I have a bookmarks folder called "run the numbers." It's not glamorous. It's the folder I open before I sign anything, accept anything, or buy anything expensive. Over the years it's saved me from at least two bad car deals and one truly awful job offer that looked great on paper.
Here's what's in it, and the real math behind why each one matters.
A friend was offered a $95,000 salary or a $70-an-hour contract. The contract sounded richer. It wasn't, quite.
Here's the thing most people miss: a W-2 salary quietly includes about 7.65% in employer payroll taxes you'd pay yourself as a contractor, plus benefits worth roughly 20 to 30% of salary. So $95,000 salary is really a $115,000 to $120,000 package. At $70 an hour for 2,000 hours, the contract is $140,000 gross, but after self-employment tax and buying your own health insurance (call it $6,000 to $9,000 a year for an individual), the gap shrinks fast.
$70 × 1,000 = $70,000 effective vs $95,000 salary. He took the salary. Run your own numbers with the 1099 vs W-2 calculator.
Dealerships love the monthly payment conversation because it hides the total. "Only $489 a month!" sounds fine until you realize it's 84 months at 9% and you're paying $41,000 for a $28,000 car.
The honest math: keep total car costs (payment, insurance, gas, maintenance) under 15% of take-home pay. On $5,000 a month take-home, that's $750 all-in. If insurance is $180 and gas is $150, your payment ceiling is $420, which at 7% for 60 months buys roughly a $21,000 car. That's the real answer, and it's almost always less than what the salesperson suggests.
I check the car affordability calculator before I ever set foot on a lot. It keeps me honest.
Everyone knows refinance. Almost nobody knows recast. If you come into a lump sum — a bonus, an inheritance, a sale — you can hand your lender $30,000 or $50,000 and ask them to re-amortize your existing loan. Same rate, same term, lower payment. The fee is usually a few hundred dollars, not the thousands a refinance costs.
Example: $320,000 balance at 7% with 25 years left. Drop $50,000 on a recast and the payment falls by roughly $330 a month, and you keep your original rate. A refinance to 6.5% would save a similar amount monthly but cost $4,000 to $6,000 in closing costs. The recast wins on payback in under two months.
Compare all three paths — recast, refinance, or just making extra payments — with the mortgage recast calculator.
If you're offered a pension buyout, this is a one-way door. Take the $180,000 lump sum or $1,100 a month for life?
The break-even math: $1,100 a month is $13,200 a year. Divide the lump sum by the annual payment: $180,000 ÷ $13,200 ≈ 13.6 years to break even, ignoring investment returns. If you can invest the lump sum at 5% after tax, the break-even stretches past 17 years. So the real question is how long you'll live and how disciplined an investor you are. There's no shame in admitting you'd rather have the guaranteed check — that's what the annuity is for.
Work through your own numbers, including joint-and-survivor options, with the pension lump sum calculator.
Coast FIRE is the idea that you save enough early that compound growth alone carries you to retirement, and then you just cover your bills with work you don't hate. No more aggressive saving required.
The math surprised me: $150,000 invested at 30, growing at 7% real, becomes about $1.6 million by 65 without another dollar added. That's the whole trick. If you've already got the principal, the pressure's off — you can take the lower-stress job.
Check where you stand with the Coast FIRE calculator.
None of these decisions should be made on vibes. Ten minutes with a calculator beats ten years of regret. That's the whole philosophy behind this directory: the math is free, so use it before the money's gone.