Monthly repayment, total interest and a full schedule for a fixed-rate loan — including extra payments.
Compound Interest Calculator
Grow a lump sum and monthly contributions over time, with a year-by-year breakdown.
Runs in your browser — nothing you enter is sent to our servers
How it works
The calculator steps through every month. The annual rate r with n compounding periods per year is converted to an equivalent monthly rate: (1 + r/n)n/12 − 1. Each month, interest is added to the balance, then that month’s contribution is added (contributions at the end of the month).
With no contributions this matches the classic formula A = P(1 + r/n)nt.
Example
Start with R10,000, add R1,000 a month for 20 years at 9% compounded monthly. You contribute R250,000 in total, and the balance grows to about R728,000 — nearly R478,000 of it interest.
Frequently asked questions
What is compound interest?
Interest earned on both your original amount and on interest already added. Over long periods, this “interest on interest” can become larger than your contributions.
How does compounding frequency affect growth?
More frequent compounding (monthly rather than yearly) adds interest sooner, so it grows slightly faster at the same nominal rate. The difference is usually smaller than the effect of the rate itself or time.
Does this account for inflation, tax or fees?
No. Results are nominal. Fees and tax reduce returns, and inflation reduces what the final amount can buy. To approximate a real (after-inflation) return, subtract expected inflation from the rate.
Is the return guaranteed?
Only for fixed-rate products such as fixed deposits. Investment returns vary year to year; this calculator assumes a constant rate for illustration.
Related tools
Work out percentage change, apply an increase or decrease, or reverse one to find the original value.
Convert pay between hourly, monthly and annual, and estimate South African PAYE and UIF.
Find X% of a number, or what percent one number is of another.
Related guides
CSS colour formats explained: HEX, RGB, HSL, HWB and OKLCH
What each CSS colour format means, when to use it, why OKLCH is useful for palettes, and how to check colour contrast for accessibility.
· 7 min read
Ecommerce tools available in South Africa
Selling online in South Africa: hosted stores vs WooCommerce, local payment gateways such as PayFast and Yoco, delivery, VAT on prices and the consumer rules to check.
· 8 min read
JWTs explained: structure, claims, decoding vs verifying, and common mistakes
How JSON Web Tokens work, what each part and claim means, why decoding isn't verifying, and the mistakes that cause real security bugs.
· 8 min read