Savings Calculator

Building savings realistically usually involves both a starting lump sum and ongoing monthly contributions together, not one or the other in isolation — and how much of the growth you actually keep depends on whether the interest earned is taxable for you specifically. This calculator combines all three factors — initial deposit, monthly additions, and your tax position — into one growth projection.

Why Combining Both Matters More Than Either Alone

A calculator that only handles a lump sum, or only handles monthly contributions, forces you to approximate your actual real-world situation, which for most people genuinely involves both — an existing savings balance you already have, plus an ongoing amount you intend to keep adding. This calculator models both together specifically because the combined trajectory is not simply the sum of running each separately; the existing lump sum compounds throughout the full term while the monthly additions each compound for a progressively shorter remaining period, and seeing both effects combined gives a genuinely more accurate picture of where your actual savings plan leads.

This is worth using directly with your own real, current numbers rather than a rounded, approximate version of them — the compounding effect on an existing lump sum is genuinely sensitive to the exact starting figure, and a materially inaccurate starting balance can meaningfully skew the final projection over a longer term.

The Tax Simplification Worth Understanding

This calculator applies a simplified adjustment based on your tax position — assuming interest is untaxed, taxed at a rate reflecting a basic-rate taxpayer, or taxed at a rate reflecting a higher-rate taxpayer — rather than calculating your exact Personal Savings Allowance position year by year. This is a reasonable simplification specifically for someone who already knows their savings interest will exceed their PSA throughout the projection, but it is worth being aware that for a smaller balance where interest genuinely stays within your PSA for some or all of the term, the “none” (untaxed) option will give a more accurate picture than assuming full taxation from the very first year.

A genuinely useful approach for many people sits between these two extremes: run the calculation with the “none” setting first to see the full, untaxed potential growth, then separately estimate roughly how many years into the term your balance is likely to exceed your PSA, and treat the true outcome as somewhere between the untaxed and taxed projections for that specific stretch of the term.

Seeing the Tax Position Change the Outcome

£15,000 initial deposit plus £150 a month at 4.5% over 8 years grows to roughly £38,780 if the interest is genuinely untaxed throughout. Assuming basic-rate tax applies to all the interest instead brings this down to roughly £36,657, and assuming higher-rate tax brings it down further still to roughly £34,665 — a difference of over £4,000 between the untaxed and higher-rate scenarios, on identical contributions and headline rate. This is exactly why checking which tax scenario genuinely applies to your own situation, rather than defaulting to whichever feels safest to assume, meaningfully changes the realistic final figure.

The gap between these scenarios widens the longer the term runs, since tax is effectively reducing the rate that compounds every single month rather than being a one-off deduction at the end — the difference between the three tax scenarios would be proportionally even larger over a longer term than the 8 years shown here.

When a Tax-Free Wrapper Changes the Whole Calculation

If your savings interest is genuinely likely to be taxed — because your balance and rate combined produce interest above your Personal Savings Allowance — holding the same savings inside a Cash ISA instead removes this tax consideration entirely, since ISA interest is never taxed regardless of amount or your tax band. This is worth checking directly against your own numbers before assuming a standard taxable account and simply accepting the tax reduction this calculator models: for anyone genuinely facing the higher-rate tax scenario, an ISA at a comparable rate would produce the full untaxed growth instead, which based on the earlier worked example could be worth over £4,000 more by the end of the same 8-year term, purely from avoiding tax on the interest altogether. Our ISA Growth Calculator covers this tax-free alternative directly, and our Compound Interest Calculator is useful for the simpler lump-sum-only version of this same growth modelling, without the monthly contribution or tax adjustment layered on top.

Frequently Asked Questions

Important Information

This calculator provides an estimate based on the figures and assumptions you enter and does not constitute financial advice. Actual savings account terms and rates vary by provider and may change over time. See our Disclaimer for further information.