What Is Resistance? Ohms, Resistors, and Why It Matters
Resistance is the opposition to the flow of electric current, measured in ohms. Learn what resistance is, how it works, what affects it, and how to calculate it in a circuit.
Resistance is the opposition to the flow of electric current in a circuit. It controls how much current flows for a given voltage. Resistance is measured in ohms (Ω) and is one of the three fundamental properties of any circuit, alongside voltage and current.
How resistance works
When electrons flow through a material, they collide with atoms in the material's structure. These collisions slow the electrons down and convert some electrical energy into heat. The more collisions, the higher the resistance.
In the water analogy: if voltage is water pressure and current is flow rate, then resistance is the narrowness of the pipe. A narrow pipe resists the flow of water — even with high pressure, less water gets through. Similarly, a high-resistance component limits how much current can flow even when voltage is high.
The relationship between resistance, voltage, and current is defined by Ohm's Law:
R = V / IWhere:
- R = resistance in ohms (Ω)
- V = voltage in volts (V)
- I = current in amperes (A)
Calculate resistance, voltage, or current instantly with our Ohm's Law Calculator.
The ohm explained
One ohm is the resistance that allows exactly one ampere of current to flow when one volt is applied. In real circuits, resistance values range from fractions of an ohm to millions of ohms:
| Unit | Symbol | Value | Example |
|---|---|---|---|
| Milliohm | mΩ | 0.001 Ω | Wire connections, PCB traces |
| Ohm | Ω | 1 Ω | Current sense resistors |
| Kilohm | kΩ | 1,000 Ω | Pull-up/pull-down resistors, LED current limiters |
| Megohm | MΩ | 1,000,000 Ω | Input impedance, leakage paths |
What affects resistance
The resistance of a conductor depends on four factors. This relationship is expressed by the resistivity formula:
R = ρ × L / AWhere:
- ρ (rho) = resistivity of the material (Ω·m)
- L = length of the conductor (m)
- A = cross-sectional area (m²)
1. Material
Different materials have different resistivities. Conductors like copper and silver have very low resistivity. Insulators like rubber and glass have extremely high resistivity.
| Material | Resistivity (Ω·m) | Category |
|---|---|---|
| Silver | 1.59 × 10⁻⁸ | Conductor |
| Copper | 1.68 × 10⁻⁸ | Conductor |
| Aluminum | 2.65 × 10⁻⁸ | Conductor |
| Carbon (graphite) | 3–60 × 10⁻⁵ | Semi-conductor |
| Silicon (pure) | 640 | Semiconductor |
| Glass | 10¹⁰ – 10¹⁴ | Insulator |
| Rubber | 10¹³ | Insulator |
2. Length
Longer conductors have more resistance. Doubling the length of a wire doubles its resistance, because electrons must travel through more material and encounter more collisions.
3. Cross-sectional area
Thicker conductors have less resistance. Doubling the cross-sectional area halves the resistance, because there are more paths for electrons to flow through simultaneously. This is why power cables use thick wire and signal wires can be thin.
4. Temperature
In most metals, resistance increases with temperature. As temperature rises, atoms vibrate more and cause more collisions with flowing electrons. This is why incandescent light bulbs have much higher resistance when hot than when cold. Some materials (like carbon and silicon) behave differently — their resistance decreases with temperature.
Resistors: components designed for resistance
A resistor is a passive component manufactured to provide a precise, known resistance value. Resistors are among the most common components in electronics. Their main uses include:
- Limiting current: A series resistor protects LEDs and other sensitive components from excessive current. See our LED resistor calculator.
- Dividing voltage: Two resistors in series create a voltage divider, producing a lower voltage from a higher source.
- Setting bias points: Resistors set the operating conditions for transistors and op-amps.
- Pull-up and pull-down: Resistors connected to power or ground ensure digital inputs have a defined logic level when no signal is present.
Resistor values are marked using color bands or printed numbers. Use our resistor color code calculator to decode them. To learn about the special case of resistors with no resistance at all, see our zero ohm resistor guide.
Resistance in series and parallel circuits
How resistance combines depends on whether components are in series or parallel.
Resistance in series
Resistors in series add directly. The total resistance is the sum of all individual resistances:
R_total = R₁ + R₂ + R₃ + ...Three 1 kΩ resistors in series = 3 kΩ total. Series resistance is always higher than any individual resistor.
Resistance in parallel
Resistors in parallel combine using the reciprocal formula:
1/R_total = 1/R₁ + 1/R₂ + 1/R₃ + ...For two resistors, this simplifies to:
R_total = (R₁ × R₂) / (R₁ + R₂)Three 1 kΩ resistors in parallel = 333 Ω total. Parallel resistance is always lower than the smallest individual resistor. For detailed calculations with voltage drops, see our voltage drop guide.
How to measure resistance
Resistance is measured with an ohmmeter or a multimeter set to resistance mode (Ω). The meter sends a small known current through the component and measures the resulting voltage to calculate resistance.
Steps to measure resistance
- Disconnect the component from the circuit (measuring resistance in-circuit gives false readings because of parallel paths).
- Set the multimeter to resistance mode (Ω).
- Touch the probes to each end of the component.
- Read the resistance value on the display.
For a full walkthrough with photos, see our guide to reading resistors, which covers both color code identification and multimeter measurement.
Resistance and power dissipation
When current flows through resistance, electrical energy is converted to heat. The power dissipated is calculated by:
P = I² × ROr equivalently:
P = V² / RThis is why resistors have power ratings (typically ¼W, ½W, 1W, etc.). If the actual power dissipation exceeds the rating, the resistor overheats and can fail. For example, a 220Ω resistor carrying 100 mA dissipates: P = (0.1)² × 220 = 2.2W — far too much for a standard ¼W resistor. You would need a 3W or higher rated resistor, or redesign the circuit to reduce the current.
Common mistakes with resistance
- Measuring resistance in-circuit. Other components create parallel paths that change the reading. Always disconnect the component or at least power off the circuit before measuring.
- Ignoring power ratings. A resistor with the correct ohm value but insufficient power rating will overheat. Always check that actual power dissipation stays below the resistor's rating with margin.
- Confusing resistance with impedance. Resistance applies to DC circuits. Impedance is the AC equivalent and includes the effects of capacitors and inductors. At DC, impedance equals resistance.
- Forgetting wire resistance. Long or thin wires have measurable resistance that can cause unexpected voltage drops, especially in high-current circuits. This is why power distribution uses thick cables.
Summary
Resistance is the opposition to current flow, measured in ohms (Ω). It depends on the material, length, cross-sectional area, and temperature of the conductor. Ohm's Law (R = V / I) connects resistance to voltage and current. Resistors in series add directly; in parallel, the total is always less than the smallest individual value. Resistors are components designed to provide specific resistance values for current limiting, voltage division, and biasing. Understanding resistance is essential for designing circuits that are safe, efficient, and functional.