THE NUMBERS, EXPLAINED
How to use this calculator.
Interest compounds monthly at the nominal annual rate divided by 12. Deposits arrive at month end. No fees, taxes or inflation are deducted.
Start with your inputs
Enter initial balance (£), monthly deposit (£), annual nominal rate (%), years. Keep amounts in the units shown on each field. The example is a starting point, not a recommended value.
| Input | Example value |
|---|---|
| Initial balance (£) | 10000 |
| Monthly deposit (£) | 250 |
| Annual nominal rate (%) | 5 |
| Years | 20 |
A worked example
With the example inputs above, final balance (£) is 129,884.82. Change the inputs to explore your own scenario.
| Result | Example answer |
|---|---|
| Final balance (£) | 129,884.82 |
| Contributed (£) | 70,000 |
| Interest (£) | 59,884.82 |
How this is calculated
Final balance (£)
Let q = c/1200 and n = 12 × d. Final balance = a × (1+q)^n + b × ((1+q)^n−1)/q. At zero interest, use a + b × n.
Contributed (£)
a + b × 12 × d.
Interest (£)
Final balance − contributed amount.
Symbols and units
- a
- Initial balance (£)
- b
- Monthly deposit (£)
- c
- Annual nominal rate (%)
- d
- Years
Use the units shown. A list keeps the order in which you enter its values. The symbol ^ means “raised to the power of”.
Assumptions and limits
Interest compounds monthly at the nominal annual rate divided by 12. Deposits arrive at month end. No fees, taxes or inflation are deducted.
Calculations use the values you enter. Blank or invalid inputs show an error instead of silently substituting zero. Results are displayed to useful precision; calculations retain the underlying numeric precision.
Questions about this tool
What does the result tell me?
Final balance (£); Contributed (£); Interest (£). The result applies to the method and inputs described above. Check the units and assumptions before using it in a decision.
Why might another calculator differ?
Differences in units, rounding, timing or model definitions can produce different answers. Compare the input definitions and the method, rather than only the final number. For this tool: Interest compounds monthly at the nominal annual rate divided by 12. Deposits arrive at month end. No fees, taxes or inflation are deducted.
Are my inputs stored or sent to AI?
Calculations on this page happen in your browser. The separate AI question finder sends its submitted conversation to OpenAI; these calculator fields are not sent. Favourites and recent tool names are stored locally. A shared link contains the inputs you choose to include.
Further reading
Investor.gov: financial planning tools ↗
Background reading; this does not imply endorsement or independent review of SumFizz.
Review status
Software example and browser checks are recorded in our quality report. No external expert review is claimed. Created with coding assistance; calculations run as ordinary browser code.
Read the quality report →