Gigaohm to Microohm (GΩ to μΩ)

Resistor values are usually written without a decimal point, because decimal points disappear on smudged schematics and photocopies. BS 1852 notation puts the multiplier where the point would go: 4R7 is 4.7 Ω, 4K7 is 4.7 kΩ, and 4M7 is 4.7 MΩ. A value written plainly as "4.7" with no unit is ambiguous and worth checking.

The scale is decimal throughout: 1 MΩ = 1,000 kΩ = 1,000,000 Ω, and 1 Ω = 1,000 mΩ = 1,000,000 µΩ. On this page, 1 GΩ = 1.000000e+15 μΩ.

Milliohms and microohms are the domain of shunts, contact resistance, busbars and battery internal resistance. At these values the resistance of your test leads is no longer negligible, which is why they are measured with a four-wire (Kelvin) connection rather than a standard multimeter.

Megaohms and gigaohms show up in insulation testing and high-impedance input specifications. Readings at this scale are sensitive to humidity and to surface contamination on the board, so they are quoted with test voltage and conditions attached.