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Foundations

Calculators and tools

Pomegra Learn

Calculators and tools

Understanding the mathematics of compounding is one thing. Building a spreadsheet that correctly implements it is another. Excel has powerful functions—FV, PV, RATE, NPER, IRR, XIRR—that solve compounding problems. But they have quirks. The RATE function sometimes fails to converge if you give it bad initial guesses. XIRR can give confusing results with certain cash-flow patterns. A spreadsheet that looks right, with proper formatting and reasonable numbers, can be subtly wrong in ways that don't become apparent until you're projecting a decade out.

This chapter is a practical guide. We'll walk through the built-in financial functions, show you how to build your own projection spreadsheet from scratch, and explain what Monte Carlo simulation is and why you might want it for understanding uncertainty. We'll also cover the online tools and mobile apps that do this work for you: Portfolio Visualizer, FIREcalc, cFIRESim, Empower, and others.

Each tool has a purpose. A simple calculator tells you how much money you'll have in 30 years at a fixed return. A Monte Carlo simulator shows you the range of outcomes and the probability of success under varied market conditions. Neither is perfect, but together they give you a mental map of the landscape.

Building your own spreadsheet​

You don't need fancy software to model compounding. Excel and Google Sheets have everything you need. We'll build a basic projection model from scratch that takes your starting balance, your contribution schedule, and a return assumption, and produces your projected future balance. A simple row-by-row calculation: previous balance, multiply by (1 + return rate), add contribution.

Then we'll extend it: add inflation to see what your purchasing power will actually be, add taxes if you're investing in a taxable account, add varying return rates to see how the model behaves under different scenarios. Each addition teaches you something about how the pieces interact. You'll see viscerally how a 1% change in return rate affects your 30-year projection.

The limitations of tools​

Every tool makes assumptions, and you need to understand what those assumptions are. Most assume constant returns (the real world varies significantly). Most assume you follow the plan (you might panic-sell during a crash). Most assume no market timing (you might not). Most assume you have a single income stream and regular contributions (your life might be messier).

Tools are useful for building intuition and exploring scenarios, not for predicting the future. A projection model that says you'll have $500,000 at retirement is not a prediction; it's a point estimate assuming everything goes according to plan. A good tool will also show you the range of outcomes: you might have anywhere from $400,000 to $650,000 depending on market conditions. We'll explore where tools shine and where they mislead, and how to use them responsibly.

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