This calculator works backwards from your goal: given a target amount, a timeframe, any existing savings, and an interest rate, it finds the monthly contribution needed to get there.
The interest rate compounds monthly, so your existing savings and each monthly payment start earning interest immediately. The calculator uses the future-value-of-an-annuity formula to work out the level monthly payment that, together with your existing savings growing at the given rate, reaches your goal by the deadline. Even a modest rate makes a meaningful difference over several years — a 4.5% rate on a three-year goal reduces the required monthly saving by roughly 7% compared to keeping the money under the mattress.
The interest rate to use depends on where you're saving. Easy-access savings accounts and cash ISAs typically offer 3–5% at current rates, though this varies and can change. Fixed-term bonds may offer slightly more in exchange for locking your money away. If you're saving into investments rather than cash, returns are less predictable — use a conservative estimate and revisit the calculation annually. For goals within one to two years, the interest effect is small enough that the exact rate matters less than the consistency of your contributions.
If your current savings alone will grow to meet or beat your goal by the deadline — with no further contributions — the calculator tells you that directly instead of showing a monthly figure, along with how much of a surplus you'd end up with.
This is a planning estimate, not a guarantee — real savings rates change over time, and this calculator assumes a single fixed rate for the whole period. It doesn't account for tax on interest (relevant if you're saving outside a tax-free wrapper like an ISA) or for inflation eroding the real value of your goal.