The Cable Size Calculator is a free tool that helps you calculate cable size using the formula Pick the smallest copper cross-section keeping voltage drop within about 3% (simplified). Enter your values above and press Calculate to get suggested copper size, estimated drop instantly, with a full step-by-step breakdown.
What is the Cable Size Calculator?
The cable size calculator is a free online tool that helps you calculate cable size quickly and accurately. It applies the standard formula Pick the smallest copper cross-section keeping voltage drop within about 3% (simplified) to the values you enter and, when you press Calculate, displays the result together with a clear, step-by-step explanation of how the answer was reached.
Also known as: wire size calculator.
How to use the Cable Size Calculator
- Enter the load current.
- Enter the cable run length.
- See a suggested cable size.
Formula used
The cable size calculator is based on this formula:
Pick the smallest copper cross-section keeping voltage drop within about 3% (simplified)Each term in the formula corresponds to one of the inputs explained below. The calculator performs the arithmetic for you and shows every step so you can check the working by hand if you wish.
Worked example
Suppose you enter 20A, 30m, 230V. The calculator works as follows:
| Inputs | 20A, 30m, 230V |
|---|---|
| Working | drop-based sizing |
| Result | about 4 mm2 |
Step-by-step calculation
- Apply the formula:
Pick the smallest copper cross-section keeping voltage drop within about 3% (simplified) - Substitute your values: drop-based sizing
- Read the result: about 4 mm2
Inputs explained
- Load current (A) — the load current used in the calculation.
- Cable length (m) — the cable length used in the calculation.
- Voltage (V) — the voltage used in the calculation.
Understanding your result
- Suggested copper size — the calculated suggested copper size in mm2.
- Estimated drop — the calculated estimated drop in %.
Benefits and practical uses
The cable size calculator saves time and removes the risk of manual errors when you need to calculate cable size. It is useful for students, professionals and anyone who wants a fast, dependable answer. Results update the moment you press Calculate, work on any device, and can be copied or shared in a single click — and because the maths runs in your browser, your figures stay completely private.
Tips for an accurate result
- Keep all related inputs in the same units.
- Use the worked example above to confirm you are entering values in the right places.
- Review the assumptions below so the result fits your situation.
- Press Reset to clear everything and try a different scenario.
Common mistakes to avoid
- Entering values in the wrong unit (mismatched units give wrong answers).
- Leaving a required field empty — the calculator will ask you to fill it in.
- Rounding inputs too early, which can slightly change the result.
Assumptions and limitations
- Copper conductor; about 3% target drop; simplified - not a code calculation.
Frequently asked questions
Is this a substitute for an electrician?
No. This is a rough guide; always follow local wiring regulations and consult a licensed electrician.
How do I use the Cable Size Calculator?
Enter your values in the fields, then press the Calculate button to see the result. Use Reset to clear the form and start again. Nothing is calculated until you click Calculate.
What formula does the Cable Size Calculator use?
It uses the standard formula: Pick the smallest copper cross-section keeping voltage drop within about 3% (simplified). The full step-by-step working is shown with every result so you can verify it.
What information do I need to use the cable size calculator?
You need: load current, cable length, voltage. Sensible example values are pre-filled so you can see how it works.
Is the Cable Size Calculator free to use?
Yes. Every DigiToolkit calculator is completely free, with no sign-up, no login and no usage limits.
Does the Cable Size Calculator work on mobile phones?
Yes. The calculator is mobile-first and works on phones, tablets and computers in any modern browser.
Do I need to install an app?
No. The cable size calculator runs directly in your web browser — there is nothing to download or install.
Is my data private when I use this calculator?
Yes. All calculations run inside your browser. The numbers you enter are never uploaded to a server or shared with anyone.
How accurate is the Cable Size Calculator?
It uses the generally accepted formula for cable size and computes to several decimal places. Results are accurate for the assumptions listed on this page.
Can I copy or share my result?
Yes. After calculating, use the Copy button to copy the result, or Share to copy the page link.
Why is nothing showing until I press Calculate?
By design, the calculator only computes when you press Calculate, so you can finish entering all your values first. Press Reset any time to clear everything.
What units does the Cable Size Calculator use?
It works with A, V, m. Keep your inputs in consistent units for an accurate result.