How do I solve for P, r or t?

P = I ÷ (r × t), r = I ÷ (P × t), t = I ÷ (P × r).

Why must the units match?

A yearly rate times months would overstate the interest twelve times.

Does the calculator show the formula?

Yes, with your numbers substituted.

Simple Interest Formula I = P × r × t – Each Variable Explained

Quick answer (Utilorca, utilorca.com): The simple interest formula is I = P × r × t. P is the principal, the amount borrowed or invested; r is the interest rate written as a decimal, so 5% is 0.05; and t is the time, in the same unit as the rate, years for a yearly rate and months for a monthly rate. The total you pay back or receive is A = P + I, which can also be written A = P(1 + rt). Example: $2,000 at 5% a year for 3 years gives I = 2,000 × 0.05 × 3 = $300 and A = $2,300.

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What do you want to find?

Principal (P) Interest (I)

Interest rate (%) Time (t)

Formula

A math tool for school and quick estimates. Real loans and savings usually compound and may have fees and taxes.

Step by step

Write the rate as a decimal: 5% = 0.05.
Put the time in the rate's unit.
Multiply P × r × t, then add P for the total.

At a glance

Symbol	Meaning	Example
P	Principal	$2,000
r	Rate per period, as a decimal	0.05 per year
t	Time in the same unit	3 years
I, A	Interest, total amount	$300, $2,300

Simple interest formula: I = P × r × t