Every few days I catch myself doing the same thing: opening a search tab to find yet another single-purpose calculator site. One for a loan EMI, another for GST, a third for working out my exact age for a form. Most of them are buried under pop-ups, some want an email address before showing a result, and all of them ask me to type personal numbers into a stranger's server.

So I collected the twelve calculators I actually reach for and put them on one page of ToolNest, a small tools site I built. Full disclosure, because this is DEV and you'd ask anyway: there is no backend doing this maths. Every calculator is plain JavaScript running in your browser tab. Nothing you type is uploaded, stored, or logged — close the tab and the numbers are gone. No account, no install, no "premium" tier.

Here they are, grouped the way I use them.

Loan EMI, with the maths shown

Before signing any loan — bike, phone, home, personal — it is worth knowing three numbers: the monthly instalment, the total interest, and the total amount you will actually hand over. The EMI Calculator gives all three from the loan amount, interest rate, and tenure.

It uses the standard reducing-balance formula:

EMI = P × r × (1 + r)^n / ((1 + r)^n − 1)

where r is the monthly rate (annual rate ÷ 12) and n is the number of months. A worked example: borrow ₹5,00,000 at 9% for 5 years and you get:

  • Monthly EMI: ₹10,379
  • Total paid over 5 years: ₹6,22,751
  • Total interest: ₹1,22,751

That last line is the one banks don't print in large type — the loan costs you roughly 25% on top of what you borrowed. Seeing it before you sign changes decisions.

GST: adding tax, and the trickier reverse

India's GST slabs are 5%, 12%, 18%, and 28%, and billing constantly needs one of two operations: adding GST to a base price, or pulling it back out of a tax-inclusive price. The second one trips people up because you can't just subtract the percentage. The GST Calculator does both directions.

Adding is simple: 18% on a ₹10,000 service is ₹1,800 of tax, so the invoice total is ₹11,800.

Removing needs division, not subtraction: a ₹10,000 price that already includes 18% GST splits into a base of ₹8,474.58 plus ₹1,525.42 of tax. If you had just knocked 18% off, you'd have understated the base by over ₹300 — a small error that adds up across a month of invoices if you're a freelancer or run a small shop.

Sale maths: percentages and discounts

The Percentage Calculator covers the three shapes this question takes — what is X% of Y, X is what percent of Y, and percentage increase or decrease — which between them handle exam marks, salary hikes, and price changes.

Its close cousin is the Discount Calculator, for sale tags. A "40% off" board on a ₹2,500 jacket means you pay ₹1,500 and save ₹1,000 — obvious at 40%, much less obvious at 35% off ₹1,899, which is exactly when a calculator earns its keep.

Age and BMI: forms and a health snapshot

Government and exam forms in India regularly demand your age in years, months, and days as of a specific date. Counting that by hand is error-prone, so the Age Calculator does it from your date of birth in one step, and the Date Difference Calculator counts the exact days between any two dates — deadlines, notice periods, countdowns.

For a rough health check, the BMI Calculator divides your weight by the square of your height. Someone at 70 kg and 1.70 m lands at 24.2. Measured against the WHO's ranges, that reads as: underweight below 18.5, normal from 18.5 to 24.9, overweight from 25 to 29.9, and obese at 30 or above. BMI is a screening number, not a diagnosis — but as a screening number it is genuinely useful.

The small converters at the end

The remaining five are helpers rather than headline acts. The Unit Converter flips between length, weight, temperature, and other everyday units. Number to Words spells a figure out in English words, which anyone who has written a cheque or an invoice amount-in-words will appreciate. The Number Converter switches values between decimal, binary, octal, and hexadecimal — the one tool here aimed squarely at students and developers. And the Random Number Generator draws fair random numbers inside a range you set, while the Color Picker — handy for anyone tweaking a UI — lets you grab an exact colour and copy its value.

Questions people usually ask

Is it really free? Yes. All twelve, unlimited use, no watermark, no trial clock.

Where do my numbers go? Nowhere. The calculations happen in your browser with JavaScript, so a salary figure or a loan amount stays on your own device.

Can I trust the results? They use the standard published formulas — the same EMI and GST arithmetic shown above — so you can verify any result by hand. For a large loan, still compare the output with your bank's sanction letter; that is just good practice with any calculator.

If you build or maintain tools like these, I'd genuinely like to know: do you keep ten bookmarks too, or is there one calculator site that has earned your trust? Drop it in the comments.