🧮 Calculators

A calculator is only as good as the assumptions behind it, which is why the useful ones show their working rather than just a final figure. The value is in seeing how a result was reached — where the interest goes, which slab applies, what the break-even actually depends on.

The arithmetic people get wrong

Percentages do not behave the way intuition suggests, and the errors are consistent. A 50 percent increase followed by a 50 percent decrease does not return to the start, because each change applies to a different base. Successive discounts multiply rather than add, so two 30 percent reductions total 51 percent off, not 60. Finding an original price means dividing by (1 − rate), not adding the percentage back. And margin is not markup: a £50 item sold for £100 carries a 50 percent margin but a 100 percent markup, a confusion that quietly underprices a lot of businesses.

Nominal figures are not real figures

Every projection here is nominal. Subtract inflation to get purchasing power, and remember that interest is usually taxable in the year it accrues, which reduces the amount left to compound. A 7 percent return with 3 percent inflation is roughly 4 percent in real terms before any tax.

Compounding frequency, meanwhile, matters far less than people expect. £10,000 at 6 percent for a year returns £600 compounded annually and £618 compounded daily. The rate and the time horizon dominate; the frequency is a rounding detail.

Regional rules and their limits

Several finance calculators here model Indian statutory rules — GST, TDS, HRA, PPF, NPS, gratuity and income tax. Those rates and thresholds change with most budgets, so treat any output as a planning estimate. For a filing or investment decision, confirm against the current year's rules or a qualified professional. These tools are not financial, tax or medical advice.