The dilemma
Every surplus dollar has two jobs. Only one can win.
The fixed interest saving
Every dollar you put toward debt saves you its interest rate — after-tax, based on the rates you enter. Paying off a 22% APR card saves you an estimated 22 cents of interest on every dollar, a saving no investment return is guaranteed to match.
The compounding bet
Historically, stock returns have averaged about 10% a year nominal — roughly 7% after inflation. Money you invest now compounds for decades — and every year your cash sits in a 1% savings account, or a 4% loan, is a year of compounding you never get back.
The honest answer? It depends on your rates, your tax bracket, and your cash buffer — not on a rule of thumb. That's exactly what the engine calculates.
The solution
Real math, your numbers
Enter your numbers
Debt balances and APRs, monthly surplus, emergency fund, expected market return — about 60 seconds, no bank connections.
The engine runs the math
Debt-first, balanced, and invest-heavy strategies are simulated year by year, with tax-adjusted returns and inflation baked in.
See which is projected to come out ahead
Compare 30-year net worth trajectories, crossover points, and risk-adjusted outcomes — for your situation, not an average one.
The visualizer
See your net worth 30 years out
Debt-first jumps ahead early; investing pulls ahead over decades. Your personal crossover point is the whole answer — and it moves with every number you change.
What you get
A complete allocation toolkit
Tax-Adjusted Return Math
Investment returns are netted for capital-gains drag and compared against the true after-tax cost of your debt.
Emergency Cash Buffer Checks
Every strategy is screened for how exposed it leaves you if life happens mid-plan — before you commit a dollar.
Strategy Simulations
Debt-first, balanced, and invest-heavy plans simulated year by year on your actual balances and rates.
Optimized & Debt-First Comparison
Pro lines overlay the optimized allocation against debt-first so you can compare where each plan is projected to come out ahead.
Inflation-Adjusted Overlay
Toggle between nominal dollars and real purchasing power so optimism can't hide behind 3% a year.
PDF Projection Reports
Export your full 30-year projection as a clean report to share with a partner, planner, or future you.
Why Surplus
Built for real decisions
See your numbers, not a rule of thumb
Compare debt-first vs. invest-heavy side by side
Export a PDF to share with a partner or planner
Surplus Pro
One price. Every simulation, comparison, and export. Forever.
$27 one-time
- Optimized & Debt-First comparison lines
- Inflation-adjusted real net worth overlay
- Unlimited saved scenarios (stored on this device)
- Export PDF projection reports
- Risk-adjusted return analytics
- Lifetime access — pay once, done
Secure checkout via Base44 Payments · No subscription · 30-second delivery
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