Calculators
Every Calculators tool on Toolz.dev: 14 free utilities that run entirely in your browser. No signup, no uploads, no downloads.
A calculator answers a question that has one right number and a formula nobody memorises. The twelve here fall into four groups: money, where the arithmetic compounds and small inputs move large outputs; body metrics, where the formula is a population estimate rather than a measurement of you; school marks; and the everyday arithmetic that is faster to type into a purpose-built field than into a general calculator.
The money tools are the ones worth being careful with. The loan and EMI calculator produces a monthly payment and the full amortisation schedule behind it, which is the only way to see how little of an early payment touches the principal. The compound interest calculator shows the same effect from the other side, and lets you set how often interest compounds, because annual and monthly compounding on the same rate do not give the same answer.
What each calculator works out, and what it needs from you
| Calculator | You supply | You get |
|---|---|---|
| Loan / EMI | Amount, rate, term | Monthly payment, total interest, amortisation schedule |
| Compound Interest | Principal, rate, term, frequency | Final balance and interest earned |
| Discount | Price and percentage off | Sale price and the amount saved |
| Tip | Bill, tip percentage, party size | Tip, total, and per-person split |
| Percentage | Any two of three values | Percent of, percent change, or what percent |
| BMI | Height and weight | BMI and its WHO category |
| TDEE | Age, sex, height, weight, activity | BMR, maintenance calories, macro split |
| Age | Birth date, optional target date | Age in years, months, days |
| GPA | Grades and credit hours | Weighted grade point average |
| Average | A list of numbers | Mean, median, mode and range |
| Fraction | Two fractions and an operator | Simplified result, as a fraction and a decimal |
| Aspect Ratio | A width or a height, and a ratio | The matching dimension |
| Profit Margin | Cost and revenue, or either plus a margin | Gross margin, markup, and the price that hits it |
| Pace | Any two of pace, distance and time | The third, in pace and in speed |
Where the money calculators surprise people
The monthly payment on a loan is the number everyone asks for and the least informative one. What the amortisation schedule shows is that early payments are almost entirely interest: on a 25-year mortgage, the first payment can put under a fifth of itself against the principal, and the ratio only crosses halfway years in. This is also why an overpayment is worth so much more than its size suggests, since it comes off the principal directly and removes every future month of interest that principal would have earned. The loan calculator models an extra payment for exactly this reason.
Compounding frequency is the other quiet variable. The same nominal rate compounded monthly rather than annually produces a higher effective rate, which is why lenders quote one and savings accounts advertise the other. Over a year the gap is small; over thirty it is not, and the difference is entirely in a dropdown most calculators do not offer. If you want the effect in a single sentence: interest on interest is the whole product, and frequency decides how often it starts.
The body calculators are estimates, and say so
BMI is weight divided by height squared, and the World Health Organization bands it at 18.5, 25 and 30. It was designed by a statistician to describe populations, not individuals, and it does not know the difference between muscle and fat: a heavily trained athlete lands in the overweight band routinely. The bands also fit some ancestries poorly, which is why several health bodies use lower thresholds for South and East Asian populations. Read it as one number among several rather than a verdict.
The TDEE calculator uses the Mifflin-St Jeor equation, which research has found to be the most accurate of the common predictive formulas, and multiplies the resulting BMR by an activity factor between 1.2 and 1.9. The activity factor is where the error lives: most people place themselves a band too high, and the difference between two adjacent bands is a few hundred calories a day. Treat the output as a starting point to be corrected by what the scale does over three or four weeks, not as a measured quantity.
The everyday arithmetic
The word percentage covers three different questions, and confusing them is the single most common arithmetic error in ordinary use. What is 15% of 80, what percent is 12 of 80, and what is the change from 80 to 92 all have different formulas and all get called a percentage calculation. The percentage calculator puts the three side by side rather than making you decide which one you are asking. The same distinction matters in reporting: a rate moving from 3% to 5% has risen 2 percentage points and 67 percent, and both numbers are correct.
Elsewhere the value is in the input format rather than the arithmetic. The fraction calculator simplifies as it goes, so you never see 84/144 as an answer. The age calculator does the calendar arithmetic that trips people up around leap days and month lengths. The aspect ratio calculator turns one known dimension into the other, which is the daily arithmetic of anyone laying out images or video. Two more solve for whichever value you are missing: the profit margin calculator works backwards from a target margin to a selling price, and the running pace calculator gives you the third of pace, distance and time from any two. Everything runs client-side: nothing you enter, including salary, loan amount or body measurements, is sent anywhere.
Frequently asked questions
- Are the numbers I enter into these calculators stored?
- No. Every calculator here runs client-side, in your browser, and computes from the values in the form without sending them anywhere. That matters more than usual for this category, since the inputs are salaries, loan balances, body weights and grades, all of which are ordinarily typed into a form that posts them to a server.
- Is EMI the same as a monthly loan payment?
- Yes. EMI stands for Equated Monthly Instalment, the term used across South Asia for the fixed monthly payment on an amortising loan, and it is the same figure a mortgage or car loan calculator elsewhere calls the monthly payment. The split between interest and principal inside that fixed amount changes every month, which is what the amortisation schedule shows.
- How much does making extra loan payments save?
- More than the size of the payment suggests, because an overpayment reduces the principal directly and cancels all the future interest that principal would have accrued. The saving is largest early in the term, when almost all of a scheduled payment is interest. The loan calculator shows the revised payoff date and total interest alongside the original.
- How accurate is a BMI calculation?
- The arithmetic is exact and the interpretation is not. BMI is weight over height squared, a population measure that cannot distinguish muscle from fat, so muscular people are routinely classed as overweight. The WHO thresholds of 18.5, 25 and 30 also fit some ancestries poorly, and several health bodies apply lower cut-offs for South and East Asian populations.
- Which formula does the TDEE calculator use?
- Mifflin-St Jeor for basal metabolic rate, multiplied by an activity factor from 1.2 for sedentary up to 1.9 for very heavy training. Mifflin-St Jeor is generally the most accurate of the common predictive equations, but every one of them is a population estimate, so use the result as a starting point and adjust from three or four weeks of real data.
- What is the difference between a percentage and a percentage point?
- A percentage point is an absolute difference between two percentages; a percentage is a relative one. A rate rising from 3% to 5% has risen 2 percentage points, and also risen about 67 percent. Both are true, which is why the two are so often mixed up in reporting, and why the percentage calculator labels which question it is answering.