Saving for something specific raises one of two questions, and which one you have depends on what is fixed. If the monthly amount is what you can spare, you want to know the date. If the date is fixed — a wedding, a deposit deadline, a car that will not last another winter — you want to know what it costs a month.
They are the same arithmetic run in opposite directions. This calculator does both from one form, and switching between them keeps everything you have already typed.
This is for a specific, dated goal — a deposit, a car, a wedding, a sabbatical. It is not for open-ended saving, where the useful question is what share of your income you keep rather than what a particular target costs a month.
This calculator needs JavaScript. Ignoring interest, which for a short goal barely moves the answer: subtract what you have saved from what you need, then divide by the months you have to get the monthly figure — or divide by the monthly figure to get the months. Interest makes both slightly better than that.
Why a weekly figure is worth seeing
A monthly number is the one the arithmetic produces, but it is not always the one that changes behaviour. Five hundred a month is a decision you make once and then either keep or quietly abandon. A hundred and twenty a week is a decision you make every few days, against things you are actually choosing between.
Neither framing is more accurate — they are the same money. But people are consistently better at holding a small recurring figure in mind than a large one, which is why the calculator shows both.
A worked example
A $15,000 goal, two years away, with $2,000 already saved and 3% interest on the balance:
The $2,000 grows to $2,124 over the period on its own, leaving $12,876 to find. Spread across 24 months with interest working on each deposit, that is $521 a month, or about $120 a week.
The weekly figure is deliberately the one to look at. $521 a month sounds like a budgeting exercise; $120 a week sounds like a decision about specific things you would otherwise buy, and that is the level at which people actually change behaviour. It is the same number either way — but only one of the two is comparable to the things it is competing with.
Where the interest actually matters
Over a short goal, interest is close to irrelevant. On two thousand pounds saved towards a fifteen thousand pound target over two years, a good rate moves the monthly figure by a few pounds. It is worth having, but it is not the lever.
Over a long one it stops being a rounding error. The further out the date, the more of the total comes from growth rather than from you, and past roughly a decade the rate starts to matter more than small changes to the monthly amount. That is the same effect the compound interest calculator is built to show directly.
The practical consequence is about where the money sits. A goal eighteen months away belongs somewhere instant-access and safe, and chasing return with it is a mistake. A goal a decade away held in cash is a different mistake.
What this assumes
A flat interest rate for the whole period, monthly compounding, contributions landing at the end of each month, and no tax on the interest. Real accounts vary the rate, and an introductory bonus that expires after twelve months will not behave like the projection.
It also assumes you keep going, which is the assumption most likely to break. If the monthly figure comes out higher than feels sustainable, that is worth treating as information rather than as a target to grit your teeth against — a smaller amount you actually maintain beats a larger one you abandon in month four.
If the number is the problem, the subscription audit is the usual place to find the first hundred a month, and our saving guides cover the habit side.