The money math guide: loans, investments, tax, and pay

Four numbers decide your money

Most personal-finance advice is about behaviour — spend less, invest early, avoid debt. Underneath all of it are four calculations that decide what you actually pay, earn, and keep.

Get these four right and the decisions get easier, because you stop arguing about headlines and start looking at the real number.

1. What a loan really costs

A loan's interest rate tells you almost nothing on its own. What you pay is decided by the reducing-balance formula:

EMI = P × r × (1+r)^n ÷ ((1+r)^n − 1)

P is the amount borrowed, r is the monthly rate (annual ÷ 12 ÷ 100), and n is the number of months. The rate applies to the outstanding balance, not the original loan — which is why a longer tenure costs far more than its length suggests.

A ₹10,00,000 loan at 8.5% over 20 years has an EMI of about ₹8,678, and the interest alone comes to more than the amount borrowed. Shorten the tenure and the EMI rises, but the total interest falls sharply. Run yours in the Loan EMI Calculator, then read what a loan actually costs for the tenure trade-off in detail.

2. What an investment really earns

A SIP grows on the future value of an annuity — every month you add money, and the whole pot compounds:

FV = P × ((1+r)^n − 1) ÷ r × (1+r)

The surprise is which variable matters most. It is not the return; it is time. A ₹5,000 monthly SIP at 12% reaches about ₹11.6 lakh in 10 years, roughly ₹50 lakh in 20, and near ₹95 lakh in 25. The monthly amount barely changes — the extra years do nearly all the work.

That is why the question to ask is not "what return can I get?" but "how much can I invest, for how long?" The SIP Calculator shows the invested amount and the gains separately, and how much to invest monthly works through the numbers to ₹1 crore.

3. What tax sits on a price

Tax is where a good quote quietly becomes a bad one. The rule people mix up is inclusive versus exclusive:

• Exclusive (add tax): Total = Amount × (1 + rate ÷ 100)
• Inclusive (remove tax): Base = Amount ÷ (1 + rate ÷ 100)

Quoting ₹10,000 "plus GST" when you meant ₹10,000 inclusive moves your margin by the whole tax rate. The GST Calculator handles both directions at every slab, and GST and EMI calculators every Indian business needs covers the billing mistakes to avoid.

4. What your time is really worth

Converting a salary to an hourly rate is simple division — until you remember that paid leave means you work fewer weeks for the same money:

Hourly = Annual salary ÷ (hours per week × weeks per year)

Use the weeks you actually work. Four weeks of leave means 48, not 52, and the same salary converts to a higher hourly figure. For freelancers the number is only a floor: you also carry your own taxes, tools, and unpaid hours. The Salary to Hourly Rate Calculator gives the benchmark, and how to convert salary to hourly rate explains the gap between that number and what you should charge.

Use them together

The four are not separate. A loan EMI is a SIP running in reverse: the same monthly payment, one building your money and one costing you money. A GST figure changes what a project is worth, which changes the rate you need. A salary benchmark is the floor a freelance rate is built on.

Every money calculator and guide on Plainso is collected in one place at the money and finance guides. Start there, pick the calculation that is in front of you, and get the real number before you commit to anything.