Free Online Calculators Guide: Percentages, Loans, Interest, Tax, Dates and Health

Most everyday maths questions are small, but getting them wrong is costly: a loan that looks affordable until you see the total interest, a "50% off" deal that is not the best price, or a margin that is really a markup. Our Calculator Hub has 24 calculators for these questions. Every result updates as you type, many show the formula they used, and almost all of them run entirely in your browser^[1].

This guide groups them by the kind of question you are asking.

Percentages and everyday maths

Percentage Calculator answers the three common questions: what is X% of a number, what percent one number is of another, and what the total is when you know a part and its percentage. Each result shows the working. A handy trick: X% of Y equals Y% of X, so 8% of 50 is the same as 50% of 8.

Percentage Increase/Decrease finds the change between two values, or applies a change such as a 15% raise. Always divide by the original value. Remember that a 50% fall followed by a 50% rise ends 25% lower, not where you started.

Discount Calculator shows the sale price and your savings, or the discount percentage from two prices. Stacked discounts multiply rather than add: 20% plus 10% off is 28% off, not 30%. Compare final prices, not percentages.

Tip Calculator works out the tip, the total and each person's share in one step. Check whether a service charge is already included first.

Average Calculator gives the mean, median, mode, range and standard deviation of numbers pasted from anywhere, and lists anything it ignored. Use the median when a few extreme values, like salaries or house prices, would distort the mean.

Ratio Calculator simplifies ratios and solves proportions such as A: B = C: X, for scaling recipes, mixing materials and resizing images without distortion.

Dates and age

Age Calculator gives an exact age in years, months and days, plus totals and the countdown to your next birthday. Set "Age on" to an application closing date to check age limits for jobs, exams and admissions.

Date Difference Calculator counts days, weeks, months and business days between two dates, handling leap years for you. Tick "include the end date" for inclusive periods such as leave from the 1st to the 5th, which is five days. Business days exclude weekends but not public holidays.

Loans and borrowing

Loan Calculator estimates the payment and total interest for monthly, bi-weekly or weekly payments, with a year-by-year schedule. EMI Calculator does the same in the equated monthly instalment format banks in Pakistan, India and elsewhere use, and lets you expand any year to see the monthly split between interest and principal.

That split is called amortisation: early payments are mostly interest, and later ones mostly principal. A fixed-payment amortizing loan works this way because each payment first covers the interest on the remaining balance, and only the rest reduces the debt^[3]. Two practical points: a longer term lowers each payment but raises the total interest, and even a small prepayment early in a loan saves a lot of interest.

Saving and investing

Simple Interest Calculator works out interest on the original amount only, as used for many short-term loans and school maths. Compound Interest Calculator earns interest on interest too, with yearly, quarterly, monthly or daily compounding and optional monthly contributions. The difference grows dramatically over time, which is why starting early matters more than the amount. The standard compound interest formula shows why: the balance is multiplied by the growth factor once per period, so time has an exponential effect^[2].

Investment Calculator projects how a starting amount and monthly investments could grow at an expected return, and adds an inflation rate to show the result in today's money. Use conservative return estimates, and always look at the inflation-adjusted figure for long periods.

Average Balance Target Calculator is a specialist tool for bank branch and relationship managers measured on average balances. Enter your current average, its growth over the base month and your target, and it calculates the amount you need to hold every remaining day to reach it.

Salary, tax and pricing

Salary Calculator converts pay between hourly, daily, weekly, monthly and yearly figures, so you can compare a salaried offer with an hourly contract. Freelancers should add 20–40% to cover unpaid time, tools, taxes and holidays.

Tax Calculator applies income tax brackets (slabs) for you and shows total tax, effective rate, take-home pay and the tax in each bracket. It includes Pakistan's salaried slabs and lets you enter your own brackets for any country. Moving into a higher bracket only taxes the extra income at the higher rate.

GST Calculator and VAT Calculator add tax to a net price or remove it from a tax-inclusive amount, with common rates preset. To remove tax, divide by (1 + rate) rather than subtracting the percentage.

Profit Margin Calculator and Markup Calculator turn cost and price into profit, margin and markup. The two are easy to confuse: a 50% markup is only a 33% margin. Use markup when you price from cost and margin when you plan profit from sales, and include shipping, packaging and payment fees in your cost.

Health

BMI Calculator compares weight with height to give a screening measure and the healthy weight range for your height, in metric or imperial units. BMI is a screening tool, not a diagnosis; the CDC's guide to BMI explains what it can and cannot tell you^[4].

BMR Calculator estimates the calories your body uses at rest with the Mifflin–St Jeor equation, and Calorie Calculator estimates daily needs for maintaining, losing or gaining weight. Choose your activity level honestly; overestimating it is the most common mistake. For medical decisions, talk to a doctor.

Currency

Currency Converter converts between more than 160 currencies using daily mid-market reference rates, with a quick conversion table. Your bank or card may offer a slightly different rate; compare the two to see the real fee.

Where to start

Calculators are only as good as their inputs, so check units and periods before trusting a result. If you are weighing up a loan, start with the EMI Calculator: seeing the total interest and the yearly schedule is often what changes the decision.