The formula
How to calculate portfolio growth
This projects a portfolio forward from what you hold now plus what you add each month. The split between contributions and growth is the part worth watching: it shows when the market starts doing more work than you do.
Returns are applied monthly, and contributions are assumed to arrive at the end of each month. Real portfolios do neither exactly, but over a twenty-year horizon those assumptions move the answer by less than the error in the return estimate.
The calculator asks for:
- Starting balance
- Monthly contribution
- Annual return (%)
- Years (years)
Results appear immediately — there is nothing to submit. Changing a field rewrites the link, so you can share the exact scenario you are looking at.
Some of the fields above will accept figures that seem unusual for your own situation, and that is deliberate: the formula behind portfolio growth works the same way regardless of scale, so the calculator does not stop you testing a hypothetical scenario a long way from your actual numbers — often the fastest way to see which input the result is most sensitive to.
Why portfolio growth matters
A calculation like this usually gets used at a decision point rather than out of curiosity — comparing two real options, checking a number a lender or adviser has quoted, or working out whether a plan that sounded fine in conversation still holds up once it is written down with actual figures. The maths itself is rarely complicated; what is hard is remembering which figures to use and in what order, which is exactly what a dedicated calculator is for.
It is also useful as a sense check before signing anything. A quote, an offer letter or a spreadsheet from someone else can contain an error, an optimistic assumption, or simply a different convention for rounding — running the same inputs through an independent calculator is a quick way to confirm a number before relying on it.
This kind of calculation rarely stands entirely alone. A portfolio growth figure usually feeds into a wider decision — how it compares with a competing offer, whether it fits inside a monthly budget, what it does to a longer-term plan — and the value of having it as an exact number rather than a rough guess is that those follow-on comparisons stop being guesswork too. Once one figure in a decision is precise, it is worth making the effort to get the others precise as well, rather than mixing an exact calculation with several estimates and treating the result as equally reliable.
Where the same calculation needs to be run for several different scenarios side by side — three loan offers, two savings plans — the fastest approach is usually to open the calculator in a second browser tab for each one, so that the results can be compared directly rather than overwriting each other in a single set of fields.
Worked example
A concrete run-through, using the values already in the fields:
- Starting balance: 50,000
- Monthly contribution: 750
- Annual return: 7 %
- Years: 20 years
That gives:
- Portfolio value: 592,631.94
- Total you put in: 230,000
- Growth on top: 362,631.94
These figures are only the calculator's own starting values, included so the working is visible rather than hidden inside the tool above. Replace them with your own numbers and the same arithmetic applies — nothing about the method changes, only the inputs feeding it.
Reading the result
Find the crossover point — the year when accumulated growth passes total contributions. At 7% with these figures it lands around year fifteen, and it is the clearest illustration of why the last decade of investing does more than the first.
Where this goes wrong. A single average return implies a smooth line, which no portfolio delivers. Sequence risk is not visible here: the same average return with bad years early produces a different outcome once withdrawals start.
A useful check on any unfamiliar result is to compare it against a rough mental estimate first — round the inputs to convenient numbers and see whether the calculator's answer lands in roughly the same territory. A wildly different figure usually means one of the fields was entered in the wrong unit, most often a percentage typed as a whole number where a decimal was expected, or the reverse.
For a globally diversified equity portfolio, 5% after inflation or around 7% before it is a defensible planning assumption. Bond-heavy portfolios should assume less. Running the projection again at two points lower is a useful sanity check.
Real, if you want the answer to mean something. A nominal £1.2 million in twenty years sounds transformative; at 2.5% inflation it buys what about £730,000 buys today.
The answer it gives you is portfolio value. With 50,000 starting balance, 750 monthly contribution and 7 % annual return, that comes to 592,631.94. Change any field and the figure moves with it.
Whenever one of the underlying figures changes — a new interest rate, a different balance, an updated term — since the result only reflects what is currently in the fields. There is no need to keep a separate record of past results; the web address for a filled-in version already carries the figures used to produce it.
Not unless a tax rate or a fee is explicitly one of the inputs above. Where it is not, the figure shown is a gross calculation, and any tax due depends on your personal circumstances and current tax rules, which are worth checking separately.
The arithmetic itself is exact — the calculator applies the formula shown above precisely, with no rounding until the final figure is displayed. The uncertainty, where it exists, is entirely in the inputs: an estimated rate or an approximate balance carries that same approximation through to the result.