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For Students · Retirement Planner

Retirement Planner for a college fund

Built for students learning it for a class or exam. See the concept move instead of memorizing formulas — and check your homework intuition. Simulate a college fund live below — adjust the inputs and watch it respond, right in your browser.

1

Compound Interest

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Compound Interest & InvestingLive

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Compounding means earning returns on your past returns, so growth accelerates over time. The gap between the balance line and your total contributions is pure compound interest — and it widens dramatically in the final years. Educational tool, not investment advice.

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Data Inspector

Final balance$691,150
Total contributed$190,000
Interest earned$501,150

Governing equation

Reading this result: Rule of 72: at 7.0% a year, money roughly doubles every 10.3 years — about 2 doublings across your 30-year horizon. Each $500/mo contribution then earns returns on its own past returns, so the balance curve pulls away from your flat contribution line. Because compounding is exponential, most of the $501,150 in interest is created in the final years — time in the market matters far more than the amount. Educational tool, not investment advice.

Runs locally in your browser — free forever. Scale to the cloud when reality gets heavy.

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2

Monte Carlo

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Monte Carlo Price SimulationLive

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Geometric Brownian motion models a price with constant drift and random volatility — the assumption behind Black-Scholes. Running hundreds of simulated paths reveals the full distribution of outcomes, not just an average. Educational tool, not investment advice.

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Data Inspector

Median outcome$0
5th percentile$0
95th percentile$0

Governing equation

Reading this result: Median outcome is about $0, just under the drift-only $107 — GBM compounds in log-space, so volatility always pulls the typical path below the average.

Runs locally in your browser — free forever. Scale to the cloud when reality gets heavy.

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3

Amortization

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Loan / Mortgage AmortizationLive

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Presets

A fixed-rate loan has a level monthly payment, but its split shifts over time: early payments are mostly interest, later ones mostly principal. That is why paying extra early, or choosing a shorter term, saves so much interest. Educational tool, not financial advice.

▶ Run in Python

Data Inspector

Monthly payment$2,212
Total interest$446,406
Total paid$796,406

Governing equation

Reading this result: At 6.50% APR over 30 years, the monthly payment is $2,212 and you pay $446,406 in interest — about 128% of the amount borrowed. The payment is level, but its split shifts: your first payment is mostly interest (86% of it), while the last is almost all principal. A higher rate or a longer term sharply raises the total interest, and paying extra early attacks principal before interest can accrue on it.

Runs locally in your browser — free forever. Scale to the cloud when reality gets heavy.

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Also for:ResearchersEducatorsEngineersHobbyists & MakersK-12 StudentsFirst RespondersGeneral version →

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Frequently asked questions

Is this good for students?
Yes — this version of "Retirement Planner for a college fund" is framed for students learning it for a class or exam. See the concept move instead of memorizing formulas — and check your homework intuition.
Do I need to install anything?
No. It runs in any modern browser, free, with no account required.