Wire Size Calculator

Calculate the correct wire size for a circuit based on amps, distance, and voltage. See AWG, ampacity, and voltage drop for copper or aluminum.

Mason ReidFounder and Lead Estimator at MeasureThen

Last updated 2026-08-21

What Is a Wire Size Calculator?

A wire size calculator finds the smallest safe conductor gauge for a circuit based on the current, the run distance, the voltage, and the allowed voltage drop. It prevents undersized wire that overheats and oversized wire that wastes money. Use it for branch circuits where long runs make voltage drop a real concern.

How It Works

Required cmil = (2 × 12.9 × Current × Distance) ÷ Voltage drop. Pick the smallest AWG whose cmil meets it.

Example

A 20 A circuit, 100 ft run at 120 V with 3% drop: ≈14,333 cmil → 8 AWG copper.

How to Calculate

Enter the circuit current, for example 20 A for a kitchen counter circuit, and the voltage, commonly 120 V.
Measure the one way distance from the panel to the load, say 100 ft.
Set the allowed voltage drop, usually 3% for a branch circuit.
The tool computes the required cross sectional area in circular mils, about 14,333 for this example, and returns the smallest AWG that meets it, which is 8 AWG copper. Confirm the result with a licensed electrician before buying wire.

AWG copper wire ampacity

AWG size	Ampacity at 60°C	Ampacity at 75°C
14 AWG	15 A	15 A
12 AWG	20 A	20 A
10 AWG	30 A	30 A
8 AWG	40 A	50 A
6 AWG	55 A	65 A

Sources

Frequently Asked Questions

What is voltage drop?

The voltage lost along a conductor. Excess drop can damage equipment and is limited by code to ~3% for branch circuits.

Why does distance matter?

Longer runs have more resistance, so they need larger wire to keep drop in check.

Is this for copper or aluminum?

This calculator assumes copper (K = 12.9). Aluminum needs larger sizes.

Mason ReidFounder and Lead Estimator at MeasureThen

Mason Reid founded MeasureThen after two decades of hands-on work in construction and estimating. Across residential and light commercial projects, he sized concrete pours, framed walls, hung drywall, and priced out roofing, fencing, and deck work — learning the exact math that every job depends on. At MeasureThen he writes the formulas, checks every calculator against real-world assumptions, and makes sure each one is simple enough for a homeowner to trust and a contractor to use. When he is not measuring twice, he is helping projects stay on budget.

Every MeasureThen calculator uses simple, transparent formulas and standard industry assumptions drawn from common construction practice. We show the math behind each result and include worked examples so you can follow along and double-check the numbers. Results are estimates: materials, labor, and pricing vary by region and market, so always confirm important figures with a licensed professional and your local building codes.