№ 01
Compound Interest
Project investment growth over time with annual contributions and compounding returns.
Foundations · 4 min
Most personal‑finance tools optimize for sign‑ups. This one optimizes for the number you actually leave with. Every assumption is visible. Every formula is linked. Save your scenarios to your own Drive — I never see them.
Each calculator shows its work. Every assumption is visible — the formula, the source, and why it matters.
Project investment growth over time with annual contributions and compounding returns.
Break down monthly mortgage payments into principal and interest with a full amortization schedule.
Model a life. Savings, employer match, drawdown, inflation — benchmarked against S&P 500, Nasdaq, and a bond floor.
Calculate monthly payments and total interest for any auto, personal, or student loan.
The 50/30/20 rule, with room to disagree. Needs, wants, savings — proportional, adjustable, honest.
How much runway you really need, and how many months until you get there at the pace you're saving now.
The single most useful chart in personal finance: what every year of waiting to start actually costs you.
Snowball vs Avalanche across every debt you owe. Find the fastest, cheapest route to zero — and what they each feel like.
Find out which IRA wins for your tax situation — now and at retirement. Apples-to-apples with break-even analysis.
A finance tool that doesn't explain its assumptions is just a vibe.
I got tired of calculators that quietly assumed 10% returns forever, ignored inflation, and hid their math behind slick charts. So I wrote these — for myself first, for anyone else who wants to see the gears.
The data lives in your Drive or browser. The opinions are, regrettably, mine.
There's no single right allocation — but there is a direction of travel. Here's the decade-by-decade framework I wish I'd had from the start, and why the $100K milestone changes the math entirely.
I dropped $7,000–$14,000 into a Roth IRA early in my career, picked a boring S&P 500 ETF, and forgot about it. Fifteen years later it's worth over $40,000 — entirely tax-free. Here's what I'd tell my younger self.
I bought GE in the $30s, then the $20s, then kept buying all the way down to $5. What that 85% loss taught me about ego, laziness, and why the most expensive investing mistakes are the ones you make late.