What this calculator does
This is a future-value-of-annuity calculator with a single twist: you decide which of the three core variables is the unknown. Pick “solve deposit” and we tell you how much you need to set aside each period to reach a target on time. Pick “solve time” and we tell you how many years it will take at your current savings rate. Pick “project balance” and we just compound everything forward and show you where you'll land.
All three modes use the same engine — the classic compound interest formula combined with an end-of-period ordinary annuity: FV = P × (1 + r)n + PMT × ((1 + r)n − 1) / r, where P is your starting amount, PMT is each deposit, r is the per-period rate (annual rate ÷ periods per year), and n is the total number of periods. For zero-rate accounts we degenerate cleanly to P + PMT × n, so cash-under-a-mattress savings still give a sensible answer.
Why all three modes matter
People come at savings goals from three different directions and most calculators only do one of them. If you've already set a budget (“I can put aside €400/mo”) the right question is “how long until I hit my target?” — solve time. If you've already set a deadline (“I want a house deposit in 5 years”) the right question is “what monthly deposit does that require?” — solve deposit. And if you're weighing whether a habit is worth keeping (“does saving €50/week for a decade even add up to anything?”) the right question is just “what will my balance be?” — project.
The compounding trap most people fall into
On short horizons the deposits dominate. €400/mo for 2 years at 4% only earns about €100 in interest — the bulk of your final balance is the deposits themselves. On long horizons the opposite is true: €400/mo for 30 years at 7% compounds to over €450,000, of which more than €300,000 is interest. The growth chart visualises the moment where the amber band (contributions) is overtaken by the gap above it (compounded interest). That crossover usually happens around year 12-15 at 5-7% rates — much sooner than people expect.
This is the single most important lesson the calculator delivers: time is the dominant variable. Doubling your monthly deposit doubles your final balance. Doubling your time horizon roughly quadruples it (at typical rates). That asymmetry is why “start early” advice is so universal among financial planners.
What rate should I use?
- EU high-yield savings accounts (2026): roughly 2.5-3.5% on instant-access products, slightly more for locked term deposits. Use the headline rate.
- US high-yield savings / money market: 4-5% is common on online-bank products. CDs lock in slightly higher rates in exchange for less liquidity.
- UK ISAs and fixed-rate bonds:4-5% on easy-access cash ISAs, 5%+ on 1-2 year fixed bonds. The tax-shelter benefit isn't modelled here — bake it in by using the post-tax rate if your savings live outside an ISA.
- Diversified index investing:long-run real returns for global equity indices have averaged 5-8% after inflation. For a 20+ year horizon, 6-7% is a defensible planning assumption — but the path is bumpy, so don't assume it for short-horizon goals.
How accurate is the projection?
The math is exact for the model: a fixed nominal rate, equal periods, end-of-period deposits, no taxes, no fees. Real savings products deviate from this in a few predictable ways and the size of the deviation is usually small. The biggest ones:
- Tax drag on interest income outside a tax-shelter — model it by reducing the rate proportionally (e.g. 5% pre-tax × (1 − 0.3 marginal rate) = 3.5% post-tax).
- Inflation erodes the real value of your future balance. The number this calculator gives is nominal. To get the real-terms balance, subtract your expected inflation rate from your nominal rate before entering it (this is the “Fisher equation” in one line: real rate ≈ nominal rate − inflation rate).
- Variable rates — savings account rates change. Model with the current rate to see what the schedule would look like if conditions stayed exactly as they are now.
- Feeson managed-investment products eat into compounded returns more than people realise. A 1% annual fee on a 7% return is not a 1/7th = 14% reduction over 30 years — it's closer to 25%. If you're modelling an investment account, subtract the expense ratio from the rate.
Reconciling against a real account statement
If you're cross-checking projected balances against what your bank is actually showing on month-end statements and the numbers are close but not identical, it's almost always day-count convention (some banks compound daily on the outstanding balance instead of monthly) or fees you've forgotten to subtract. Drop the actual statement PDF into our main bank statement converter to extract every interest credit as line items — then compare cumulative interest from our chart against the real numbers.
For a one-shot compounding question (“what does my €10k grow into at 5% over 8 years with no deposits?”), the interest rate calculator is more targeted — it's really a present-value / future-value tool rather than a goal-planner.
Worked example: 25 years to a million
Set the calculator to “solve deposit”, starting amount €0, target €1,000,000, time horizon 25 years, rate 7% (a long-run equity index assumption), monthly compounding. You get a required deposit of roughly €1,234/mo. Of the final million, only about €370,000 is contributions — the other ~€630,000 is compounded growth. Now switch to 35 years instead of 25: the required deposit drops to about €582/mo— less than half. That's a ten-year difference doing more work than doubling your monthly deposit would.