Ohm's Law Calculator
Enter any two known values, voltage, current, resistance or power, and the remaining two are calculated instantly. Handles all formula paths: V=IR, P=VI, P=I²R and V²/R.
Last updated: May 2026
Enter any two values above to calculate the remaining two.
Handles all formula combinations: V=IR · P=VI · P=I²R · P=V²/R
Ohm's Law, all four formula paths
Most calculators only solve for one variable at a time and require you to pick the formula manually. This tool takes any two known values and calculates both remaining unknowns in one step. Particularly useful when the only known pair is resistance and power (P=I²R path) or voltage and power (V²/R path), formula paths that are easy to miscalculate under time pressure. It sits alongside the rest of the Electronics Hub tools, so a result here flows naturally into resistor, LED and power-supply calculations.
Formula reference
| Known | Solve for I | Solve for V | Solve for R | Solve for P |
|---|---|---|---|---|
| V + I | - | - | V ÷ I | V × I |
| V + R | V ÷ R | - | - | V² ÷ R |
| V + P | P ÷ V | - | V² ÷ P | - |
| I + R | - | I × R | - | I² × R |
| I + P | - | P ÷ I | P ÷ I² | - |
| R + P | √(P ÷ R) | √(P × R) | - | - |
Typical use cases
- Checking a current-limiting resistor before wiring an LED circuit
- Verifying a resistor's power rating before soldering (P = I²R)
- Calculating load current from a known supply voltage and impedance
- Reverse-engineering component values from measured bench readings
Where Ohm's Law sits in a circuit build
This is the calculation everything else on the bench grows out of. Voltage, current, resistance and power are the four numbers every later tool either needs as an input or hands you as an output, so a circuit usually starts here and branches outward:
- You are here: the four base numbers. Pin down any two of V, I, R and P and the rest follow. Everything below is a special case of what this calculator just gave you.
- Tame an LED with a series resistor. Supply minus forward voltage, divided by target current, is pure Ohm's Law. The LED resistor calculator does it and rounds to the nearest real part.
- Split a voltage with two resistors. A divider sets a reference or scales a signal into range. The voltage divider calculator gives Vout and the current the divider wastes.
- Match the number to a physical part. The bin gives you bands and codes, not values. Decode them with the resistor color code and the capacitor code calculator.
- Size the supply for the whole load. Add up the power and check the rail can deliver the current. The watts, volts and amps calculator covers DC and AC.
- Watch the wire on a long run. Thin cable over distance drops voltage and wastes power. The voltage drop calculator sizes it for LED strips, solar and automotive runs.
- Predict how long a battery lasts. Load current against pack capacity gives runtime. The battery life calculator adds a real-world efficiency factor.
For anything driven by AC rather than steady DC, resistance gives way to reactance and impedance: the impedance calculator and the reactance tools take over where this page leaves off.
A note from my own bench: the number I reach for most is the LED current-limiting resistor, because a microcontroller pin punishes a mistake instantly. An Arduino pin gives 5 V and tolerates about 20 mA; an ESP32 pin gives 3.3 V and only about 12 mA. Get the resistor wrong and you either get a sad dim LED or a dead GPIO. I work it from Ohm's Law every time, supply minus the LED forward voltage divided by the current I want, then round up to the nearest resistor I actually own so the pin stays inside its limit.
Frequently Asked Questions
What is the Ohm's Law formula?
Ohm's Law defines the relationship between voltage (V), current (I) and resistance (R): V = I × R, I = V / R, R = V / I. Power is linked through P = V × I, P = I² × R and P = V² / R. Knowing any two of the four quantities lets you solve the other two. Enter any two values above to calculate all four simultaneously.
How do you calculate current using Ohm's Law?
Divide voltage by resistance: I = V / R. For a 12 V supply across a 470 Ω resistor, I = 12 / 470 = 0.0255 A (25.5 mA). Alternatively, if you know power: I = P / V or I = √(P / R). Enter any two known values in the calculator above to get current and the remaining quantities instantly.
How do you calculate power dissipation in a resistor?
Use P = I² × R or P = V² / R. For a 220 Ω resistor on a 5 V supply: P = 25 / 220 = 0.114 W, well within a standard 1/4 W (0.25 W) rating. On a 12 V supply: P = 144 / 220 = 0.655 W, which exceeds 1/4 W and requires a 1 W rated component. Always verify wattage rating before soldering.
What happens if I enter all four values?
The calculator uses the first valid pair it finds in priority order: V+I, then V+R, then V+P, then I+R, then I+P, then R+P. To get a specific formula path, clear the fields you don't want used as inputs. For most bench checks, entering exactly two values gives the clearest result.
How do I convert mA to A before entering the value?
Divide milliamps by 1000. A 20 mA LED becomes 0.02 A. The quick-value buttons handle common mA values for you, click "20 mA" and 0.02 is entered automatically. For microamps (µA), divide by 1,000,000 (e.g., 500 µA = 0.0005 A).
Why does power dissipation matter when choosing a resistor?
Standard 1/4 W resistors fail if they dissipate more than 0.25 W continuously. A 220 Ω resistor on a 12 V supply dissipates 0.655 W, that needs a 1 W or larger rated component. Use P = V²/R or P = I²R to check before ordering. Running a resistor above its rated wattage causes drift, overheating or permanent failure.
Does Ohm's Law apply to AC circuits?
For resistive loads (heaters, incandescent bulbs, resistors) yes, apply the same formulas with RMS voltage and current. For reactive components (capacitors, inductors, motors) the relationship involves impedance (Z) rather than pure resistance (R), and the formulas become V = I × Z. For AC power calculations including power factor, use the Watts / Volts / Amps Calculator.
How do I check if a resistor value is in the E12 or E24 series?
Common E12 values (10% tolerance): 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82 Ω (and ×10 multiples). E24 adds: 11, 13, 16, 20, 24, 30, 36, 43, 51, 62, 75, 91. If your calculation gives 185 Ω, the nearest E12 is 180 Ω or 220 Ω. Pick the one that keeps current within safe limits, typically the higher value for LED circuits.
Methodology and sources
This tool applies Ohm's Law and the electrical power relationships to any two known quantities among voltage (V), current (I), resistance (R) and power (P), then solves for the remaining two. It is direct algebra on the established physics, not a lookup of tabulated values.
- Method: Ohm's Law, V = I × R, combined with the power law P = V × I. Derived paths follow algebraically: R = V / I, I = V / R, P = V² / R, P = I² × R, R = P / I², V = √(P × R) and I = √(P / R). The calculator picks the first complete pair in the order V+I, V+R, V+P, I+R, I+P, R+P.
- Standards and sources: Standard electrical physics, Ohm's Law and Joule's power law. No wiring code or ampacity table is involved; the math is the same in every reference text. Nearest standard resistor suggestions use the IEC 60063 E12 and E24 preferred-value series.
- Assumptions and limits: Assumes a purely resistive, linear, direct-current (DC) circuit where resistance is constant. For AC with reactive components (capacitors, inductors, motors) you must use impedance (Z) and account for power factor, so results here will not apply directly. Values are taken as ideal and exact, with no allowance for component tolerance, temperature drift or wiring losses.
Reviewed and maintained by Rick Oosterling, who builds and wires 12 V, solar and EV systems hands-on. Last reviewed: June 2026. This calculator is a planning and learning aid, not a substitute for a qualified professional or your local wiring and building code. Always confirm component ratings and circuit safety before building.
Next step in this workflow
Resistance solved: now check the full power picture.