Eight finance calculators, free to use
Free, no sign-up, and everything runs in your browser so your numbers stay private. Every engine is checked against the standard closed-form financial formulas, and how that checking works is written up in full.
Paycheck Calculator
Your salary is not your paycheck. See gross-to-net with federal brackets, Social Security, Medicare, and pre-tax 401(k) and HSA deductions.
Open calculator → C02Investment Calculator
See what your money becomes. Model contributions, compounding, and expected return with a live growth chart and inflation-adjusted totals.
Open calculator → C03Loan & Mortgage
Know the real cost of borrowing. Monthly payments, total interest, PMI, and a full amortization schedule, plus what extra payments save you.
Open calculator → C04Auto Lease
Understand a car lease. See how depreciation, the money factor, and residual value shape your monthly payment, due at signing, and total cost.
Open calculator → C05Rent vs Buy
Should you rent or buy? Compare your net worth over time for each path, including what your down payment could earn if you invested it instead.
Open calculator → C06Debt Payoff
Get out of debt on a plan. Compare the snowball and avalanche methods side by side and see exactly which debt to attack first.
Open calculator → C08Net Worth Tracker
Know where you stand. Categorize what you own and owe, check your health ratios, and save snapshots to watch your progress over time.
Open tracker → C07Budget Builder
Give every dollar a job. Build a 50/30/20 or zero-based budget with editable targets and a live picture of where your money goes.
Open calculator →Economics and finance you can actually poke at
The calculators above answer questions about your own money. These are different: working models of how a market, a firm, or a whole economy behaves, running from supply and demand through monopoly, tariffs and game theory to DCF valuation and retirement risk. Pull the levers yourself and watch what breaks.
Retirement
Will the money last? A thousand simulated lives instead of one smooth line, showing why the order your returns arrive in can matter more than the average.
Test a plan → S02DCF Valuation
Value a company the way an analyst does. Forecast the cash, build the discount rate from its parts, then see how far the answer moves when two assumptions shift.
Value a company → S03Run a Business
Five years to build a company. Set price, headcount, marketing and R&D each quarter against a rival who fights back, and find out what actually pays for the fixed costs.
Take the job → S05Supply and Demand
Drag a rent ceiling, a minimum wage, or a sales tax across the market and watch shortages, tax incidence, and deadweight loss appear on the diagram.
Open the market → S04Macro Policy Simulator
You run the economy. Set interest rates, spending, and taxes, then steer inflation and unemployment through shocks you do not control.
Run the economy → S12Bonds & Yield Curve
Price a bond, see duration and convexity checked against a full repricing, then invert the yield curve and read the forward rates hiding inside it.
Price a bond → S13Portfolio Diversification
The only free lunch in finance. Drag correlation and watch two risky assets combine into a portfolio safer than either one alone.
Build a portfolio → S09Tariffs
Who actually pays for protection? Split the cost into producer gain, government revenue, and the part that reaches nobody at all.
Set a tariff → S10Inequality
Where the Gini coefficient comes from, how far redistribution moves it, and two economies that score identically while looking nothing alike.
Read the curve → S11Game Theory Solver
Type any payoff matrix and it finds the dominant strategies, every Nash equilibrium including the mixed ones, and which outcomes waste value.
Solve a game → S06Production Costs
When is losing money the right call? Cost curves, the shutdown point, and where a firm's supply curve actually comes from.
Find the shutdown point → S07Monopoly
Why a firm with no competitors deliberately sells less than it could, and what that restraint costs everyone else.
Open the model → S08Externalities
Why markets overproduce pollution, and how a tax sized to the damage raises total welfare instead of destroying it.
Open the model → S14Comparative Advantage
A country worse at making everything still gains from trade. Set the productivity of two economies and watch both consume beyond what they could produce alone.
Open the model →Money questions, worked out rather than argued about
Most financial advice tells you what to do. These take one question people actually ask, compute the answer from the models above, and say plainly where that answer stops being reliable. Every figure is traceable to a calculator on this site.
Pay off the mortgage or invest?
Prepaying earns exactly your interest rate, guaranteed. So the break-even is one number, and at 6.5% the two paths finish $3 apart on $221,000.
Read the article →How much house can I afford?
A lender approves you on gross income. You pay from take-home. The 28% rule is really 39% to 42% of the money that actually arrives.
Read the article →How much do I need to retire?
25 times your spending fails about one time in four. A flat-return calculator says $2.00M; allow returns to vary and the median life ends with $725k.
Read the article →Snowball or avalanche?
Avalanche saves $728 here, and the gap scales with how far apart your rates are. But the extra $250 a month is worth five times either method.
Read the article →Is leasing a car throwing money away?
You pay for depreciation either way. Three routes to the same car land within $751 of each other over three years, and the answer changes in month 37.
Read the article →All articles
Every article in one place, each one answering a single question with numbers you can reproduce.
Browse articles →Where the ideas behind the tools get taught
A calculator answers a question once you already know how to ask it. This is the part before that: opportunity cost, compounding, risk, insurance and earnings, each module giving you the vocabulary the next one assumes. Free, no account, and there are slides and answer keys if you are teaching it.
I bring aerospace-floor precision to financial analysis.
I spent nearly two years laying up composite parts bound for SpaceX rockets, the Boeing V-22, and the 737 MAX, where a misread spec is not an option. Then I earned a mathematics A.S. and an economics B.A. Now I point that same discipline at financial models: build them carefully, then check the math.
Let's talk.
If you're hiring, want to build something together, or just want to talk shop about the tools and the math behind them, I'd like to hear from you.
Colophon
- Where it runs
- Entirely in your browser. Nothing you type into a calculator is sent anywhere, because there is nowhere for it to be sent.
- Checked against
- The standard closed-form financial formulas, and in several cases a second independent implementation written to disagree with the first.
- Tax figures
- 2026 brackets and standard deduction, with the Social Security wage base at $184,500. These change annually and are the first thing to go stale.