Financial Concepts Explained
Every term behind the calculators, guides, and strategies on this site — explained plainly, the way it should have been the first time.
🔥 FIRE
The 4% Rule
The single number behind almost every FIRE calculation — and where it actually comes from.
Safe Withdrawal Rate
How much you can withdraw each year without outliving your portfolio — and why 4% isn’t the only answer.
FIRE Number
Your personal retirement target in a single calculation — and why changing the rate changes everything.
Coast FIRE
Already invested enough to retire on compound growth alone? Here’s how to know if you can stop saving and just coast.
Barista FIRE
Not fully retired, not still grinding — how part-time work covers what your portfolio doesn’t yet.
Lean FIRE
The fastest path to financial independence: a smaller target, reached by deciding to need less — not earn more.
Fat FIRE
Retiring without downgrading your lifestyle — same formula as FIRE, just a bigger number and a longer runway.
Sequence of Returns Risk
Why two retirees with identical average returns can end up in completely different places, depending on when the bad years hit.
📈 Investing
Index Funds vs. Picking Stocks
Why most professional stock pickers fail to beat a fund that just buys the market.
Expense Ratios — The Silent Fee
The one fee you’ll never see a bill for — because it’s already been taken out.
Dollar Cost Averaging
Investing the same amount on a fixed schedule — and why the market’s worst stretches are often when it helps most.
Diversification
Spreading across assets is the only free lunch in investing — here’s what the math behind it actually shows.
Asset Allocation
How you split stocks and bonds determines most of your long-run return — and most of your risk.
Rebalancing
Your portfolio drifts away from your target over time — quietly changing how much risk you’re actually carrying.
Factor Investing
The research-backed tilts — small, value, profitability — that have historically earned a premium over the broad market.
Dividend Investing
Collecting regular income without selling shares — and the trade-offs that come with chasing yield.
ETF vs. Mutual Fund
Two ways to own the same index, often one fee apart — and why that thin difference compounds over decades.
Tax Loss Harvesting
Using a losing position to offset gains elsewhere — and why the IRS effectively subsidizes the swap.
🪺 Retirement
401(k)
The employer-sponsored plan most Americans use to save for retirement — and the free money inside it most people don’t fully claim.
IRA
A retirement account you open yourself, at any brokerage, whether or not your employer offers one.
Roth IRA
Pay the tax now, on a smaller number — so everything it grows into comes out completely tax-free.
Traditional IRA
The tax break comes first here, not last — deduct the contribution today, pay the IRS on withdrawals later.
Roth Conversion Ladder
The strategy FIRE retirees use to access retirement accounts before 59½ — without the 10% penalty.
Required Minimum Distributions
The IRS eventually requires withdrawals from tax-deferred accounts — whether you need the money or not.
Social Security
The age you claim permanently locks in your monthly benefit — the difference between earliest and latest is over $1,000/month, for life.
Medicare
Miss your 65th-birthday enrollment window and the late penalty follows you permanently — on every future premium.
💰 Budgeting
50/30/20 Rule
Split after-tax income 50% needs / 30% wants / 20% savings — a fast check on whether your spending is sustainable.
Zero-Based Budgeting
Give every dollar a job before the month starts — until income minus allocations equals exactly zero.
Pay Yourself First
Automate savings the moment income lands — so saving stops depending on whatever’s left at month’s end.
Savings Rate
The single biggest lever in FIRE math — the percentage you save moves both ends of the equation at once.
Emergency Fund
Three to six months of expenses set aside for the unexpected — and why it stays separate from everything else.
Sinking Funds
Save gradually for expenses you already see coming — so predictable costs never raid your emergency fund.
Net Worth
What you’d have left after selling everything and paying every debt — the number income alone can’t show.
High-Yield Savings Accounts
Your bank’s 0.01% savings account is quietly losing you money. Here’s the ten-minute fix.
HYSA vs. CDs
When rates are close, the real question isn’t which pays more — it’s whether locking up access is worth it.
💳 Credit
Credit Score
Five factors build the number — payment history and amounts owed alone account for nearly two-thirds of it.
Credit Utilization
The fastest lever on your score — a lower balance can show up in your next reporting cycle, not months.
Hard vs. Soft Inquiry
Checking your own score never hurts it — applying for credit does, though rate shopping within a short window usually counts as one look.
Debt Avalanche
The mathematically optimal payoff order — attack the highest interest rate first, regardless of balance size.
Debt Snowball
Not the cheapest way to pay off debt — but often the one people actually finish.
APR vs. APY
The same rate, advertised two different ways — each side of a deal picks whichever number looks better.
🧮 Math Concepts
Compound Interest
Rule of 72
Time Value of Money
Inflation
Real vs Nominal Returns
Net Present Value
Opportunity Cost
🏡 Real Estate
Mortgage Amortization
Two payments five years apart cost exactly the same — but almost none of the first one builds equity.
PMI
Required under 20% down, and it protects the bank, not you — but it’s temporary, not a fee for life.
LTV Ratio
The single ratio that decides whether you pay PMI and what rate you actually qualify for.
Rent vs. Buy
The real answer isn’t a feeling — it’s a breakeven number most people never actually calculate.
Cap Rate
Compare two rental properties on equal footing, whether one’s paid in cash or financed to the hilt.
House Hacking
Live in one unit, rent out the rest, and turn your biggest expense into a fraction of itself.
🧾 Tax
Marginal vs. Effective Tax Rate
The rate on your last dollar and the rate on all of them are different numbers — and only one matters for a 401(k) contribution.
Capital Gains Tax
Hold an investment past one year and the IRS taxes the profit completely differently — sometimes at 0%.
Tax-Advantaged Accounts
Pre-tax now or tax-free later — the two flavors behind every 401(k), IRA, and HSA.
Tax Loss Harvesting
Using a losing position to offset gains elsewhere — and why the IRS effectively subsidizes the swap.
FICA Taxes
The 7.65% withheld before you ever see your paycheck — and where it stops applying at all.
Backdoor Roth
The legal workaround that lets high earners fund a Roth IRA anyway — if the pro-rata rule doesn’t get in the way.
ACA Subsidy Cliff
One extra dollar of income can erase thousands of dollars in health insurance subsidies overnight.
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