A few financial calculations come up often enough in Bangladesh that it's worth knowing the actual formula, even if you'd rather not do the arithmetic by hand every time.

VAT (15% standard rate)

To add VAT to a price: multiply by 1.15. To find how much of an already-VAT-inclusive price is tax: divide the total by 1.15 to get the pre-VAT amount, then subtract that from the total. A lot of people get this backwards and subtract 15% directly from the inclusive price, which undercounts the actual VAT amount.

Income tax

Bangladesh's personal income tax uses tiered slabs that change from year to year in the national budget, so always check the current year's slab rates (NBR's website publishes these) rather than relying on a number that was correct last year - a calculator built around a fixed rate goes stale the moment the budget changes.

EMI (loan installments)

Monthly EMI depends on three things: principal amount, interest rate, and loan tenure, combined in a standard amortization formula. The quick mental version: a longer tenure lowers your monthly payment but increases the total interest paid over the life of the loan - worth running both a short and long tenure through a calculator before deciding, rather than just optimizing for the lowest monthly number.

Zakat

Zakat is calculated at 2.5% of qualifying wealth that's been held above the nisab threshold for a full lunar year - savings, gold, silver, and business assets typically count, while your primary residence and daily-use items generally don't. The nisab threshold itself is pegged to the current value of gold or silver, so it moves with the market rather than being a fixed number - check a current nisab value before calculating rather than reusing an old one.

A couple of calculators that help with the adjacent math

Our Unit Converter and Mortgage Calculator cover the general-purpose conversion and loan-payment math that often comes up alongside these - useful if you're sanity-checking a number before you commit to it.