Calculators and converters
How do I make a calculator for loan instalments, land area or exam GPA?
Describe the formula in Bangla and say which box holds what, and the Builder makes the calculator. Loan instalments, land area in katha, bigha and shatak, unit conversion, exam GPA and a bill splitter all work well. Just remember that changing numbers such as a dollar rate or a gold price have to be typed in by you.
What you are making
A calculator is the safest thing for a beginner to build. A number goes in a box, a sum happens, an answer appears. There is little design to fuss over, the result is visible at once, and people genuinely use it. These are the ones most in demand in Bangladesh:
- Loan instalments: how much is borrowed, at what interest, over how many months, what to pay monthly and how much interest in total.
- Land area: moving between katha, bigha, shatak, decimal and square feet, with a price worked out too.
- Unit conversion: kilograms and maunds, feet and metres, litres and gallons, Celsius and Fahrenheit.
- Currency conversion: dollars, riyals, ringgit or dirhams into taka, very useful for families with someone abroad.
- Exam GPA: put in the grade for each subject and get the overall GPA.
- Bill splitter: how a restaurant bill divides between people, who owes what, tip included.
Step by step
Step 1: Decide what boxes there will be and what answer you want. Write three columns on paper: what I put in, what gets worked out, what gets shown. With those three clear, the prompt writes itself.
Step 2: Write the formula out as openly as you can. The AI knows the common formulas, but it does not know how your area counts. Land measures differ from district to district, so put your own numbers in.
Make a calculator for converting land area. At the top I type a number in a box and beside it choose katha, bigha, shatak or square feet. Below, show what that is in all the other units at once. Use 1 bigha = 20 katha = 33 shatak, and 1 katha = 720 square feet. In another box I type the price per shatak and it shows the total price in taka. All in Bangla and in large text for a phone.
Step 3: When the first version arrives, check it against a sum you already know. Put in the one whose answer you know in advance. If it does not match, write "with these numbers the answer should be this but it is showing that, please fix the formula".
Step 4: Ask for the result in human words. Not just a bare number, but a line that explains, such as "you pay ৳9,560 a month, and ৳44,160 in interest over 3 years".
Make a loan instalment calculator. Three boxes: the amount borrowed, the yearly interest rate, and the number of months. Work out and show the monthly instalment, the total to be repaid, and how much of that is interest. Below it a month by month table showing how much of each payment is principal and how much is interest. Give a button to download that table as a PDF. Show money in Bengali numerals, with commas.
Step 5: When you are happy, press Share and give the link to whoever needs it. You can say "make it something I can put on my phone home screen", and it will sit there with its own icon and work without internet, which is a blessing in a shop or out on a plot of land.
Tricks that make it better
- Leave an example filled in. Ask for sample numbers in the boxes rather than empty ones. People understand what to do the moment they arrive.
- Keep the rates in your own hands. Dollar rates, gold prices and interest rates change. A built app cannot go and fetch today's rate, so leave a box where the rate is typed in, or later just say "change the rate to this".
- Show the date. Ask for the date the rate was entered to appear beside it, so the user knows how old the number is.
- Ask it to catch mistakes. If someone types a zero or a letter it should say "please enter a number" in red, and you will never get a nonsense answer.
- Give the answer a way out. A button that downloads the result as a PDF or a picture lets people send it on WhatsApp, and your tool travels with it.
Mistakes many people make
- Asking for the result without giving the formula. Say "work out the land area" and the AI assumes a common conversion which may not match your area. Write the numbers yourself.
- Expecting today's rate to arrive by itself. A dollar rate or a market price will not update on its own. Accept that and keep a box for the rate.
- Thinking everyone shares the same saved sums. Whatever someone types and saves stays on their own phone, and neither you nor anyone else can see it.
- Leaving box labels vague. Instead of "rate", write "yearly interest rate (%)". Clear labels stop people putting the wrong number in.
What to do next
Once one calculator stands up, adding another beside it is very easy. Say "put tabs at the top, one for land area, one for instalments, one for GPA", and all your tools live behind one link. For GPA, write out your board's rule, how many points each grade is worth and how an optional subject counts. And in a bill splitter, letting people be added by name is what makes it get passed around a table of friends.
Beginner questions
Interest, EMI, zakat, can it do them all?
Yes, state the rule and that calculator exists. The common ones the AI already knows.
Could the results be wrong?
Once the formula is right, the math never drifts, it runs the same rule every time. Verify once with a known example.
Can my customers use it?
Yes, share the link or keep it open on the shop tablet. Any number of people, no cost.