Resistance Calculator

Electrical resistance measures a component's opposition to the flow of electric current, measured in Ohms (Ω). Unlike generic calculators that only co

Electrical

Calculation Mode

Solve from circuit parameters or combine multiple resistors

Voltage (V) or Resistor 1 (Ω)

V or Ω

Voltage if Ohm’s Law mode; R1 if combination mode

Current (A) or Resistor 2 (Ω)

A or Ω

Current if Ohm’s Law mode; R2 if combination mode

Optional Resistor 3 (Ω)

Used for 3-resistor series/parallel networks

4-Band Color Code Tolerance

Resistor color band manufacturing tolerance

Formula & Methodology

Ohm's Law: R = V ÷ I; Series: Rtotal = R₁ + R₂ + R₃; Parallel: 1 ÷ Rtotal = (1 ÷ R₁) + (1 ÷ R₂) + (1 ÷ R₃)

In series circuits, resistors connect end-to-end so total resistance is the direct sum of individual values (Rtotal = R₁ + R₂ + ...). In parallel circuits, resistors connect across the same two nodes, meaning total resistance is always less than the smallest individual resistor: Rtotal = 1 ÷ [(1/R₁) + (1/R₂) + ...]. Standard electronic resistors display 4 colored bands representing two significant digits, a decimal multiplier, and a tolerance band.

Calculated Resistance (R = V ÷ I)

12.000 Ω Ohms (Ω)

Overview

Electrical resistance measures a component's opposition to the flow of electric current, measured in Ohms (Ω). Unlike generic calculators that only compute R = V ÷ I, this comprehensive Resistance Calculator solves resistance from circuit voltage and current, computes equivalent resistance for complex series and parallel resistor networks, and decodes standard 4-band electronic resistor color codes.

Safety Notice: Resistance calculations are theoretical reference estimates for circuit design and academic study. Physical electronic resistors vary with manufacturing tolerances (±1% to ±10%) and temperature coefficients. Verify all high-voltage and power circuits with a licensed professional.

Worked Examples

Calculate Resistance from 120V and 10A (Ohm’s Law Mode)

Voltage = 120 V, Current = 10 A.

Apply Formula: R = V ÷ I
Compute Resistance: 120 V ÷ 10 A = 12.0 Ω
Identify 4-Band Color Code for 12 Ω: Brown (1), Red (2), Black (×1), Gold (±5%)

Final Answer: 12.000 Ω (Color Code: Brown – Red – Black – Gold)

Two 100Ω Resistors in Parallel

R1 = 100 Ω, R2 = 100 Ω, Combination mode.

Parallel Formula: 1 ÷ Rtotal = (1/100) + (1/100) = 0.01 + 0.01 = 0.02
Invert: Rtotal = 1 ÷ 0.02 = 50.0 Ω
Series Equivalent for Comparison: 100 + 100 = 200.0 Ω

Final Answer: Parallel: 50.000 Ω | Series: 200.000 Ω

Three Resistors in Network: 10Ω, 20Ω, and 30Ω

R1 = 10 Ω, R2 = 20 Ω, R3 = 30 Ω.

Series: 10 + 20 + 30 = 60.0 Ω
Parallel: 1 ÷ [(1/10) + (1/20) + (1/30)] = 1 ÷ [0.100 + 0.050 + 0.0333] = 1 ÷ 0.1833 = 5.455 Ω

Final Answer: Series: 60.000 Ω | Parallel: 5.455 Ω

How to Use This Calculator

Choose whether to calculate resistance using Ohm’s Law (from V and I) or from series/parallel resistor combinations.

Enter your circuit voltage and current, or enter individual resistor values R1, R2, and optional R3.

Select the color code tolerance (Gold ±5%, Silver ±10%, or Brown ±1%).

Click Calculate to see the exact resistance, network equivalents, and 4-band EIA color code representation.

Reference Table

Color Band	1st & 2nd Digit Value	Multiplier Factor	Tolerance Band
Black	0	× 1 (10⁰)	—
Brown	1	× 10 (10¹)	± 1% (F)
Red	2	× 100 (10²)	± 2% (G)
Orange	3	× 1,000 (10³ / 1k)	—
Yellow	4	× 10,000 (10⁴ / 10k)	—
Green	5	× 100,000 (10⁵ / 100k)	± 0.5% (D)
Blue	6	× 1,000,000 (10⁶ / 1M)	± 0.25% (C)
Gold	—	× 0.1 (10⁻¹)	± 5% (J)
Silver	—	× 0.01 (10⁻²)	± 10% (K)

Frequently Asked Questions

Why is parallel resistance always smaller than the smallest resistor?

Connecting resistors in parallel creates multiple alternate pathways for electric current to flow. Adding an extra pathway increases overall circuit conductance, which decreases total equivalent resistance.

How do you read a 4-band resistor color code?

The first two bands represent significant digits, the third band represents the decimal multiplier (powers of 10), and the fourth band (typically Gold for ±5% or Silver for ±10%) indicates manufacturing tolerance.

What is conductance and how does it relate to resistance?

Conductance (G), measured in Siemens (S), is the reciprocal of resistance: G = 1 / R. While resistance measures opposition to current, conductance measures how easily current passes through a component.

What happens to resistor value when temperature increases?

Most standard conductors and carbon film resistors have a positive temperature coefficient, meaning their resistance rises slightly as operating temperature increases.

What is the standard tolerance of carbon film resistors?

Standard commercial through-hole carbon film resistors typically feature a gold tolerance band indicating ±5% tolerance. High-precision metal film resistors feature brown bands indicating ±1% tolerance.