Switch between finding out when you'll hit a savings goal, or how much you need to save monthly to hit it by a specific date.
Your current savings compound at the monthly rate, and your monthly contribution is added on top each month, until the running balance reaches your goal — this is simulated month by month rather than solved with a single formula, so it stays accurate even with a step-up in contributions or an unusual starting balance.
This solves the future-value-of-annuity formula in reverse: your current savings are projected forward to the target date first (current × (1+r)^n), and whatever gap remains to the goal is divided across the remaining months — accounting for compounding — to find the required contribution.
Use 0% or close to it for a plain savings account, and a market-linked rate (with realistic volatility in mind — markets don't return the same percentage every single year) only if the money is actually invested in something like a brokerage account or index fund. Mixing up the two — assuming savings-account money grows like the stock market — produces a badly wrong answer, usually one that's too optimistic.
Worked example: starting from $2,000 with a $500/month contribution at 4% annual return, reaching a $20,000 goal takes about 2 years 10 months. Working backward from the same goal over exactly 3 years instead requires about $463/month — enter these numbers in each tab to confirm.