Compound savings interest: nominal rate, effective rate and your inputs
Compounding means retained interest joins the balance used for later interest calculations. To estimate the result, specify the starting amount, rate convention, compounding period and contributions. A nominal annual rate divided into monthly periods is different from an effective annual rate. The calculator below uses the nominal monthly model.
Use one rate convention
Capitec labels its savings rate convention as nominal annual compounded monthly, or NACM [1]. For this illustration, a nominal annual rate r gives a monthly rate i = r ÷ 12. Do not enter an effective annual percentage in that field without converting it; the result would use the wrong convention.
The timing of contributions matters
The calculator adds each contribution at the end of a month after that month’s interest. An opening contribution would earn interest for an extra period. Record the timing alongside the amount when comparing a spreadsheet with a bank projection. Fees, tax, withdrawals and changing rates are outside this simple model.
Separate your money from the modelled growth
Total contributions equal the starting amount plus the monthly contribution multiplied by months. Modelled interest is the final balance less those contributions. This distinction lets you see whether progress comes mostly from deposits or growth. Replace illustrative rates with the convention in the bank’s current terms.
A calculation you can check
Fictional nominal rate: R1 000 at 12% annually compounded monthly, no contributions or charges. Monthly rate = 0.12 ÷ 12 = 0.01. After two months: R1 000 × 1.01² = R1 020.10. Interest = R20.10. This is a mathematical example, not a bank’s savings rate.
Try the monthly compounding model
Monthly rate i = nominal annual rate ÷ 12. Bₙ = P(1+i)ⁿ + D((1+i)ⁿ−1)/i; at i=0, Bₙ=P+nD. D is deposited at month end. Fixed rate, monthly compounding, no fees, tax or withdrawals. This is a gross illustration, not a quotation. JavaScript is required for interactive results; the worked example and formula remain readable without it.
Questions about this decision
What does compounding add?
It allows retained interest to form part of the next interest-bearing balance. The effect depends on rate, time and whether interest stays invested. It is not a guarantee that a product will maintain a particular future rate.
Can I enter an effective annual rate?
The field expects a nominal annual rate divided by 12. An effective annual rate requires a different monthly conversion: (1 + annual effective rate)^(1/12) − 1. Confirm which figure the bank publishes before using the tool.
When are monthly deposits added?
At month end, after that month’s interest. This is an explicit model assumption. If you deposit at the beginning of each month, this tool’s result will not reproduce that timing; use the bank’s projection for the actual product.
What if the rate is zero?
The balance is the starting amount plus all contributions. No interest is added. Zero is a valid entered rate; a blank rate is missing information and the calculator should ask you to supply it.
Why does my bank show a different figure?
Compare daily versus monthly calculation, payment dates, balance tiers, changing rates, tax and charges. The tool deliberately uses one fixed monthly model. A difference is a reason to inspect assumptions, not evidence that the bank is wrong.
Does this calculate inflation-adjusted growth?
No. It shows nominal rand balances before fees and tax. To judge future purchasing power you need a separate inflation assumption and should label it as an assumption rather than a known future price change.
Can I use it to forecast investment returns?
Do not treat its fixed rate as a prediction of market returns. The tool illustrates a stated savings-rate model. Investments with changing prices and uncertain returns need different analysis and can produce losses.
Is the result a deposit offer?
No. You enter the assumptions; SaveWise does not verify a bank rate or open a deposit through the calculator. Read the bank’s rate convention and terms before using the estimate for a decision.