Free U.S. planning tool

Wire Size Calculator

Wire Size Calculator gives U.S. homeowners, contractors, and DIY planners a transparent way to estimate a conservative planning gauge from load current and voltage-drop target. Enter the project measurements below and the tool applies the displayed formula instead of hiding the math behind a black-box result. The table intentionally simplifies ampacity and voltage drop. Insulation rating, temperature, terminals, raceway fill, bundling, conductor use, motor circuits, and local code can require a different size.

Calculate your estimate

Use the example values as a guide, then enter your own measurements.

Planning use only: This calculator is informational and does not replace code-compliant design, equipment selection, or qualified professional review.

Results

Planning wire size—

Estimated voltage drop— V

Estimated drop—

How to use this wire size calculator

Enter the project measurements in the units shown beside each field.
Choose any material, circuit, shape, or planning options that appear for this tool.
Review the result together with the visible formula and assumptions before ordering or budgeting.
Adjust waste, contingency, or project inputs when your supplier, plan, or field conditions provide better values.

What this calculator estimates

This calculator is built around the quantities that drive a real wire size decision. The result is intended to help you check a material order, compare a budget, or sanity-check a project before requesting supplier or contractor pricing. Use nameplate or measured electrical values whenever possible. A planning calculator can expose relationships, but it cannot see every code condition in the actual installation. Because field conditions and products differ, the calculator keeps assumptions visible so you can replace them with project-specific values when better information is available.

Start with field measurements rather than dimensions copied from an old plan when the site is available. Small dimension errors can become meaningful when multiplied across area, volume, or repeated components.

Keep the unit shown beside each input. If a supplier sells in a different unit, convert the final result only after the base calculation so the math stays easy to audit.

Round purchase quantities up where products are sold only in full bags, boards, sheets, pallets, bundles, or containers. The calculator marks whole-unit outputs where that behavior is appropriate.

Wire Size Calculator formula

Select the first supported AWG that meets the simplified ampacity table and keeps calculated voltage drop at or below the selected percentage.

Inputs used

Supply voltage (V)
Load current (A)
One-way conductor length (ft)
Maximum voltage drop (%)
Conductor material

Worked example

Worked example: enter Voltage: 240, Amps: 30, Length: 75, Max Drop Pct: 3, Material: copper. The calculator applies “Select the first supported AWG that meets the simplified ampacity table and keeps calculated voltage drop at or below the selected percentage.”. The example returns Planning wire size: 10 AWG; Estimated voltage drop: 4.5 V; Estimated drop: 1.87%. Use this result to understand the input relationship, then replace every example value with measurements from your own project.

Assumptions and limitations

Final conductor sizing and protection should be verified by a qualified electrician under the applicable electrical code.
This tool cannot inspect site conditions, product specifications, code requirements, workmanship, moisture, soil, structure, access, or hidden damage. Verify any assumption that materially affects safety, cost, or ordering.

Frequently asked questions

What does the wire size calculator calculate?

It estimates a conservative planning gauge from load current and voltage-drop target. The table intentionally simplifies ampacity and voltage drop. Insulation rating, temperature, terminals, raceway fill, bundling, conductor use, motor circuits, and local code can require a different size.

How accurate is this wire size calculator?

The result follows the formula shown on the page using the measurements you enter. Accuracy depends mostly on measuring the real project dimensions and choosing assumptions that match the product or installation. Round measurements only after you have captured the important dimensions, because early rounding can compound across a larger project.

Which measurements matter most for this wire size calculator?

Use nameplate or measured electrical values whenever possible. A planning calculator can expose relationships, but it cannot see every code condition in the actual installation. The field labels identify the expected U.S. units so you can avoid mixing feet, inches, square feet, or other measures.

Should I add extra material or contingency when using the Wire Size Calculator?

If the calculator includes a waste or extra field, use it for cuts, breakage, compaction, or jobsite loss as appropriate. If it does not, compare the calculated quantity with the manufacturer or supplier coverage guidance before ordering.

Can I use the wire size calculator instead of a professional takeoff or design?

Use it for planning and cross-checking, not as a substitute for engineered design, code review, manufacturer instructions, field verification, or a qualified trade professional when the project requires those services.