What is simple interest?

Interest calculated only on the original principal — not on accumulated interest. The formula is I = P × r × t, where P is the principal, r is the annual rate (as a decimal), and t is the time in years. R10,000 at 8% for 3 years earns R2,400 in simple interest — R800 per year, every year.

What's the difference between simple and compound interest?

Compound interest earns interest on interest already earned, simple interest doesn't. Over short periods the difference is small. Over long periods it's enormous: R10,000 at 8% for 30 years earns R24,000 in simple interest vs about R100,000 with monthly compounding — four times more. For anything long-term, compounding wins.

Where is simple interest actually used in South Africa?

Some short-term fixed deposits, certain promissory notes, payday-loan-style products, and quick informal lending arrangements. Most consumer products in SA — savings accounts, mortgages, vehicle finance, credit cards, business loans — use compound interest. Simple interest is most useful as a back-of-the-envelope calculation when you want a quick estimate without thinking about compounding effects.

How do I use this for a fixed deposit?

If your bank quotes a 'simple interest' rate for a fixed deposit (some 12-month and shorter deposits work this way), enter the deposit amount as principal, the quoted rate, and the term in years. The calculator gives the maturity value. If the deposit instead quotes a compound rate, use the Compound Interest tool — the answer will be a bit higher.

Can I use this for an informal loan?

Yes, for the basic case. If you lend R20,000 to a family member at 10% simple interest for 18 months, set principal = 20000, rate = 10, years = 1.5. The calculator shows the interest portion and the total they'd repay. Important caveat: any lending in the course of business may fall under the National Credit Act, with required disclosures, NCA-capped rates, and registration requirements — that goes beyond a simple-interest calculation.

How do I do months instead of years?

Convert to years. Three months = 0.25 years. Six months = 0.5. Eighteen months = 1.5. Twenty months = 1.667. The calculator handles decimals — type the converted figure in the years field.

What's the daily simple interest formula?

I = P × r × (days / 365). So R10,000 at 8% for 90 days earns 10000 × 0.08 × (90/365) = R197. This calculator does this implicitly when you enter a partial year — 90/365 ≈ 0.247 years. Some agreements use 360-day years for simplicity; check yours.

Is interest earned this way taxable?

Interest income is taxable in South Africa above the annual SARS interest exemption. Inside a Tax-Free Savings Account (TFSA), it's tax-free. Outside, anything above the exemption is added to your taxable income at your marginal rate. The current exemption amounts (different for under-65 and over-65) are on sars.gov.za.

Is the data I enter saved anywhere?

No. Every calculation runs entirely in your browser. We never see the numbers you type, and nothing is stored on a server.

Savings & Investments

Simple Interest Calculator (South Africa)

Flat-rate interest on a principal — useful for short-term deposits, quick informal lending estimates, and anywhere you just want the basic P × r × t answer.

Principal amount

Annual interest rate

Period

Use a decimal for partial years — 0.5 = 6 months.

Final balance

R 0,00

fill in the three fields above

Principal: R 0,00
Total interest: R 0,00

All calculations run in your browser. Simple interest only — for anything compounding, use the Compound Interest tool.

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Flat-rate maths

Simple interest, when it actually applies

Simple interest is the most basic interest calculation there is. Multiply principal by rate by time, that’s the interest. No reinvestment, no compounding, no curve — just a flat line over the period.

The formula

I = P × r × t

P = principal, r = annual rate (as decimal), t = years. Final balance = P + I. R20,000 at 9% for 2 years earns R3,600 in interest, so the balance ends at R23,600. The calculator above does this with whatever numbers you put in.

When it actually shows up in SA

Despite being the textbook intro to interest, simple interest is fairly rare in real South African products. Where it does crop up:

Some short-term fixed deposits quote a simple interest rate, especially for terms under 12 months. The bank tells you the maturity amount and that’s what you get.
Promissory notes and bills of exchange — commercial paper used between businesses — often use simple interest.
Quick informal lending between friends or family members tends to default to a simple-interest agreement because it’s easier to explain than compounding.
Short-term penalty interest — overdue invoices and late-payment clauses in contracts often use simple interest for the days-overdue portion.

Most consumer products (savings accounts, credit cards, mortgages, business loans) use compounding. If you’re not sure which your product uses, check the agreement.

Simple vs compound — how big is the gap?

Over short horizons, almost nothing. Over long ones, a lot. R10,000 at 8%:

1 year: simple R800, compound (monthly) ~R830. Gap: ~R30.
5 years: simple R4,000, compound ~R4,898. Gap: ~R900.
10 years: simple R8,000, compound ~R12,196. Gap: ~R4,200.
30 years: simple R24,000, compound ~R98,975. Gap: ~R75,000.

For anything you’re planning to leave for years, compounding crushes simple interest. For a 3-month deposit, the difference is rounding-error stuff. Match the maths to the horizon.

For days, not years

Some agreements quote interest on a daily basis — common for trade credit, late-payment clauses, and short-term instruments. The formula adapts: I = P × r × (days / 365). R10,000 at 8% for 90 days = 10000 × 0.08 × (90/365) = R197. Some agreements use a 360-day year for simplicity — check yours; the small difference can matter on large amounts.

In the calculator above, just enter the days as a fraction of a year. 90 days ≈ 0.247.

Frequently asked questions

What is simple interest?
Interest calculated only on the original principal — not on accumulated interest. The formula is I = P × r × t, where P is the principal, r is the annual rate (as a decimal), and t is the time in years. R10,000 at 8% for 3 years earns R2,400 in simple interest — R800 per year, every year.
What's the difference between simple and compound interest?
Compound interest earns interest on interest already earned, simple interest doesn't. Over short periods the difference is small. Over long periods it's enormous: R10,000 at 8% for 30 years earns R24,000 in simple interest vs about R100,000 with monthly compounding — four times more. For anything long-term, compounding wins.
Where is simple interest actually used in South Africa?
Some short-term fixed deposits, certain promissory notes, payday-loan-style products, and quick informal lending arrangements. Most consumer products in SA — savings accounts, mortgages, vehicle finance, credit cards, business loans — use compound interest. Simple interest is most useful as a back-of-the-envelope calculation when you want a quick estimate without thinking about compounding effects.
How do I use this for a fixed deposit?
If your bank quotes a 'simple interest' rate for a fixed deposit (some 12-month and shorter deposits work this way), enter the deposit amount as principal, the quoted rate, and the term in years. The calculator gives the maturity value. If the deposit instead quotes a compound rate, use the Compound Interest tool — the answer will be a bit higher.
Can I use this for an informal loan?
Yes, for the basic case. If you lend R20,000 to a family member at 10% simple interest for 18 months, set principal = 20000, rate = 10, years = 1.5. The calculator shows the interest portion and the total they'd repay. Important caveat: any lending in the course of business may fall under the National Credit Act, with required disclosures, NCA-capped rates, and registration requirements — that goes beyond a simple-interest calculation.
How do I do months instead of years?
Convert to years. Three months = 0.25 years. Six months = 0.5. Eighteen months = 1.5. Twenty months = 1.667. The calculator handles decimals — type the converted figure in the years field.
What's the daily simple interest formula?
I = P × r × (days / 365). So R10,000 at 8% for 90 days earns 10000 × 0.08 × (90/365) = R197. This calculator does this implicitly when you enter a partial year — 90/365 ≈ 0.247 years. Some agreements use 360-day years for simplicity; check yours.
Is interest earned this way taxable?
Interest income is taxable in South Africa above the annual SARS interest exemption. Inside a Tax-Free Savings Account (TFSA), it's tax-free. Outside, anything above the exemption is added to your taxable income at your marginal rate. The current exemption amounts (different for under-65 and over-65) are on sars.gov.za.
Is the data I enter saved anywhere?
No. Every calculation runs entirely in your browser. We never see the numbers you type, and nothing is stored on a server.

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