# Simple interest calculator

> Calculate simple interest (I = P × r × t) and the maturity amount on a principal for a term in years, months or days.

Version interactive : https://www.calcopenly.com/fr/finance/simple-interest-calculator
Sujet : Calculatrices financières

Simple interest is paid only on the original principal: I = P × r × t, where r is the yearly rate as a decimal and t the time in years. A term in months is divided by 12, and a term in days by 365 (actual/365) or by 360, the banker's year some lenders and money markets use. The amount due at maturity is A = P(1 + rt).

With the defaults, 10,000 at 6% for 3 years earns 600 a year and 1,800 in total, for a maturity amount of 11,800. Compounded yearly at the same rate, the same deposit would reach 11,910.16, which is 110.16 more.

Interest here never earns interest of its own. Savings accounts and most deposits compound, so use the compound interest calculator for them; simple interest suits short loans, flat-rate quotes and interest accrued between coupon dates.

## Données

- **Capital**
- **Interest rate (per year)**
- **Temps**
- **Time in** (options : Années, Mois, Days)
- **Days in a year** (options : 365 (actual/365), 360 (banker's year))

## Résultats

- Intérêts — résultat principal
- Total amount
- Interest per year
- Amount if compounded yearly

## Formule

$$
I = P \cdot r \cdot t,\qquad A = P(1 + r t)
$$

## Exemples détaillés

### 10,000 at 6% for 3 years

- Capital: 10,000
- Interest rate (per year): 6%
- Temps: 3
- Time in: Années
- **Intérêts: 1,800.00**
- **Total amount: 11,800.00**
- **Amount if compounded yearly: 11,910.16**
- Source de vérification : 10000 × 0.06 × 3 by hand; 10000 × 1.06³ = 11910.16 (Python decimal)

### 5,000 at 8% for 9 months

- Capital: 5000
- Interest rate (per year): 8%
- Temps: 9
- Time in: Mois
- **Intérêts: 300.00**
- **Total amount: 5,300.00**
- Source de vérification : 5000 × 0.08 × 9/12 by hand

### 20,000 at 7.5% for 90 days, actual/365

- Capital: 20,000
- Interest rate (per year): 7.5%
- Temps: 90
- Time in: Days
- Days in a year: 365 (actual/365)
- **Intérêts: 369.86**
- Source de vérification : 20000 × 0.075 × 90/365 = 369.8630… (Python decimal)

### Same loan on a 360-day year

- Capital: 20,000
- Interest rate (per year): 7.5%
- Temps: 90
- Time in: Days
- Days in a year: 360 (banker's year)
- **Intérêts: 375.00**
- Source de vérification : 20000 × 0.075 × 90/360 by hand

### Zero rate

- Capital: 1000
- Interest rate (per year): 0%
- Temps: 5
- Time in: Années
- **Intérêts: 0.00**
- **Total amount: 1,000.00**
- Source de vérification : Definition: no rate, no interest

## Questions

### How do you calculate simple interest?

Multiply the principal by the yearly rate and by the time in years: I = P × r × t. 10,000 at 6% for 3 years earns 10,000 × 0.06 × 3 = 1,800, so 11,800 is repaid. For a term in months, divide by 12 first: 5,000 at 8% for 9 months earns 5,000 × 0.08 × 9/12 = 300.

### How do you calculate simple interest for a number of days?

Divide the days by the day-count basis before multiplying. 20,000 at 7.5% for 90 days earns 20,000 × 0.075 × 90/365 = 369.86 on an actual/365 basis, and 375.00 on a 360-day banker's year. Excel's ACCRINTM function uses basis 3 for actual/365 and basis 2 for actual/360.

### How do you find the rate or the time from simple interest?

Rearrange I = P × r × t. The rate is r = I ÷ (P × t): earning 1,800 on 10,000 over 3 years means 1,800 ÷ 30,000 = 6% a year. The time is t = I ÷ (P × r): earning 3,000 on 10,000 at 6% takes 3,000 ÷ 600 = 5 years.

### Is simple interest better than compound interest?

For a borrower, yes; for a saver, no. Simple interest grows in a straight line, while compound interest also earns interest on interest. On 10,000 at 6%, the gap is only 110.16 after 3 years (1,800 against 1,910.16), but after 20 years simple interest totals 12,000 while yearly compounding adds 22,071.35.

### Quelle est la précision de « Simple interest calculator » ?

La précision dépend de vos données et des hypothèses de la méthode. Le calcul décimal utilise 50 chiffres significatifs, mais les estimations, méthodes numériques et données sources peuvent être moins précises ; l’arrondi affiché ne supprime pas ces limites. Exemples résolus vérifiés à partir de sources indépendantes : 5. Par exemple, « 10,000 at 6% for 3 years » est vérifié à l’aide de 10000 × 0.06 × 3 by hand; 10000 × 1.06³ = 11910.16 (Python decimal).

### D’où vient cette méthode ?

OpenStax Prealgebra 2e — Simple Interest Applications (I = Prt); Brealey, Myers & Allen — Principles of Corporate Finance, ch. 2 (simple vs compound interest); Microsoft Excel ACCRINTM function (simple interest accrued to maturity, basis 2 = actual/360, 3 = actual/365).

## Sources

- [OpenStax Prealgebra 2e — Simple Interest Applications (I = Prt)](https://openstax.org/books/prealgebra-2e/pages/6-4-solve-simple-interest-applications)
- Brealey, Myers & Allen — Principles of Corporate Finance, ch. 2 (simple vs compound interest)
- [Microsoft Excel ACCRINTM function (simple interest accrued to maturity, basis 2 = actual/360, 3 = actual/365)](https://support.microsoft.com/en-gb/office/accrintm-function-f62f01f9-5754-4cc4-805b-0e70199328a7)

_Pour la planification uniquement. Prêteurs, administrations fiscales et marchés appliquent leurs propres arrondis, frais et règles ; confirmez les chiffres auprès d’eux avant de vous engager._