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.
The required contribution is solved from the future-value-of-an-annuity formula, which assumes you deposit the same amount at the same interval and earn a constant return. Reality is messier on all three counts, so treat the output as a target rather than a guarantee.
To save $30,000 in five years at 4%, the calculator asks for $452.50 a month. Over 60 months you contribute $27,149.74 of your own money and the interest supplies the remaining $2,850.26 — about 9.5% of the goal. That ratio is worth internalising: over a five-year horizon, saving does almost all the work and returns do very little. Chasing an extra percentage point of yield on a five-year goal is largely wasted effort compared with increasing the deposit.
Stretching the same goal from five years to ten more than halves the monthly requirement, because you both make twice as many deposits and give compounding meaningfully longer to contribute. Shortening it has the opposite and equally dramatic effect. If the required monthly figure looks impossible, the horizon is almost always a more powerful lever than the return assumption.
The common mistake is entering an equity-like return on a two-year goal. The arithmetic will happily accept 10%; the deadline will not.
Set the target slightly above what you need. Goals with a fixed date and a fixed cost — a deposit, a wedding, a move — almost always cost more than the original estimate, and discovering the shortfall in the final month leaves no time to fix it.
If your account pays flat non-compounding interest, the Simple Interest Calculator is the closer match. The same ground is covered chapter by chapter in our book, From Paycheck to Portfolio.