Electrical Resistance Converter

Electrical resistance measures opposition to current flow, in ohms (Ω), defined by Ohm's law V = IR. Real-world resistance spans twelve orders of magnitude — from a bond-wire at 10 mΩ to an insulator at 10 GΩ. A typical through-hole resistor ranges from 0.1 Ω to 10 MΩ. Copper wire has very low resistance (0.017 Ω·mm²/m at 20°C); a toaster heating element runs 10-30 Ω; a good insulator is MΩ or higher. Every wire, connector, trace, and component has resistance — keeping track of which ones matter is the daily work of any electrical engineer.

The ohm (Ω) is the SI unit, defined since 2019 via the fixed elementary charge and Planck's constant rather than via a physical standard resistor. One ohm is the resistance that lets 1 A flow at 1 V. Resistance depends on material (copper, aluminum, nichrome for heating, carbon for resistors), length, cross-section, and temperature. The relationship R = ρ × L / A — resistivity times length over area — is the geometric backbone. Copper resistance rises about 0.393% per °C — a hot motor winding has higher resistance than a cold one, which is why motors draw inrush current at startup before thermal equilibrium.

Resistors come in preferred values following E-series (E12, E24, E96 depending on tolerance). E24 gives 5% resistors in 24 values per decade; E96 gives 1% resistors in 96 values. Datasheets also specify temperature coefficient (ppm/°C — parts per million per degree), power rating (1/8 W, 1/4 W, 1/2 W, 1 W are common through-hole; SMD goes down to 1/16 W), and max voltage. Precision metrology uses Vishay or Caddock foil resistors stable to parts per million; everyday electronics uses thick-film chips.

Kilohm to ohm: multiply by 1,000. Megaohm to ohm: multiply by 1,000,000. Milliohm to ohm: divide by 1,000. Series resistors add: R_total = R1 + R2 + R3. Parallel resistors: 1/R = 1/R1 + 1/R2, so two equal parallel resistors give half the resistance — a useful shortcut. Power dissipation: P = I²R = V²/R = VI. A 10 kΩ resistor across 5 V dissipates 2.5 mW — well inside 1/8 W rating. The same 10 kΩ across 100 V dissipates 1 W — needs a beefier part.

• Ohm (Ω)
• Milliohm (mΩ)
• Microhm (µΩ)
• Kilohm (kΩ)
• Megohm (MΩ)
• Gigohm (GΩ)
• Abohm
• Statohm (esu)

• Ohm vs kiloohm vs megohm. Factor-of-1,000 differences. Schematics and datasheets use capitals for multipliers; a '10k' resistor is 10 kΩ = 10,000 Ω. Reading '10M' as '10k' is a 1000× error that will either burn out your resistor or short your circuit.
• Resistor color codes. 4-band standard: first two bands are value, third is multiplier, fourth is tolerance. 10 kΩ 5% = brown/black/orange/gold. Mistakes here create orders-of-magnitude errors. Always verify with a meter if unsure — especially old stock where colors fade.
• Temperature dependence. Carbon film drifts ~500 ppm/°C; metal film ~50-100 ppm/°C; precision foil <5 ppm/°C. A 10 kΩ carbon resistor can shift 50 Ω over a 10°C swing — irrelevant for a pull-up, fatal for a precision ADC reference.
• Power rating vs continuous dissipation. A 1/4 W resistor can dissipate 1/4 W indefinitely only in free air at 25°C. At 70°C ambient, derate 50%; at 100°C, don't exceed 10%. Packaged tightly in an enclosure, derate further.
• Contact resistance. Measuring mΩ-level shunts requires 4-wire (Kelvin) connections. Regular 2-wire measurement includes probe and clip resistance — typically 20-500 mΩ, larger than the target.
• PTC vs NTC thermistors. Positive-temperature-coefficient devices (PPTC fuses) rise in resistance with heat; negative-temperature-coefficient thermistors (common temperature sensors) drop. Swap them by accident and a protective fuse circuit becomes a runaway.

• Current-limiting LED resistor: 220-1,000 Ω typical.
• Pull-up resistor on data lines (I²C, GPIO): 1-10 kΩ.
• Input impedance of an op-amp non-inverting stage: 10 kΩ-1 MΩ.
• Feedback resistor in precision amp: 10 kΩ, 1% metal film.
• Heating element in a 1500 W toaster (120 V): 9.6 Ω.
• Incandescent bulb (cold): a few ohms; hot: 100+ Ω (big temperature coefficient).
• Human body resistance (dry skin): 100 kΩ-1 MΩ.
• Human body (wet or injected current): 1-5 kΩ.
• Copper wire 1 m of 12 AWG (3.31 mm²): 5.2 mΩ.
• 12 V car battery internal resistance: 5-20 mΩ.
• Fresh AA alkaline internal resistance: 150-300 mΩ.
• CAT6 Ethernet cable (per 100 m pair): 9.4 Ω.
• PCB trace resistance (1 oz copper, 10 mil wide, 10 cm long): 50 mΩ.

• For LEDs, calculate R = (V_supply - V_LED) / I_target. A red LED (~2V, 20mA) on 5V needs (5-2)/0.020 = 150 Ω.
• Series resistors sum; parallel resistors' reciprocals sum.
• For current-sensing, low-value shunts (10 mΩ-1 Ω) in series with the load let you measure current by Ohm's law. Use 4-wire measurement below 10 Ω.
• For high voltages, check the resistor's max working voltage — a 1/4 W 1 MΩ might be rated only 200 V despite the low power it dissipates.

Related: Electric Current Converter · Capacitance Converter · Inductance Converter.