gaps, and assumptions before using the chart result.
  • Record known valuesWrite the known voltage, current, resistance, or power values with units before choosing the equation.
  • Mark the solved variableDocument which variable was solved so the result can be checked against the matching calculator output.
  • Add load contextNote whether the load is resistive, DC, or an AC load needing power-factor or impedance review later.
Common mistakes to avoid. Review these before turning chart current into an equipment decision.
  • Mixing watts, volts, amps, and ohms without first identifying which variable is being solved.
  • Using simple resistance formulas for AC equipment where impedance, phase, or power factor changes the result.
  • Entering milliamps, kilohms, or kilowatts into a base-unit formula without converting the prefix first.

Frequently asked questions

These answers explain how to use the chart without turning a quick reference into a final design decision.

Can this chart replace the calculator?
No. Use the chart to choose the right equation, then use the calculator when you need a documented input set, units, and result record.
Does Ohm's Law include power factor?
No. The basic formula chart covers simple voltage, current, resistance, and power relationships. AC loads with power factor need a dedicated calculator path.
Why show more than one power formula?
Different project notes may provide voltage and current, current and resistance, or voltage and resistance. The chart helps choose the formula that matches the values actually known.
Which Ohm's Law formula should I use first?
Start with the row that matches the two known values. If voltage and resistance are known, solve current with I = V / R; if voltage and current are known, solve power with P = V x I or resistance with R = V / I.

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