Assumptions & Uncertainty
Every financial calculator uses assumptions. Understanding what they are — and what happens when they change — is more important than the number it shows you.
Key Definitions
Assumption
An input treated as true inside a model. For example, assuming a fixed 8% annual return.
Uncertainty
The future can vary, so results are not guaranteed. Real returns fluctuate year to year.
Model
A simplified tool that turns assumptions into outcomes. It calculates, it does not predict.
Why This Matters
A model is not a prediction machine. It calculates results based on the assumptions you choose.
Small changes in assumptions can significantly change results. A 1% difference in return compounded over years creates a surprisingly large gap.
Understanding the assumptions behind a number matters more than just reading the output. The output is only as reliable as its inputs.
Try It: Sensitivity Demo
Formula:
FV = P × (1 + r)tClick a button on the left to adjust the return rate and see how the result changes.
Trade-Offs to Remember
Higher Return = More Uncertainty
Investments promising higher returns carry more uncertainty about actual outcomes
Time Helps But Doesn't Remove Risk
Longer time horizons reduce some volatility but can never eliminate uncertainty entirely
Models Compare, Not Guarantee
Use models to compare choices and understand trade-offs — not as guarantees of what will happen