Deadlines and amounts: the only two honest questions
Every savings goal reduces to one of two questions. Either the deadline is fixed — a house deposit in twelve months, a tax bill in April, a wedding in June — and you need the monthly amount that gets you there. Or the amount is fixed, because it is what your budget can spare, and you need to know when you will arrive. This calculator answers both: leave the monthly amount at zero and it divides the remaining distance by your deadline, adjusted for interest; enter an amount instead, and it counts forward month by month until the goal is reached.
The interest line surprises people in both directions. At a 4% APY, a year's saving toward $15,000 earns a few hundred dollars — real money, worth having, and the reason the required monthly amount is a little less than simple division suggests. But interest is a tailwind, not the engine: on household timescales, the amount you deposit matters far more than the rate you earn, which is why this tool shows the required amount per week and per biweekly paycheck as well as per month. A goal that looks impossible monthly often reads as an achievable amount per paycheck, and paycheck-sized numbers are the ones budgets are actually built from.
Two practical notes. If you have already saved part of the goal, enter it — the tool only makes you fund the distance remaining, and watching "still to save" shrink is the best motivation the math offers. And if the required monthly amount comes back larger than your budget allows, the honest options are the three the formula contains: a later deadline, a smaller goal, or a larger starting amount. Changing the APY barely moves the answer.