Resistance Calculator
Find the resistance from the voltage across a component and the current through it — R = V ÷ I.
Any two of volts, amps and ohms give the third — and the watts.
How the resistance calculator works
Ohm's law rearranged for resistance: divide the voltage across something by the current through it. This is how you size a resistor when you know the drop you need and the current you want — the LED case, where R is the difference between supply and forward voltage divided by the target current. The power row tells you what rating that resistor has to be, which is the part people forget.
Formula: V = I R · P = V I
Worked examples
| Inputs | Result | Note |
|---|---|---|
| 12 V at 2 A | 6 | 6 Ω |
| An LED resistor: 3.2 V drop at 20 mA | 160 | 160 Ω — use the next size up, 180 Ω |
| 230 V at 10 A | 23 | 23 Ω, and 2.3 kW of heat |
FAQFrequently asked questions
How do I calculate resistance?
Divide the voltage across the component by the current through it: 12 V at 2 A is 6 Ω.
How do I choose an LED resistor?
Subtract the LED's forward voltage from the supply, then divide by the current you want. For 5 V, a 1.8 V LED and 20 mA: (5 − 1.8) ÷ 0.02 = 160 Ω.
What resistor rating do I need?
Check the power row and pick a resistor comfortably above it — a quarter-watt part in a half-watt job will cook.
Why is my measured resistance different?
Meter leads add a fraction of an ohm, and resistance rises with temperature — noticeable in filaments and motor windings.
What are the colour bands?
They encode the value and tolerance: two or three significant digits, a multiplier, then a tolerance band.
Where these figures come from
- NIST — CODATA 2018 fundamental physical constants — G, g₀, R, c
- NIST Special Publication 811 — Guide for the use of the International System of Units — unit conversions
- The Engineering ToolBox — material properties — specific heats, expansion coefficients, densities
- National Measurement Institute — Australia's measurement authority
Last checked: September 2026. Constants are the CODATA 2018 values; formulas are the standard textbook forms.