Ohm's Law Calculator
Enter any two known values to calculate voltage, current, resistance and power
Leave the values you want to find blank. Enter any two known values — the other two will be calculated.
// All Ohm's Law Formulas
// Resistor Colour Code Chart
| Colour | Digit | Multiplier | Tolerance |
|---|---|---|---|
| Black | 0 | ×1 | — |
| Brown | 1 | ×10 | ±1% |
| Red | 2 | ×100 | ±2% |
| Orange | 3 | ×1k | — |
| Yellow | 4 | ×10k | — |
| Green | 5 | ×100k | ±0.5% |
| Blue | 6 | ×1M | ±0.25% |
| Violet | 7 | ×10M | ±0.1% |
| Grey | 8 | ×100M | ±0.05% |
| White | 9 | ×1G | — |
| Gold | — | ×0.1 | ±5% |
| Silver | — | ×0.01 | ±10% |
Ohm's Law Explained
Ohm's Law states that voltage across a conductor is directly proportional to the current flowing through it, provided temperature remains constant. The constant of proportionality is resistance.
The Four Quantities
Practical Examples
A 12V car battery powering a 60W headlight: I = P/V = 60/12 = 5A. The filament resistance: R = V/I = 12/5 = 2.4Ω. A USB charger supplying 5V at 2A delivers: P = V×I = 5×2 = 10W.
Ohm's Law: The Water-Pipe Guide to Electricity
Built and verified by Andrius R. · Updated June 2026
Three quantities — voltage, current, resistance — and one relation between them: V = I × R. Add the power formula P = V × I and you can reason about nearly every plug, fuse, cable and electricity bill in your life.
The analogy that makes it click
Think of a water pipe: voltage is the pressure pushing water, current is the flow rate, resistance is how narrow the pipe is. More pressure → more flow; narrower pipe → less flow. Ohm's law is just that sentence in symbols: I = V ÷ R. (The analogy even extends: a battery is a pump, a switch is a valve, and a short circuit is a burst pipe — huge flow, nothing useful done.)
A real appliance, fully worked
Current: I = P ÷ V = 2,000 ÷ 230 ≈ 8.7 A.
Element resistance: R = V ÷ I = 230 ÷ 8.7 ≈ 26.5 Ω (or directly: R = V² ÷ P).
Now the same kettle's logic explains your fuse box: a UK 13 A plug fuse passes the kettle's 8.7 A happily, but a fault that drops resistance toward zero sends current soaring (I = V ÷ R with tiny R), and the fuse melts first — by design. It also explains why US kitchens (120 V) struggle with big kettles: the same 2,000 W would need 16.7 A, beyond a standard 15 A circuit, which is why American kettles are weaker and electricians treat voltage as destiny.
The power family — and your electricity bill
P = V × I combines with Ohm's law into P = I²R and P = V²/R. The I²R form is the important one for wiring: heat in a cable grows with the square of current, which is why high-power devices need thick cables, why transmission lines run at hundreds of kilovolts (raise V, slash I, slash I²R losses), and why a loose, high-resistance connection gets hot — the household fire mechanism. Your bill is just power × time: the 2 kW kettle running 6 minutes uses 0.2 kWh — at €0.25/kWh, five cents per boil. Energy units and conversions live in the energy calculator.
Series vs parallel in one minute
Series (one path): resistances add, current is shared by all, and one break kills the chain — old Christmas lights. Parallel (multiple paths): each branch gets full voltage, currents add, total resistance drops (1/R = 1/R₁ + 1/R₂). Homes wire everything in parallel — every socket sees 230 V regardless of what else is on — at the cost that plugging in more things draws more total current, which is what the circuit breaker is counting.
Honest fine print
Ohm's "law" is really a material property: it holds beautifully for metals at steady temperature ("ohmic" conductors) but bends elsewhere — a bulb filament's resistance climbs as it heats, LEDs and diodes don't obey it at all, and batteries have internal resistance that sags their voltage under load (why headlights dim while cranking the engine). And the safety footnote worth repeating: current, not voltage, is what harms — tens of milliamps across the chest can be lethal, which is why dry skin's high resistance is doing you a quiet favor every day, and why wet hands and mains electricity must never meet.
From the Blog
// The VIR Triangle
Cover the value you want to find in the V/I/R triangle. What's left is the formula: V on top, I and R on the bottom (multiply or divide).
// Safe Current
The human body's resistance is roughly 1,000–100,000 Ω. At 230V mains: I = 230/1000 = 230mA — well above the 100mA lethal threshold.
// Series vs Parallel
Resistors in series: Rtotal = R1+R2. In parallel: 1/Rtotal = 1/R1 + 1/R2. Parallel circuits split current; series circuits split voltage.
// Power Dissipation
P = I²R is important for heat calculations. Double the current → four times the heat. This is why fuses blow when current gets too high.