Daily Interest Calculator

Use this daily interest calculator to estimate interest earned by day using simple interest or daily compounding. Enter principal, annual rate, time or exact dates, then view daily rate, balance, and APY.

This daily interest calculator determines the exact amount of interest earned or owed on a principal balance over a specific timeframe. You can evaluate scenarios using either simple interest or daily compounding by entering your initial amount, the annual interest rate, and your time period as either a set duration or exact start and end dates.

The tool outputs your total interest, final balance, daily interest rate, and average interest per day. When evaluating a daily compound interest calculator scenario, it also provides the Annual Percentage Yield (APY) to reflect the true earning rate. All calculations fully support both the standard 365-day year and the 360-day banker’s year conventions.

How this daily interest calculator works

This tool processes your inputs through several specific calculation paths depending on your selected settings:

• Simple interest: Applies the annual rate strictly to the original principal without adding accumulated interest back into the base.
• Daily compounding: Adds earned interest to the principal every single day, creating a growing balance for subsequent calculations.
• Date-based calculation: Functions as an interest between dates calculator by finding the exact number of days between a chosen start and end date.
• Duration-based calculation: Allows direct input of years, months, and days, converting them into a single total day count.
• 365 vs 360 day-count basis: Adjusts the divisor used to determine the daily rate, aligning with either standard calendar years or commercial banking standards.

Daily interest formula

When the tool is set to evaluate simple interest, it relies on the following direct calculation:

$$I=P\times{r}\times\frac{d}{B}$$

Where:

• $I$ = total interest
• $P$ = principal amount
• $r$ = annual interest rate as a decimal
• $d$ = number of days
• $B$ = day-count basis, either 365 or 360

This specific simple interest calculator formula ensures that interest only accrues on the initial deposit, scaling linearly with the number of days.

Daily compounding formula

For scenarios where interest builds upon itself, the calculator utilizes this exponential equation:

$$A=P\left(1+\frac{r}{B}\right)^d$$

$$I=A-P$$

Where:

• $A$ = final balance
• $I$ = interest earned
• $P$ = principal
• $r$ = annual rate as a decimal
• $B$ = 365 or 360
• $d$ = number of days

Daily compounding means interest is added to your balance at the end of each day. Because tomorrow’s interest is calculated on today’s slightly larger final balance, your money grows at an accelerating pace compared to flat-rate methods.

Daily interest rate formula

To find the exact percentage applied to your balance each day, the calculator converts the annual rate using this formula:

$$\text{Daily Rate}=\frac{r}{B}$$

The daily interest rate calculator output is always displayed as a percentage, giving you a clear view of the fractional amount driving your daily growth.

APY formula for daily compounding

The APY calculator function determines the true annual return using this formula:

$$\text{APY}=\left(1+\frac{r}{B}\right)^B-1$$

Annual Percentage Yield (APY) includes the mathematical effect of compounding over a full year. Because interest generates its own interest, the APY will always be slightly higher than the stated nominal annual interest rate, giving you a standardized metric to compare different accounts.

What each input means

What each result means

Daily interest calculator example using simple interest

Suppose you place $10,000 into an account paying a 5% annual rate for 90 days, using a standard 365-day year.

$$I=10000\times{0.05}\times\frac{90}{365}$$

$$I=10000\times{0.05}\times{0.246575}$$

$$I=123.29$$

Using simple interest, the total interest earned over the 90 days is $123.29, resulting in a final balance of $10,123.29.

Daily interest calculator example using daily compounding

Now imagine investing that same $10,000 at a 5% annual rate for 90 days (365-day basis), but with daily compounding.

$$A=10000\times\left(1+\frac{0.05}{365}\right)^{90}$$

$$A=10000\times\left(1+0.000136986\right)^{90}$$

$$A=10000\times{1.012406}$$

$$A=10124.06$$

The final balance reaches $10,124.06. Your total interest earned is $124.06, and the APY for this compounding setup is 5.13%.

Interest between two dates example

If you need to calculate interest from January 1 to January 15, the tool counts the exact difference between these two points in time.

The span from January 1 to January 15 equals exactly 14 days of interest accrual. If you enter January 1 as both the start and end date, the calculator returns 0 days, as no full daily cycle has elapsed to generate interest.

360 vs 365 day count explained

The day count convention dictates how the annual rate is divided into a daily rate:

• Standard 365-day basis: Divides the annual rate by 365. This is the most common method for consumer savings accounts and standard retail banking.
• Banker’s year 360-day basis: Divides the annual rate by 360. Often used in commercial lending and specific corporate bonds, this convention results in a slightly higher daily interest rate since the denominator is smaller.

When to use simple interest vs daily compounding

Common assumptions and limitations

When using this tool, keep the following scope parameters in mind:

• Calculates flat scenarios with no recurring deposits or withdrawals.
• Operates sequentially and performs no reverse solving for principal, rate, or time.
• Provides gross figures and calculates no taxes or fees.
• Month-based duration inputs use an average month length; use exact dates for strict accuracy.
• Exact bank accrual can vary slightly depending on the specific institution’s rounding rules.
• APY output is exclusively relevant to compound mode only.

FAQs

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