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Resistance Unit Converter

Convert resistance between milliohms, ohms, kilohms, megaohms, and gigaohms.

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Formula

1kΩ=1000Ω,1MΩ=106Ω1 kΩ = 1000 Ω, 1 MΩ = 10⁶ Ω

How It Works

This calculator converts between ohms, milliohms, kilohms, megaohms, and gigaohms for electronics engineers, circuit designers, and test engineers. Per SI Brochure (BIPM), the ohm is defined as V/A = kg·m^2/(A^2·s^3), traceable to quantum Hall resistance R_K = h/e^2 = 25,812.80745 ohm exactly (2019 SI redefinition). Resistance spans 15 orders of magnitude: milliohms for current shunts (1-100 mohm), ohms for signal resistors (10-1000 ohm), kilohms for pull-ups (1-100 kohm), megaohms for feedback networks (1-10 Mohm), and gigaohms for insulation testing (>1 Gohm required per IEC 60664). The standard RF impedance is 50 ohm per IEEE 802.3, a compromise between minimum loss (77 ohm) and maximum power handling (30 ohm) in coaxial transmission lines.

Worked Example

Problem

Design a 50 mohm current sense resistor for a 10 A motor driver with 0.5% accuracy. Calculate power dissipation and select appropriate component.

Solution
  1. Resistance: 50 mohm = 0.050 ohm = 50,000 uohm
  2. Voltage at full current: V = I × R = 10 A × 0.05 ohm = 0.5 V (500 mV)
  3. Power dissipation: P = I^2 × R = 100 × 0.05 = 5 W
  4. Accuracy requirement: 0.5% of 50 mohm = 0.25 mohm tolerance
  5. Temperature coefficient: at 100 ppm/C and 50 C rise, drift = 50 × 100e-6 × 50 mohm = 0.25 mohm (matches accuracy budget)
  6. Selection: Vishay WSL2512 (1%, 50 ppm/C, 1 W) with parallel array of 5 × 250 mohm resistors for 5 W total power handling

Practical Tips

  • ✓Current sense resistors per Vishay/Bourns application notes: use 1-100 mohm with 0.1-1% tolerance. 4-terminal Kelvin design eliminates lead resistance error. Power derating: 50% at max temp per MIL-STD-199
  • ✓RF impedance standards: 50 ohm (IEEE 802.3, most RF), 75 ohm (video/CATV per SMPTE), 93 ohm (some legacy ARCNET), 100 ohm differential (Ethernet per IEEE 802.3), 120 ohm (RS-485 per TIA-485-A). Match system impedance exactly
  • ✓Insulation resistance per IEC 60664: Class I equipment > 2 Mohm, Class II > 7 Mohm dry / 2 Mohm humid. Use megohmmeter (500-1000 VDC) for testing. < 1 Mohm indicates contamination or degradation

Common Mistakes

  • ✗Confusing kohm (10^3 ohm) with Mohm (10^6 ohm) - they differ by 1000x. A 10 kohm pull-up draws 100x more current than a 1 Mohm pull-up from the same supply (0.33 mA vs 3.3 uA at 3.3 V)
  • ✗Misreading European notation: 4R7 = 4.7 ohm (R is decimal point), 4K7 = 4.7 kohm, 4M7 = 4.7 Mohm. On schematics, 'R' prefix often means ohms while 'K' means kilohms per IEC 60063
  • ✗Not using 4-wire (Kelvin) connection for mohm measurements - lead resistance of 10 mohm per wire causes 40% error on a 50 mohm measurement. Always use Kelvin clips for < 1 ohm measurements

Frequently Asked Questions

Milliohm-range appears in: current sense resistors (1-100 mohm per Vishay/Bourns), PCB trace resistance (0.5 mohm/square for 1 oz copper per IPC-2152), contact resistance (1-50 mohm for relays/connectors per MIL-STD-1344), and battery internal resistance (10-100 mohm for Li-ion). Low R minimizes power loss: 100 mohm at 10 A = 10 W dissipation.
Divide by 1000: 4700 ohm = 4.7 kohm = 0.0047 Mohm. Multiply by 1000 for reverse: 4.7 kohm = 4700 ohm. Standard E-series values per IEC 60063: E24 gives 5% tolerance values (4.7k, 5.1k, 5.6k...), E96 gives 1% values (4.70k, 4.75k, 4.81k...).
Insulation resistance measures current leakage through insulating materials, specified in Mohm or Gohm. Per IEC 60664 and UL 60950: good insulation > 1 Gohm, acceptable > 100 Mohm, degraded < 10 Mohm. Test at 500-1000 VDC. Surface contamination, moisture, and aging reduce insulation resistance - monitor trending for predictive maintenance.
Per Andrew/CommScope application notes: 50 ohm is a compromise between minimum attenuation (77 ohm in air-dielectric coax) and maximum power handling (30 ohm). At 50 ohm: coax loss is 10% above minimum, power handling is 80% of maximum - optimal for most RF systems. 75 ohm video uses lower loss (closer to 77 ohm) since power is low.

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