Financial Guides & Resources
Clear, in-depth guides that explain the ideas behind every number our calculators produce — from compound interest and the Rule of 72 to retirement targets, emergency funds and financial independence — plus worked scenarios that answer specific money questions.
A calculator gives you an answer in seconds, but the answer only matters if you understand the question behind it. That is what this learning hub is for. Every guide here takes a single financial idea — how money compounds, how much you really need to retire, why an emergency fund changes everything — and explains it from first principles, with worked examples and practical steps you can act on today.
The guides and the tools are designed to work together. Read a guide to build the intuition, then jump straight into the matching calculator to run your own numbers. Start with the fundamentals in how compound interest works, or head directly to the free compound interest calculator if you already know the theory and just want to model your plan.
The Library
All Financial Guides
Every guide is original, in-depth and written to stand on its own. Pick a topic below.
How Does Compound Interest Work?
The formula, the intuition and the examples behind exponential growth — and why starting early beats investing more.
Read guide →Simple Interest vs Compound Interest
A straight line vs a curve — the widening gap on $10,000 at 7% over 30 years.
Read guide →How to Calculate Future Value
The future value formula step by step — lump sums, monthly contributions, inflation and real planning examples.
Read guide →Future Value Formula Explained
Every variable in FV = PV(1+r)n, the annuity version, and how to rearrange it to solve for rate, time or present value.
Read guide →Future Value of Money Formula
FV = PV(1+r)n applied to money, with monthly-compounding examples and a calculator to run it.
Read guide →Present Value vs Future Value
Compounding forward vs discounting back — one example run both directions.
Read guide →Future Value of Annuity Due Formula
Why beginning-of-period payments earn extra interest, with the formula and ordinary-vs-due examples.
Read guide →Monthly Compound Interest Formula With Monthly Contributions
The two-part formula for a starting balance plus monthly deposits, with verified examples.
Read guide →Time Value of Money in Excel
Do TVM in Excel or Google Sheets with FV, PV, NPER, RATE and PMT — syntax, sign convention and worked formulas.
Read guide →Time Value of Money in Google Sheets
FV, PV, NPER, RATE and PMT in Sheets — copy-ready formulas, sign convention and a worked example.
Read guide →Future Value With Monthly Contributions
How $100 to $1,000 a month grows over 10 to 40 years, rate sensitivity, and lump sum vs steady investing.
Read guide →The Investment Growth Formula
Total return vs annualized return (CAGR), the growth multiple, and growth with contributions.
Read guide →CAGR vs Average Annual Return
Why the average overstates volatile returns — a +50%/-50% swing averages 0% but a -13.4% CAGR.
Read guide →How Much Will $10,000 Grow in 20 Years?
The future value of $10,000 over 20 years by return rate, with monthly contributions, and after inflation.
Read guide →How Much Will $25,000 Grow in 15 Years?
$25,000 by return rate, over time, with contributions, and after inflation — all verified.
Read guide →How Much Will $500 a Month Grow in 20 Years?
$500 monthly for 20 years by rate, by contribution amount, and after inflation.
Read guide →How Much Will $250 a Month Grow in 30 Years?
$250/month by return rate, over time, at different amounts, and after inflation — all verified.
Read guide →How Much Will $10,000 Grow in 30 Years?
$10,000 over 30 years by return rate, with contributions, and after inflation.
Read guide →How Much Should You Invest Every Month?
How much experts recommend by age and income, the 15% rule, and why consistency beats timing.
Read guide →What Is Dollar-Cost Averaging?
Investing a fixed amount on a fixed schedule — and the worked example that shows why it lowers your average cost.
Read guide →How to Reach Your Savings Goals Faster
Set targets, automate deposits, use high-yield accounts and budget smarter to hit any goal sooner.
Read guide →How Much to Save Monthly to Reach $100,000
$1,397 a month over 5 years down to $192 over 20 — and how much comes from growth.
Read guide →How to Calculate Your Savings Rate
Savings ÷ income — and why your rate, not income, sets your time to FIRE.
Read guide →What Is the Rule of 72?
The mental shortcut that tells you how long money takes to double — where it comes from, when it is accurate, and how to use it.
Read guide →Rule of 72 vs Rule of 70 vs Rule of 69
Which doubling-time shortcut to use — compared at 4%, 6%, 8% and 10%.
Read guide →How Long to Double Your Money at 7%?
About 10.2 years — the Rule of 72 says 10.3, overshooting by only two weeks.
Read guide →How Much Money Do You Need To Retire?
The 25x rule, the 4% withdrawal guideline and a step-by-step method to turn your spending into a concrete savings target.
Read guide →How Much Do You Need to Retire on $50,000 a Year?
About $1.25M at the 4% rule — with 3%/5% variants and a Social Security version.
Read guide →How Much Do You Need to Retire on $75,000 a Year?
About $1.875M at the 4% rule — withdrawal-rate variants and a couple’s Social Security version.
Read guide →How Much Emergency Fund Do You Need?
How to size, build and store the cash buffer that keeps a bad month from becoming a financial disaster.
Read guide →3-Month vs 6-Month Emergency Fund
A head-to-head decision guide: who each target suits, a scorecard, and how long each takes to build.
Read guide →6-Month Emergency Fund Calculator Guide
How to calculate a six-month fund from your expenses, targets by spending level, and how long it takes to build.
Read guide →Emergency Fund for $3,000 a Month
How big a fund to build at $3,000 in monthly expenses, with targets by coverage and savings timelines.
Read guide →What Is FIRE (Financial Independence, Retire Early)?
The movement, the math and the variations — Lean, Fat, Barista and Coast FIRE — plus how to find your own number.
Read guide →FIRE Number by Expenses
Your FIRE number is annual expenses times 25 — full tables by spending level and withdrawal rate.
Read guide →What Is a Safe Withdrawal Rate?
The 4% rule explained, with the portfolio you need at 3%, 3.5%, 4% and 5%.
Read guide →How to Calculate Your FIRE Number in 3 Steps
Total your expenses, pick a withdrawal rate, divide — with a full worked example.
Read guide →What Is Coast FIRE?
When compounding alone reaches your target — about $104,292 at 30 for a $1.2M goal.
Read guide →Lean FIRE vs Fat FIRE vs Barista FIRE
How the FIRE variants differ by spending and working status, with the target for each.
Read guide →Category
Investing & Growth
How money grows over time, and the simple math that explains it. Read the guide, then model it with the matching calculator.
How Does Compound Interest Work?
The formula, the intuition and the examples behind exponential growth — and why starting early beats investing more.
Read guide →What Is Dollar-Cost Averaging?
Investing a fixed amount on a fixed schedule — and the worked example that shows why it lowers your average cost.
Read guide →Simple Interest vs Compound Interest
A straight line vs a curve — the widening gap on $10,000 at 7% over 30 years.
Read guide →What Is the Rule of 72?
The mental shortcut that tells you how long money takes to double — where it comes from, when it is accurate, and how to use it.
Read guide →Rule of 72 vs Rule of 70 vs Rule of 69
Which doubling-time shortcut to use — compared at 4%, 6%, 8% and 10%.
Read guide →CAGR vs Average Annual Return
Why the average overstates volatile returns — a +50%/-50% swing averages 0% but a -13.4% CAGR.
Read guide →Present Value vs Future Value
Compounding forward vs discounting back — one example run both directions.
Read guide →Future Value of Money Formula
FV = PV(1+r)n applied to money, with monthly-compounding examples and a calculator to run it.
Read guide →Future Value Formula Explained
Every variable in FV = PV(1+r)n, the annuity version, and how to rearrange it to solve for rate, time or present value.
Read guide →How to Calculate Future Value
The future value formula step by step — lump sums, monthly contributions, inflation and real planning examples.
Read guide →The Investment Growth Formula
Total return vs annualized return (CAGR), the growth multiple, and growth with contributions.
Read guide →Monthly Compound Interest Formula With Monthly Contributions
The two-part formula for a starting balance plus monthly deposits, with verified examples.
Read guide →Future Value of Annuity Due Formula
Why beginning-of-period payments earn extra interest, with the formula and ordinary-vs-due examples.
Read guide →Time Value of Money in Excel
Do TVM in Excel or Google Sheets with FV, PV, NPER, RATE and PMT — syntax, sign convention and worked formulas.
Read guide →Time Value of Money in Google Sheets
FV, PV, NPER, RATE and PMT in Sheets — copy-ready formulas, sign convention and a worked example.
Read guide →Category
Retirement & Independence
Setting a target, reaching it, and deciding when work becomes optional.
How Much Money Do You Need To Retire?
The 25x rule, the 4% withdrawal guideline and a step-by-step method to turn your spending into a concrete savings target.
Read guide →How Much Do You Need to Retire on $50,000 a Year?
About $1.25M at the 4% rule — with 3%/5% variants and a Social Security version.
Read guide →How Much Do You Need to Retire on $75,000 a Year?
About $1.875M at the 4% rule — withdrawal-rate variants and a couple’s Social Security version.
Read guide →What Is a Safe Withdrawal Rate?
The 4% rule explained, with the portfolio you need at 3%, 3.5%, 4% and 5%.
Read guide →What Is FIRE (Financial Independence, Retire Early)?
The movement, the math and the variations — Lean, Fat, Barista and Coast FIRE — plus how to find your own number.
Read guide →How to Calculate Your FIRE Number in 3 Steps
Total your expenses, pick a withdrawal rate, divide — with a full worked example.
Read guide →What Is Coast FIRE?
When compounding alone reaches your target — about $104,292 at 30 for a $1.2M goal.
Read guide →Lean FIRE vs Fat FIRE vs Barista FIRE
How the FIRE variants differ by spending and working status, with the target for each.
Read guide →FIRE Number by Expenses
Your FIRE number is annual expenses times 25 — full tables by spending level and withdrawal rate.
Read guide →Category
Saving & Safety
The cash foundation that lets every other plan survive a bad month.
How Much Emergency Fund Do You Need?
How to size, build and store the cash buffer that keeps a bad month from becoming a financial disaster.
Read guide →3-Month vs 6-Month Emergency Fund
A head-to-head decision guide: who each target suits, a scorecard, and how long each takes to build.
Read guide →6-Month Emergency Fund Calculator Guide
How to calculate a six-month fund from your expenses, targets by spending level, and how long it takes to build.
Read guide →Emergency Fund for $3,000 a Month
How big a fund to build at $3,000 in monthly expenses, with targets by coverage and savings timelines.
Read guide →How to Reach Your Savings Goals Faster
Set targets, automate deposits, use high-yield accounts and budget smarter to hit any goal sooner.
Read guide →How Much to Save Monthly to Reach $100,000
$1,397 a month over 5 years down to $192 over 20 — and how much comes from growth.
Read guide →How to Calculate Your Savings Rate
Savings ÷ income — and why your rate, not income, sets your time to FIRE.
Read guide →How Much Should You Invest Every Month?
How much experts recommend by age and income, the 15% rule, and why consistency beats timing.
Read guide →Category
Scenarios
Real-world worked examples that answer a specific money question with numbers — pick the one closest to your situation, then run your own figures in the matching calculator.
How Much Will $10,000 Grow in 30 Years?
$10,000 over 30 years by return rate, with contributions, and after inflation.
Read guide →How Much Will $10,000 Grow in 20 Years?
The future value of $10,000 over 20 years by return rate, with monthly contributions, and after inflation.
Read guide →How Much Will $25,000 Grow in 15 Years?
$25,000 by return rate, over time, with contributions, and after inflation — all verified.
Read guide →How Much Will $500 a Month Grow in 20 Years?
$500 monthly for 20 years by rate, by contribution amount, and after inflation.
Read guide →How Much Will $250 a Month Grow in 30 Years?
$250/month by return rate, over time, at different amounts, and after inflation — all verified.
Read guide →Future Value With Monthly Contributions
How $100 to $1,000 a month grows over 10 to 40 years, rate sensitivity, and lump sum vs steady investing.
Read guide →How Long to Double Your Money at 7%?
About 10.2 years — the Rule of 72 says 10.3, overshooting by only two weeks.
Read guide →Put It Into Practice
Explore the Calculators
Theory is only half the job. Run your own numbers with the free tools that pair with these guides.
Compound Interest Calculator
See how savings and investments grow with contributions, tax and inflation.
InvestingRule of 72 Calculator
Estimate how many years it takes your money to double from a single rate.
InvestingFuture Value Calculator
Calculate the future value of a lump sum plus recurring payments.
SavingsSavings Growth Calculator
Watch a savings account grow over time with regular deposits and an APY.
RetirementRetirement Calculator
Estimate the nest egg you could have at retirement from contributions and returns.
RetirementFIRE Calculator
Find your financial independence number and years to early retirement.