The Compound Interest Calculator Pack — Eight linked calculators, every formula checked by recalculation

Eight calculators for money that grows: monthly contributions, a single deposit, compounding frequency, the monthly amount a goal needs, the years it takes, how long withdrawals last, what a future balance is worth after inflation, and what waiting costs. Start with sheet 1 -- in the worked example $10,000 plus $300 a month for 20 years ends at $171,714, and $89,714 of that is interest. Every rate is an example for you to overwrite; none is a forecast and returns are not guaranteed. Every formula was recalculated and checked against an independent implementation before this shipped.

What is in it

8 calculators, one per sheet. Overwrite the blue cells with your own numbers and everything else follows — nothing is locked, there are no macros, and it opens in Excel, Numbers, LibreOffice and Google Sheets.

• Monthly contributions — How much of what you end up with is interest you never paid in?
• Lump sum growth — What does one deposit grow to if you never add another dollar?
• Compounding frequency — How much does daily compounding beat yearly, in dollars?
• Monthly amount for a goal — How much do you need to add each month to reach your number?
• Years to reach a goal — How many years until your balance reaches a target?
• How long withdrawals last — How many years will a balance last if you take money out every month?
• Inflation-adjusted value — What is that future balance worth in today's dollars?
• Cost of starting later — What does waiting ten years to start cost you?

Every formula was checked

This pack contains 56 formulas. Before it was published, every one of them was recalculated by a spreadsheet engine and compared against an independent implementation of the same arithmetic written in Python. All 56 agreed to within a cent, and the workbook carries a Verification sheet saying so.

That is an unusual thing to be able to claim about a spreadsheet, and it is the main reason to buy this one rather than download a free template. A formula that looks right and is wrong is worse than no formula at all, because you will act on it.

What each calculator assumes

There is a sheet listing this, because a calculator that hides its assumptions is just a number generator:

• Rate compounds monthly at the annual rate divided by 12, and stays the same every year. Additions land at the end of each month. No tax, fees or inflation (sheet 7 handles inflation).
• One deposit, no additions or withdrawals, a fixed rate. The rate must be above zero for the doubling time.
• The same stated annual rate in every row. When two accounts quote an APY, the frequency is already inside it: compare APYs directly.
• Same rate every month, additions at the end of each month. If what you have now already grows past the goal, the monthly amount shows zero.
• Same rate every month, additions at the end of each month. The goal must be above what you have now, and the monthly addition above zero if the rate is zero.
• A fixed rate every month, a fixed withdrawal that does not rise with inflation, withdrawals at the end of each month, no tax. Real returns vary year to year, and a bad run early on shortens the life of the money; this sheet cannot show that.
• One steady inflation rate for every year. The real rate is exact, (1 + return) / (1 + inflation) - 1, not return minus inflation.
• Both paths add the same amount each month at the same fixed rate, starting from nothing. Contributions at the end of each month; no tax or fees.

What this is not

Not financial, tax or legal advice, and not a recommendation to buy, sell, borrow or sign anything. These are calculators: they show what a choice costs, not which choice is right for you. Where a published rate or threshold is used it is the one current when the pack was built — those change, the arithmetic does not.

Honest about the format

One .xlsx workbook, 8 calculator sheets plus Start Here, Assumptions and Verification. Not a PDF, not a physical product, not a Notion template. Available immediately after purchase; nothing ships.

Refunds

If this isn't what you expected, email me within 30 days and I'll refund you in full — no need to explain. Because it's an instant download I can't take the file back, so I'd rather you just had your money back.