Free Electrical Calculators
Free calculators for the numbers electricians and estimators look up every day. Each one shows the formula, cites the NEC section it follows, and works through an example, so you can check the result instead of trusting a black box. No signup needed.
Code and design calculators
| Calculator | What it answers | Code basis |
|---|---|---|
| Voltage drop calculator | Volts and percent lost on a run, and the wire size that keeps you under 3% | Chapter 9, Table 8; 210.19 and 215.2 informational notes |
| Wire size calculator | Minimum conductor for the load and the distance | Table 310.16, 240.4(D), 110.14(C) |
| Conduit fill calculator | How many conductors fit, or what size conduit you need | Chapter 9, Tables 1, 4 and 5 |
| Box fill calculator | Minimum box volume for the conductors, devices and fittings | 314.16(B) |
| Electrical load calculator | Dwelling service size by the optional method, and room for an EV charger or heat pump | 220.82, 220.87 (Article 120 in the 2026 NEC) |
Everyday math
| Calculator | What it answers |
|---|---|
| Watts to amps | Current from power at 120 V, 240 V or three-phase |
| Ohm's law calculator | Volts, amps, ohms and watts from any two values |
Estimating and paperwork
| Tool | What it does |
|---|---|
| Electrical bid calculator | Turns material, labor hours, overhead and margin into a price |
| Panel schedule template and generator | Builds a printable panel schedule with balanced phases |
Reference charts
- Ampacity chart: Table 310.16 by size and temperature column.
- Wire size by amps: breaker-to-wire answers from 15 to 200 A.
- Conduit fill chart: maximum THHN conductors in EMT and PVC.
- Wire gauge chart: AWG diameters, circular mils and areas.
- Voltage drop formula: the formula, solved three ways.
- Electrical symbols: how to read the symbols on electrical plans.
How these calculators are built
Every calculator is built from the NEC tables it cites and tested against published values; for example, the conduit fill engine matches a distributor's published PVC fill chart on all 20 cells we compared. Each page states the code edition it uses. Read how we build our calculators for the full method, including what the tools don't account for.
The calculators give code minimums under stated assumptions. Always confirm with the code edition your jurisdiction has adopted and your inspector, and have electrical work done by qualified people.
Use them on your own site
Supply houses, schools and associations can embed any calculator for free. Embeds carry a plain brand credit, and we never require a link.
From calculator to estimate
The calculators answer one question at a time. When you're pricing a whole job from a plan set, SparkQuote counts the devices on the drawings, applies your labor units and prices, and builds the proposal. See the electrical estimator.
Based on NEC 2023. Last updated: October 6, 2026.
Related
- Voltage Drop Calculator
- Wire Size Calculator
- Conduit Fill Calculator
- Box Fill Calculator
- Residential Electrical Load Calculator
- Watts to Amps Calculator
- Ohm's Law Calculator
- Panel Schedule Template and Generator
- Electrical Bid Calculator
- Wire Ampacity Chart and Wire Size Chart
- Conduit Fill Chart for EMT and PVC
- Wire Gauge Chart (AWG)
- How to Calculate Voltage Drop
- Electrical Symbols for Blueprints
- Wire Size by Amps
- How We Build and Check Our Calculators
- Electrical Estimating Software That Does the Takeoff for You