2 calculators in one
Resistor colour code
Read 4-band and 5-band resistors, or find the colours for any value, on a drawn resistor.
- Colours to value
- Value to colours
Class 6 to 12, diploma, makers
Free tools
Free calculators for projects, practicals and exams
12 calculators for students, makers and parents: pick the right resistor, light an LED safely, gear a robot, convert any unit, add up your marks, test your typing and work out interest, EMI and GST. Every one shows its working.
6 free calculators
Electronics and projects
Pick resistors, light LEDs safely, size batteries and gear motors for working models and robots.
Free ebooks to read alongside
Ohm's law
Enter any two of voltage, current, resistance and power and get the other two, with the working.
Class 10 to 12, diploma, makers
LED resistor
Supply, LED colour and current in; the exact resistor, the next E12 value and its power out.
Class 6 to 12, makers, Arduino users
Series and parallel
Combine any number of resistors in series or parallel; see the total, the current and each share.
Class 10 to 12, diploma
Battery life
Capacity and load current to running time, with an allowance for real-world losses.
Class 6 to 12, makers, robotics
Gear ratio
Driver and driven teeth to gear ratio, output speed, approximate torque and robot speed.
Class 6 to 12, robotics, diploma
2 free calculators
Science and units
Convert any unit with exact factors and turn watts and hours into electricity units and cost.
Free ebooks to read alongside
Unit converter
Length, mass, time, temperature, speed, area, volume, energy and pressure, with exact factors.
Class 6 to 12, diploma, makers
Power and energy cost
Watts and hours to units (kWh) and cost, at the per-unit rate on your own electricity bill.
Class 6 to 12, families, makers
Where students use them
- Physics numericals (Unit converter)
- Project measurements (Unit converter)
- Temperature experiments (Unit converter)
- Maps and land (Unit converter)
- Energy audit project (Power and energy cost)
- Comparing bulbs (Power and energy cost)
- Solar model sizing (Power and energy cost)
- Data for maths and EVS (Power and energy cost)
2 free calculators
Study and exams
Percentages, marks you need and typing speed, for report cards, revision plans and typing practice.
Free ebooks to read alongside
- 2 calculators in one
Percentage and marks
Marks to percentage across subjects, and the marks you need in the exams still to come.
- Marks to percentage
- Marks I need
Class 6 to 12, college, parents
Typing speed
Characters or words, errors and time to gross WPM, net WPM and accuracy, plus a one-minute test.
Computer course students, job seekers
Where students use them
- Checking a report card (Percentage and marks)
- Planning the finals (Percentage and marks)
- Admission forms (Percentage and marks)
- Tracking the year (Percentage and marks)
- Computer course practice (Typing speed)
- Typing tests for jobs (Typing speed)
- Hindi and English typing (Typing speed)
- A quick daily check (Typing speed)
2 free calculators
Money and business
Interest, EMI and GST arithmetic for commerce students, small shops and family decisions.
Free ebooks to read alongside
- 3 calculators in one
Interest and EMI
Simple interest, compound interest by frequency, and a loan EMI with a yearly schedule.
- Loan EMI
- Simple interest
- Compound interest
Class 7 to 12, commerce, families
GST calculator
Add GST to a price or take it out, with the CGST and SGST split, at a rate you enter.
Commerce students, shopkeepers, families
Where students use them
- Maths chapters on interest (Interest and EMI)
- Commerce and business maths (Interest and EMI)
- Comparing loan offers (Interest and EMI)
- Seeing savings grow (Interest and EMI)
- Commerce and accountancy (GST calculator)
- Small shop billing (GST calculator)
- Checking a bill (GST calculator)
- Tally practice (GST calculator)
With your projects
How to use these tools in a project
A calculator gives you a number. A good project explains where the number came from. Four steps make the difference.
Write down what you know
The supply voltage, the LED colour, the marks so far, the loan amount: list the numbers and their units before you start.
Calculate and read the working
Each tool shows every step. Follow it once by hand so you can explain it, not just copy the answer.
Build, then measure
Check the real circuit with a multimeter or the real bill with the calculator. A difference tells you something worth knowing.
Explain it in your report
Copy the result and the working into your project file. Judges and examiners ask how you chose each part.
Example: a traffic-light model
Four tools, used in order, take the guesswork out of a classic working model.
More free help
Free ebooks and project ideas
The calculators work best beside a good explanation and a good idea. Both are free.
100 free PDF ebooksElectronics, robotics, science projects, computer courses, accounting and more, in easy English. Download, print and share.Browse the libraryThe project idea finder99 science project ideas to filter by class, subject and type, each with the science it shows, the materials and a safety note where one is needed.Open the idea finder
Good to know
About these calculators
Standard formulas
Every tool uses the textbook formula or the exact defined unit factor, and each page shows the formula with a worked example.
You type the rates
GST rates and electricity tariffs change, so the tools never assume one: enter the rate from the official GST portal or your own bill.
No sign-up, no record
Use any tool straight away. The calculators keep no record of what you type.
Made for phones
Big buttons, instant results as you type, and the forms still work when JavaScript is switched off.