How Much Should I Invest Each Month?

Start with the destination, not a rule of thumb

Advice such as “invest 10% of your income” can be a reasonable prompt, but it cannot know your starting balance, deadline or target. A better process begins with the outcome: define an amount, choose a timeframe and use a cautious return assumption. The gap between your current savings and the target determines the monthly contribution the mathematics requires.

That result is a planning figure, not an instruction. If it is unaffordable, change one variable honestly: extend the timeframe, lower the target, contribute a lump sum, or reconsider the assumed return. Increasing the return merely to make the answer comfortable hides the trade-off.

A worked example

Suppose you invest £500 at the end of every month for 20 years and illustrate growth at 7% a year, compounded monthly. The contributions total £120,000. Under that smooth-return assumption, the projected balance is about £260,463, of which roughly £140,463 is growth.

Actual returns will not arrive smoothly and may be lower. Fees, tax and inflation can also materially reduce what the future balance can buy.

Test affordability separately

A mathematically sufficient contribution can still be financially fragile. Before committing, allow for essential bills, expensive debt, emergency savings and irregular annual costs. A smaller amount that continues through difficult months can be more useful than an ambitious amount repeatedly stopped and restarted.

A practical decision process

1. Choose a specific goal and target date.
2. Enter your current balance and a cautious return range.
3. Calculate the required monthly amount.
4. Compare it with the amount your budget can sustain.
5. Test a lower-return scenario and include likely fees.
6. Review the plan annually and after major life changes.

The calculator is best used for scenarios. Try conservative, central and optimistic assumptions rather than treating one projection as a forecast.

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