An interactive reader · compound interest

Source code ↗

What time does with money.

Each sentence below is a small financial story you can rewrite. Amber numbers are yours to edit: click one, type, or nudge it with ↑ ↓. Green numbers are what time does with them.

Currency symbols only; amounts don't convert

A $4 coffee, 365 days a year, invested instead at 7% a year, would grow to $0 after 20 years, on just $0 of skipped coffee.

I

Small habits

What everyday spending quietly adds up to

A $5 coffee, 300 days a year, is $0 a year, and $0 over 10 years.

A $4 latte, 7 days a week, sets you back $0 a week, $0 a month, and $0 a year.

A commute coffee, 5 days a week, 50 working weeks a year, is 0 cups: a $4 habit that really averages $0 a day, not $0, and costs $0 a year.

Even a modest $2 latte, 4 days a week, means $0 spent after 10 years.

A $8 pack of cigarettes a day burns through $0 a year, and $0 in a decade.

Paying a $35 late fee every month flushes $0 a year down the drain, and $0 over ten.

II

Saving

Steady deposits, given time

Save just $1 a day at a 7% return and after 30 years you'll be holding $0.

Start with $25,000, add $500 a month at 7% compounded monthly, and in 15 years the account holds $0.

A single $1,000 invested at 5% becomes $0 after 10 years, without adding a cent.

$150 a month at 10% quietly grows into $0 in 10 years.

At $5 a day and 10% returns, reaching $1,000,000 takes 0 years. Double it to $10 a day and you get there 0 years sooner.

III

Investing

Growth, rate, and patience

$1,000 in an index fund growing 10% a year becomes $0 after 40 years.

A $1,000 bet on a stock compounding at 31% is worth $0 ten years later, a total return of 0% over 10 years.

Choose the smaller house, invest the $6,000 yearly difference at 6%, and after 30 years you'd have $0. Hold it 20 more years and it's $0.

Rate matters more than it looks: $10,000 for 30 years at 10% becomes $0, but at 12.5% it becomes $0.

Withdraw the gains on $10,000 at 7% every year and you'll earn $0 in 40 years. Let them compound instead, and you'll earn $0.

IV

Present value

What future money is worth today

At 10% interest, $1,100 a year from now is worth exactly $0 today.

A promise of $500 next year, discounted at 10%, is worth $0 right now.

$900 arriving in 3 years, at 10%, equals $0 in today's money.

A $10,000 lump sum due in 5 years, discounted at 7%, is the same as $0 today.

Every $1 you spend today is really $0 taken out of your pocket 20 years from now, at an 8% return.

V

Inflation & taxes

The quiet leak

An item that costs $100 today will cost $0 in 10 years at 3% inflation.

Put $100 under the mattress for 20 years at 3% inflation and it will only buy what $0 buys today.

To match the purchasing power of $60,000 today, in 20 years you'll need $0, assuming 3% inflation.

$100 at 5.05% APY shows $0 on your statement after 20 years, but with 3% inflation it's really worth $0.

A nominal nest egg of $350,000 arriving in 40 years, at 2.5% inflation, is only $0 in today's dollars.

Pay 28% tax on the interest and a 5.05% account really earns 0%; after 20 years, with 3% inflation, your $100 is worth just $0 in today's money.

VI

Time

The other compounding currency

20 minutes a day is 0 hours a year, the same as 0 full eight-hour workdays. And 0 minutes a day is 0 hours a year.

2 hours of scrolling a day quietly consumes 0 hours a year: 0 entire days, awake and asleep.

Read just 10 pages a day and you'll finish 0 pages a year, about 0 books of 300 pages each.

Practicing 1 hour a day, reaching 10,000 hours of mastery takes 0 years; at 3 hours a day, only 0.

Earning $60,000 a year over 2,000 working hours means $0 an hour, so a $5 coffee costs 0 minutes of your life.

VII

Starting early

Save early and often

Save $100 a month from age 18 at 6% and by 65 you'll have $0. Wait until 25 and you end up with only $0. That's $0 lost to 0 years of waiting.

Raise the return to 9% and the head start from 18 to 25, at $100 a month until 65, is worth $0, bought with just $0 of extra deposits.

Open a $350-a-month fund the day your baby is born, earning 7%, and by the time she starts university at 17.5 it holds $0.

A 25-year-old saving $200 a month at 7% retires at 65 with $0, and every single year of delay erases about $0 of it.

To retire at 65 with as much as a friend who saved $150 a month from age 20 at 7%, someone starting at 30 must put away $0 every month, as much.

VIII

Reaching a goal

Working backwards from the number you want

To reach $50,000 in 10 years at 7%, you need to save $0 a month.

Hitting $1,000,000 by age 65 at 8% takes $0 a month if you start at 30, but $0 a month if you wait until 40.

Put weekly: $1,000,000 in 40 years at 7% is $0 a week, about $0 a day.

A $30,000 down payment in 5 years at 4% means setting aside $0 a month; under the mattress, it would take $0.

Already holding $10,000? Then reaching $100,000 in 15 years at 7% needs only $0 a month, instead of $0 from zero.

IX

The big wins

The skeptic's view: decisions beat lattes

A fair objection to everything above: a handful of big decisions can outweigh years of small sacrifices.

Negotiate $1,000 off your next car and you've paid for 0 lattes at $5 each, almost 0 years of one every weekday morning.

How many lattes is that, anyway? A $2,500 impulse purchase equals 0 lattes at $5, one every day for 0 years.

Negotiating a salary just $5,000 higher, invested at 7% every year for 30 years, becomes $0, the biggest latte of all.

Overpay $20,000 on a house, at a 7% opportunity cost over 30 years, and you've lost $0, equal to 0 years of a daily $5 latte habit.

And yet: a $5 coffee, 300 days a year, is still $0. That's 0% of a $50,000 income. The honest answer is to do both: win the big negotiations and mind the habits.

X

Investing behavior

How you invest, not just how much

Slightly more advanced: three classic timing dilemmas, reduced to arithmetic.

Hand a $12,000 windfall to the market at once at 8% and it's $0 after 10 years. Drip it in over 12 months instead and you end with $0, a $0 price for the comfort of averaging in.

Stay fully invested at the market's historical 5.6% and $10,000 becomes $0 over 20 years, but investors who missed just the 10 best days earned about 2%, ending with $0. Half the outcome hid in a handful of days you can't predict.

Waiting in cash for a 10% dip while the market compounds at 8% only pays off if the dip arrives within 0 years. After that, the "expensive" market you refused to buy is still cheaper than your patience.

XI

Housing

The biggest numbers most people ever touch

A $300,000 house appreciating 3% a year is worth $0 after 20 years. Renting instead, and investing the $60,000 down payment plus $500 a month of ownership savings at 8%, grows to $0. The winner lives entirely inside these assumptions.

On a $300,000 loan at 6%, the 30-year mortgage costs $0 a month and $0 in total interest; the 15-year costs $0 a month but only $0. The shorter loan buys back $0.

Put less down and invest the difference: keeping $45,000 in the market at 8% grows to $0 over 30 years, while borrowing that same amount at 6% costs $0 more in interest. The gap between the two rates decides who wins.

Overpaying the mortgage by $200 a month is a guaranteed return at your loan's 6%, worth $0 over 25 years. The same money invested at 8% reaches $0: a $0 edge, paid for in risk.

XII

Debt

Compounding, working against you

On a $5,000 card at 24% APR, the interest alone is $0 a month; pay less than that and the debt is immortal. Pay $150 and you're free in 0 months with $0 of interest; $250 takes 0 months and $0.

Stretch a $30,000 student loan at 5% from 10 years to 25 and the payment falls from $0 to $0 a month, but the total interest jumps from $0 to $0.

Finance a $25,000 car at 8% for 5 years: $0 a month, and the car really costs $0; the $0 of interest is 0% of the sticker price.

Carrying $10,000 at a mortgage-like 4% costs $0 a year; the same balance on a 20% credit card costs $0, as much. "Debt" is one word for two very different animals.

Putting $200 a month against a 20% debt is a guaranteed return at that rate, worth $0 over 5 years, while the same money invested at 8% reaches only $0. Kill high-interest debt first; the market can't reliably beat it.