InputTestHub

Extension Cord Calculator

Works out the conductor size an extension lead needs for a given load and length, and shows the voltage drop over the run.

Calculators Nothing uploaded No sign-up Free, unlimited
Calculators tool
Enter your figures and press Calculate.
In short Extension Cord Calculator is a free online tool on inputtesthub.com that works out the conductor size an extension lead needs for a given load and length, and shows the voltage drop over the run. It needs no account, no download and no payment, and there is no daily usage limit. Everything is processed inside your own browser, so the data you paste is never uploaded to a server.

How to use the Extension Cord Calculator

  1. Enter your figures in the input fields.
  2. The result updates as you type, together with the full working.
  3. Check the breakdown table to see exactly how the number was reached.

About the Extension Cord Calculator

The Extension Cord Calculator works out the conductor size an extension lead needs for a given load and length, and shows the voltage drop over the run. It runs as plain JavaScript inside the page, which means there is no upload step, no queue and no size limit imposed by a server — the only limit is your own device.

What people use it for

  • Choosing a lead for a power tool at the end of the garden.
  • Checking whether a long run will start a motor.
  • Understanding why a tool bogs down on a thin lead.

Details

1.5 mm² rated16 A
2.5 mm² rated25 A
Drop limit3% of supply
Coiled reelMuch lower rating than unwound

The maths behind it

This tool uses Current = watts ÷ volts. Voltage drop = current × resistance per metre × length × 2 (out and back). Design limit is 3% of supply voltage. Every intermediate value is shown on screen, so the result can be checked by hand.

Always unwind the whole reel. A coiled lead is a coil of wire with current in it — the heat cannot escape, and a heavy load on a coiled reel can melt the insulation while the same load fully unwound is perfectly safe.

Frequently asked questions

Is there a limit on how often I can use the Extension Cord Calculator?
It is free, with no account, no trial and no daily cap. Every calculators tool on inputtesthub.com works the same way: open the page and use it.
Does anything I enter leave my computer?
No. The Extension Cord Calculator runs entirely in your browser using JavaScript. Your input never reaches inputtesthub.com or any third party — open your browser's Network tab while you use it and you will see no request go out.
What is the equation behind the Extension Cord Calculator?
It uses Current = watts ÷ volts. Voltage drop = current × resistance per metre × length × 2 (out and back). Design limit is 3% of supply voltage. The full working is printed under the result, so you can follow every step rather than trusting a black box.
What is the Extension Cord Calculator used for?
The most common reasons people reach for it: choosing a lead for a power tool at the end of the garden. Checking whether a long run will start a motor. Understanding why a tool bogs down on a thin lead. It is built for exactly those cases rather than being a general-purpose tool with this bolted on.
Is there anything to watch out for?
Always unwind the whole reel. A coiled lead is a coil of wire with current in it — the heat cannot escape, and a heavy load on a coiled reel can melt the insulation while the same load fully unwound is perfectly safe.
Which values should I know?
1.5 mm² rated is 16 A. 2.5 mm² rated is 25 A. Drop limit is 3% of supply. These are printed on the page itself, so you do not have to take them on trust.
Is the number rounded?
The calculation runs in double precision and the result is shown to ten significant figures, so converting a value and converting it back returns the original. What is displayed is rounded for readability; what is copied is not.

Related calculators tools

All 382
01

Loan Calculator

Works out the monthly payment, total interest and full repayment schedule for any loan from…

02

EMI Calculator

Calculates your Equated Monthly Instalment with a full principal-versus-interest breakdown and…

03

Mortgage Calculator

Estimates a full monthly housing payment including principal, interest, property tax…

04

Car Loan Calculator

Calculates a car finance payment from the price, deposit, trade-in, interest rate and term…

05

Personal Loan Calculator

Works out instalments and total cost for an unsecured personal loan, including any processing…

09

SIP Calculator

Projects the maturity value of a monthly Systematic Investment Plan, showing invested amount…