How the savings goal calculator works
Two directions, one formula
Given a monthly amount, the balance grows by (balance × monthly return) + contribution each month — walk it forward until it crosses the goal and you have the date. Given a deadline, solve the annuity formula backwards for the required payment. Same arithmetic, opposite reading directions — and the calculator does both so you can negotiate with yourself honestly.
Return choice by goal type
The return input should match where the money actually sits: 0–1% for an emergency fund (savings rates), ~4–5% for a house down payment parked in CDs/Treasuries (short horizon, capital preservation), 6–8% for retirement horizons in diversified funds. Using stock returns for a 3-year goal is how down payments meet down markets.
The +$100 column is the real answer
The timeline itself matters less than its sensitivity. If +$100/month cuts two years, the goal is behaviorally reachable — buy the coffee, automate the extra. If +$200 barely moves it, the gap is structural: raise the goal's date, the return (via allocation), or the income engine.