Resistance Calculator
Calculate electrical resistance using five methods: Ohm's Law (R=V/I), series resistance, parallel resistance, resistor color code decoding, and power dissipation. Every calculation includes step-by-step working. Free, private, no data leaves your browser.
R = V / I — Enter voltage and current to find resistance.
Rtotal = R1 + R2 + R3 — Resistors in series add up directly.
1/Rtotal = 1/R1 + 1/R2 + 1/R3 — Parallel resistance is always less than the smallest resistor.
Select the 4 color bands on your resistor to decode its resistance value.
Enter voltage and current to calculate power dissipation and resistance.
Result
How Resistance Calculations Work
Electrical resistance measures how strongly a material opposes the flow of electric current. It is measured in ohms (Ω) and is one of the most fundamental quantities in electronics. Ohm's Law, discovered by Georg Simon Ohm in 1827, states that resistance equals voltage divided by current (R = V / I). This relationship is the foundation of circuit analysis. When you know the voltage across a component and the current flowing through it, you can calculate its resistance instantly. This calculator provides step-by-step working for every calculation so you can follow the math and verify your results.
Series vs Parallel Resistance
Resistors can be connected in two fundamental ways: series and parallel. In a series circuit, the total resistance is simply the sum of all individual resistances (Rtotal = R1 + R2 + R3). The current flows through each resistor one after another, and the total resistance is always greater than any single resistor. In a parallel circuit, the reciprocal formula applies: 1/Rtotal = 1/R1 + 1/R2 + 1/R3. Parallel resistance is always less than the smallest individual resistor because the current has multiple paths to flow through. Understanding the difference is critical for designing voltage dividers, current-limiting circuits, and matching impedances in audio and RF electronics.
Reading Resistor Color Codes
Through-hole resistors use colored bands to indicate their resistance value. A standard 4-band resistor has two digit bands, a multiplier band, and a tolerance band. The first two bands represent a two-digit number, the multiplier band scales it by a power of ten, and the tolerance band indicates the precision. For example, brown-black-red-gold means 10 × 100 = 1,000Ω (1KΩ) with ±5% tolerance. The color sequence from 0 to 9 is: black, brown, red, orange, yellow, green, blue, violet, grey, white. Gold and silver multipliers are used for sub-ohm values (0.1 and 0.01 respectively). This calculator visually previews the band colors so you can verify you are reading your resistor correctly.
Resistance in Circuit Design
Resistance plays a central role in every electronic circuit. Current-limiting resistors protect LEDs from burning out. Pull-up and pull-down resistors set default logic levels in digital circuits. Voltage dividers use two resistors in series to scale voltage for sensors and microcontroller analog inputs. Power dissipation calculations are equally important: a resistor that dissipates more power than its rating will overheat and fail. The formula P = V × I gives the total power, while P = I²R and P = V²/R let you calculate power from any two known quantities. Always choose a resistor rated for at least twice the calculated power dissipation to ensure reliable operation and long component life.