How to use a retirement savings calculator
A retirement calculator is only as useful as the numbers you put into it. Here's a practical guide to choosing realistic inputs, understanding the assumptions, and getting an answer you can actually trust.
The five inputs that matter
Every retirement calculator boils down to the same handful of numbers. Get these right and the results will be meaningful:
- Current age — Where you are today.
- Retirement age — Try multiple scenarios (60, 65, 67) to see how a few years change things.
- Current savings — Sum of pensions, ISAs, 401(k)s, IRAs, taxable investments earmarked for retirement.
- Monthly contribution — Your contributions + any employer match.
- Expected return — The annualised growth rate you assume.
Choosing a realistic return rate
This is where people get tripped up. Historical equity returns in the US look closer to 10% nominal, but two things shave that down:
- Fees — Even 0.5% per year is meaningful over 30 years.
- Asset mix — Most people aren't 100% in equities. A 60/40 portfolio is closer to 6-7%.
A defensible default for planning is 5-7% nominal with 2.5% inflation.
Reading the results
A good calculator gives you three numbers:
- Future balance (nominal) — The actual dollar amount in your account at retirement.
- Future balance (inflation-adjusted) — What that balance is worth in today's spending power. This is the number that matters.
- Estimated monthly income — Usually via the 4% rule. Compare it to your expected retirement spending.
Run more than one scenario
Don't run the calculator once and call it done. Try:
- What if my returns are 1% lower than I hope?
- What if I retire 2 years earlier?
- What if I increase contributions by £100/month?
Small changes compound into large differences. A £100/month increase over 30 years at 6% is roughly an extra £100k.
What calculators can't show you
- Sequence-of-returns risk in the first decade of retirement.
- Taxes during drawdown.
- Healthcare and long-term care shocks.
- Lifestyle changes (downsizing, relocation, part-time work).
That's why this is planning, not prophecy. Use the RetireSmart calculator as a yearly check-in to see if you're on track — and adjust as life changes.
Frequently asked questions
What return rate should I use?▾
A long-term globally diversified equity portfolio has returned around 6-8% per year above inflation historically. Many planners use 5-7% nominal for projections to stay conservative.
What inflation rate should I use?▾
2-3% is a common assumption. The UK Bank of England targets 2%, the US Fed also targets 2%. Use a higher rate if you want a more cautious projection.
Should I include employer pension contributions?▾
Yes — add your full monthly contribution including any employer match. That's the actual amount being invested on your behalf each month.
How accurate are retirement calculators?▾
They give you a planning estimate, not a guaranteed result. Markets vary, contributions change, and life happens. Re-run your numbers once a year to stay on track.