A $400,000 mortgage at 7% interest costs you roughly $322,000 in interest over 30 years. Almost as much as the house itself. Most people learn this number at the closing table, when it's far too late to negotiate anything. The whole point of this page: run these numbers before you're sitting across from someone who profits from your confusion.
The mortgage math nobody walks you through
Here's the part lenders don't emphasize: in the early years of a 30-year mortgage, the vast majority of your payment is interest, not principal. On that $400,000 loan at 7%, your first monthly payment of ~$2,661 sends about $2,333 to interest and only $328 toward the house. You're essentially renting from the bank for the first decade.
That's why the mortgage calculator shows a full amortization schedule, not just the monthly number. Play with the inputs: what happens if you pay an extra $200 a month? (You'd save roughly $70,000 in interest and finish 7 years early.) What does a 15-year term do? The home affordability calculator works the other end — the 28/36 rule says your housing costs shouldn't exceed 28% of gross income or 36% of total debt. Banks will happily approve you past that. Don't let them.
Your 401(k) match is a 100% return you're ignoring
If your employer matches 50% of your 401(k) contributions up to 6% of salary and you're not contributing at least 6%, you're turning down free money with a guaranteed 50% instant return. No investment on earth offers that. Yet about 1 in 4 workers with access to a match leaves some of it on the table.
The 401(k) employer match calculator shows what capturing the full match compounds into over a career — and the compound interest calculator makes the broader point visceral. $500 a month at a 7% average return becomes about $1.2 million in 35 years. Start ten years later and it's roughly $600,000. Same monthly effort, half the result. Time is the variable that matters most, which is why "I'll start investing next year" is the most expensive sentence in personal finance.
Debt: avalanche vs. snowball, settled with numbers
Got multiple debts? Two schools of thought. The avalanche method (highest interest rate first) wins on pure math — you'll pay less interest, full stop. The snowball method (smallest balance first) wins on psychology — quick wins keep you motivated. The debt payoff calculator models both so you can see the actual dollar difference for your situation instead of arguing about it online.
Honestly? Pick the one you'll actually stick with. A mathematically perfect plan you abandon in March loses to an imperfect plan you follow for two years.
Taxes, salaries, and the self-employed
W-2 employees lose a chunk of every paycheck to taxes they barely understand. The salary calculator breaks down federal, state, Social Security, and Medicare so you see where it goes. Freelancers have it harder: the self-employment tax calculator covers the 15.3% SECA tax that surprises every new 1099 worker in April — plus the quarterly estimated payments that avoid penalties.
Also here: capital gains (short-term vs. long-term rates — holding an asset 366 days instead of 364 can cut your tax rate dramatically), the HSA triple-tax advantage (deductible in, grows tax-free, tax-free out for medical — the only triple-advantaged account in the US code), overtime pay, tip splitting, auto loans, lease-vs-buy math, and closing cost estimates for buyers and sellers.
Why financial calculators should never ask for your data
Think about what you'd type into a mortgage or salary calculator: your income, your debts, your home value, your age. That's a financial fingerprint. Many free calculator sites monetize exactly this — lead-gen forms disguised as tools, your details sold to lenders within the hour. Every calculator here runs the math in your browser and sends nothing anywhere. Your salary stays your business.
Three numbers that change decisions
Let me give you three specific numbers to run for yourself today. First: put your actual mortgage (or dream mortgage) into the calculator with one extra payment per year. On a $350,000 loan at 7%, that single extra payment annually saves roughly $60,000+ in interest and cuts ~6 years off the loan. It's the highest-return "investment" most homeowners have access to. Second: enter your credit card balance into the debt payoff calculator and look at the total interest if you pay minimums only — then look at what $100 extra a month does. The gap is usually staggering.
Third: if you're freelance, run last year's income through the self-employment tax calculator and compare it against quarterly estimated payments. The penalty for underpaying isn't huge, but the April surprise bill is brutal — knowing the number in October beats discovering it in March. These three calculations take under five minutes combined and they're worth more than most finance books.