Calculate Length from Resistance

Using a 4-wire milliohm meter, you can precisely determine the remaining length of wire on a spool without unspooling it. You must know the wire gauge and ensure the temperature is standard (20°C / 68°F), or apply a temperature correction factor.

Standard Copper Resistance at 20°C

AWG	Ohms per 1000 ft	Ohms per km
10	0.9989	3.277
12	1.588	5.211
14	2.525	8.286
16	4.016	13.17
18	6.385	20.95
20	10.15	33.31

Formula: Length = (Measured Resistance / Resistance per Unit)

Do not use a standard multimeter for this. The contact resistance of standard 2-wire test leads will introduce massive errors on low-resistance measurements. A Kelvin bridge (4-wire) is mandatory.