Wire Spool Length Calculator

Calculate Wire Length from Resistance Measurement

Measure resistance with a multimeter to determine how much wire is on a spool

Wire Information

Wire Gauge (AWG) American Wire Gauge size

Measured Resistance

Use multimeter to measure end-to-end resistance

10 AWG

12 AWG

18 AWG

Temperature Correction (Reference: 20°C / 68°F)

-20°C 20°C (68°F) - Standard 100°C

Wire resistance increases ~0.393% per °C above 20°C

Selected Wire Properties

Diameter

0.0808"

Resistance/1000ft

1.588 Ω

Cross Section

6530 CM

Max Ampacity

20 A

Measurement Diagram

📐 The Principle

Every wire gauge has a known resistance per unit length. By measuring the total resistance of the wire on the spool and dividing by the resistance per foot (or meter), you can calculate the total length.

Length = Measured Resistance ÷ Resistance per Unit Length

Calculated Wire Length

Wire Length

944.8

feet

287.9 meters

Wire Weight

18.7

lbs

8.5 kg

Resistance/Foot

1.59

mΩ/ft

At reference temperature

Temp Correction

1.00

factor

20°C reference

Calculation Formula

Length (ft) = R_measured ÷ R_{per ft} × Temp Factor

R_measured = 1.5 Ω | R_{per 1000ft} = 1.588 Ω | Temp Factor = 1.000

How to Measure Wire Resistance

Set Multimeter to Ohms (Ω)

Select the resistance measurement mode. For long wires, you may need to use a lower range (200Ω or 2000Ω).

Access Both Wire Ends

Locate both ends of the wire on the spool. You may need to unwind a small amount to access each end.

Measure End-to-End

Touch probes to bare wire at each end. Record the stable reading in ohms. Subtract lead resistance if significant.

⚠️ Important Notes

Zero your leads: Touch probes together and note the reading - subtract this from your measurement
Clean connections: Ensure good contact with bare copper - oxide layers add resistance
Temperature matters: Resistance increases with temperature - adjust if wire is hot or cold
Stranded vs Solid: This calculator uses solid wire values; stranded wire may vary slightly

AWG Wire Resistance Reference Table (Copper @ 20°C)

AWG	Diameter (in)	Diameter (mm)	Ω/1000 ft	Ω/km	Circular Mils	Weight (lb/1000ft)

* Values for solid copper wire at 20°C (68°F). Aluminum has approximately 1.6× higher resistance.