Breaker Size Calculator

Determine the correct circuit breaker size.

Mason ReidFounder and Lead Estimator at MeasureThen

Last updated 2026-08-21

What Is a Breaker Size Calculator?

A breaker size calculator finds the correct circuit breaker rating for a load based on its wattage, voltage, and whether it runs continuously. It applies the 125% safety margin that code requires for continuous loads, then rounds up to a standard breaker size. This keeps the breaker matched to both the load and the wire.

How It Works

Load current = Watts ÷ Voltage. Continuous loads derate × 1.25. Breaker = next standard size ≥ derated amps.

Example

2,400 W at 120 V continuous: 20 A × 1.25 = 25 A → 25 A breaker.

How to Calculate

Enter the total load in watts, for example 2,400 W, and the voltage, commonly 120 V.
Indicate whether the load is continuous, meaning it runs three hours or more at a time.
The tool converts watts to amps, so 2,400 ÷ 120 gives 20 A, then multiplies by 1.25 for the continuous margin to get 25 A.
It rounds up to the next standard breaker size, in this case 25 A. Always pair the breaker with the correct wire gauge and verify with an electrician.

Standard breaker sizes

Standard size	Common use	Typical wire
15 A	Lighting, general receptacles	14 AWG
20 A	Kitchen and counter circuits	12 AWG
30 A	Dryers, water heaters	10 AWG
40 A	Ranges, some EV chargers	8 AWG
50 A	Ranges, welders	6 AWG

Sources

Frequently Asked Questions

What is a continuous load?

A load expected to run 3+ hours, like lighting and some appliances — NEC requires a 125% safety margin.

What are standard breaker sizes?

15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, and 100 A.

Can I use the next size up?

No — the breaker must be rated for the wire, not just the load. Always confirm with a licensed electrician.

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.