The calculators are ordinary arithmetic in the browser. They are free, they do not need an account, and the numbers you type are not uploaded.
Percentage Calculator does a percent of a value, a ratio, or a percent change, and it rejects a zero whole where the figure is not defined. Tip Calculator applies a tip, including zero, rounds to the cent, and adds leftover cents to the last person. Tax is not split out of the bill you type. Loan Payment Calculator estimates a fixed monthly payment and the interest. Fees and extra payments are outside that estimate.
Age Calculator counts calendar years, months, and days from a date of birth. It is not a count of days divided by 365, and a future date is rejected. Date Difference counts the midnights between two dates. The time of day is ignored, and both ends are not added as extra days.
BMI Calculator divides weight by height squared and shows a WHO screening band. This is a screening number, not a diagnosis. Unit Converter moves length and weight through a base unit, and temperature through Celsius. A reading below absolute zero is rejected. Bytes Converter shows binary units of 1024 and decimal units of 1000 without treating them as the same size.
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