Use this prorated interest calculator to find accrued interest between two dates or for a set number of days. Compare Actual/365, Actual/360, and 30/360, then see payoff instantly.
A prorated interest calculator computes the exact amount of simple interest accumulated over a specific, partial time period. It functions as a precise accrued interest calculator, determining the financial cost or yield based on your principal balance, the annual interest rate, and the exact number of days involved in the transaction.
Whether you choose to input exact dates or enter manual days, this tool estimates your final figures using standard financial day-count conventions. By selecting an underlying basis—such as Actual/365, Actual/360, or 30/360—the calculator instantly provides the elapsed days, the interest per counted day, the total accrued interest, and the final payoff amount.
What Is Prorated Interest?
Prorated interest represents the portion of annual simple interest that is charged or earned for a period shorter than a full year. Instead of applying a flat annual rate, the calculation scales the interest down to match the precise timeframe of the loan or investment.
The final amount generated depends entirely on four primary variables:
- The starting principal amount
- The annual interest rate
- The total counted days
- The specific day-count convention used by the contract
How This Prorated Interest Calculator Works
Operating this simple interest by days calculator requires only a few straightforward inputs to generate a complete payoff profile. The workflow is designed to match standard lending and contract formulas.
To get your results, follow these steps:
- Enter the principal amount of the loan or investment.
- Provide the annual interest rate as a percentage.
- Choose whether to select exact start and end dates from a calendar or input manual days.
- Select your required basis (Actual/365, Actual/360, or 30/360).
- Read the output panel for your elapsed days, interest per counted day, accrued interest, and total payoff amount.
Prorated Interest Formula
The mathematics behind an interest between two dates calculator rely on scaling the annual rate to a daily rate, then multiplying by the days held. The core formula is:
$$I = P \times \frac{r}{100} \times \frac{d}{B}$$
Where:
- $I$ = prorated interest
- $P$ = principal
- $r$ = annual rate
- $d$ = counted days
- $B$ = day-count basis (such as 365 or 360)
Depending on your selected convention, the specific calculations adjust slightly as shown below.
Actual/365
This method divides the annual rate over a standard 365-day year.
$$\text{Interest Per Day} = P \times \frac{r}{100} \div 365$$
$$\text{Prorated Interest} = \text{Interest Per Day} \times d$$
Actual/360
Used frequently in commercial lending, this method divides the annual rate by a 360-day year, resulting in a slightly higher daily charge.
$$\text{Interest Per Day} = P \times \frac{r}{100} \div 360$$
$$\text{Prorated Interest} = \text{Interest Per Day} \times d$$
30/360 (US/NASD)
This convention standardizes every month to 30 days and the year to 360 days.
$$\text{Interest Per Counted Day} = P \times \frac{r}{100} \div 360$$
$$\text{Prorated Interest} = \text{Interest Per Counted Day} \times d_{30/360}$$
$$\text{Total Payoff} = P + \text{Prorated Interest}$$
Actual/365 vs Actual/360 vs 30/360
Choosing the correct basis is critical because different methods will change the final result even if the principal, rate, and calendar dates remain identical.
| Basis | How days are counted | Year divisor | Best for | Effect on interest |
| Actual/365 | Actual elapsed calendar days | 365 | Personal loans, standard retail | Baseline daily interest calculation |
| Actual/360 | Actual elapsed calendar days | 360 | Commercial lending, bank loans | Yields slightly higher total interest |
| 30/360 | Standardized 30-day months | 360 | Corporate bonds, mortgages | Smoothes out month-to-month variations |
When comparing actual 360 vs actual 365, Actual/365 uses the true number of elapsed days divided by 365. Actual/360 uses the exact same calendar days but divides by 360, which slightly inflates the daily rate. The 30/360 interest calculator normalizes all months to exactly 30 days and assumes a 360-day year, stripping out the variability of 28-day or 31-day months entirely.
How to Calculate Prorated Interest Between Two Dates
Using the exact-date mode requires understanding how the calculator counts time. For both actual-day modes (Actual/365 and Actual/360), the tool applies a standard financial counting rule: you subtract the start date from the end date. This means the calculation is start-inclusive and end-exclusive.
If you enter the exact same start and end date, the tool will return 0 days. However, if you switch to the 30/360 convention, the software abandons literal calendar-day subtraction and instead counts days based on the standardized rule that every month has exactly 30 days.
Worked Examples
To illustrate how the inputs interact with the formulas, here are three common scenarios computed by the daily interest calculator.
1. Actual/365 using exact dates
A borrower needs to calculate prorated interest on a $10,000 balance at 5% for exactly 73 days.
- Inputs: Principal = 10,000, Rate = 5%, Days = 73, Basis = Actual/365
- Formula: $$\text{Interest Per Day} = 10000 \times \frac{5}{100} \div 365$$
- Substitutions: $1.36986 \times 73$
- Final Prorated Interest: $100.00
- Total Payoff: $10,100.00
2. Actual/360 using manual days
A business calculates a short-term prorated loan interest calculator estimate on $50,000 at 6% over 30 manual days.
- Inputs: Principal = 50,000, Rate = 6%, Days = 30, Basis = Actual/360
- Formula: $$\text{Interest Per Day} = 50000 \times \frac{6}{100} \div 360$$
- Substitutions: $8.33333 \times 30$
- Final Prorated Interest: $250.00
- Total Payoff: $50,250.00
3. 30/360 using exact dates
An investor checks bond interest on $20,000 at 4.5% between January 1 and April 1, which generates exactly 90 counted days under this specific convention.
- Inputs: Principal = 20,000, Rate = 4.5%, Dates = Jan 1 to Apr 1 (90 counted days), Basis = 30/360
- Formula: $$\text{Interest Per Counted Day} = 20000 \times \frac{4.5}{100} \div 360$$
- Substitutions: $2.50 \times 90$
- Final Prorated Interest: $225.00
- Total Payoff: $20,225.00
When to Use a Prorated Interest Calculator
There are numerous practical situations where finding the exact interest per day is necessary. Standard scenarios include:
- Calculating interest generated between a loan disbursement date and the very first payment date.
- Finding the accrued interest on a partial month when closing an account.
- Managing short-term lending, promissory notes, or peer-to-peer loans.
- Creating payoff estimates between two specific calendar dates.
- Verifying bond or contract figures that legally require a specific day-count basis.
What Affects Prorated Interest Results?
Small changes to your inputs will alter the final mathematical output. Here is a breakdown of how each factor influences the calculation.
| Factor | Why it changes the result |
| Principal | A larger starting balance generates higher daily interest amounts. |
| Annual interest rate | The percentage dictates the base cost of borrowing before division. |
| Number of days | More elapsed time allows more daily interest charges to accumulate. |
| Day-count convention | Selecting 360 instead of 365 increases the daily fractional charge. |
| Start and end dates | Determines the exact calendar days counted in actual-day modes. |
Common Mistakes When Calculating Prorated Interest
Precision matters in financial calculations, and small errors can lead to inaccurate payoff amounts. Watch out for these frequent mistakes when utilizing the tool:
- Using actual calendar days when the underlying contract legally specifies the 30/360 method.
- Confusing Annual Percentage Rate (APR)—which includes fees—with the nominal simple interest rate required for this math.
- Attempting to use compound interest or amortization formulas when the scenario only calls for simple daily interest.
- Counting the wrong number of days by accidentally including both the start and end dates (violating the standard start-inclusive, end-exclusive rule).
- Ignoring basis differences and assuming all loans use a 365-day year divisor.
Prorated Interest Calculator FAQs
What is prorated interest?
It is the specific amount of simple interest that accumulates over a partial period of time, rather than a full year. It is calculated by reducing an annual interest rate down to a daily rate and multiplying it by the elapsed days.
How do you calculate prorated interest?
You multiply your principal balance by your annual interest rate, divide that number by your day-count basis (like 365 or 360) to find the daily rate, and then multiply by the total number of counted days.
How do I calculate interest between two dates?
Input your start date and end date into the calculator. The tool will subtract the start date from the end date to determine the exact elapsed days, then apply those days to the daily interest formula.
What is the difference between Actual/360 and Actual/365?
Both methods count the exact literal calendar days between two dates. The difference lies in the division: Actual/365 divides the annual interest by 365 days, while Actual/360 divides it by 360 days, which slightly increases the daily interest charge.
What does 30/360 mean in interest calculations?
This is a standard financial convention that assumes every month has exactly 30 days and a full year has exactly 360 days. It removes the mathematical variations caused by 28-day, 29-day, or 31-day months.
Is prorated interest the same as accrued interest?
Yes, in the context of this calculator, they are functionally identical. Both terms refer to simple interest that has accumulated over a specific number of days but has not yet been paid or added to the principal via compounding.
Why does the day-count convention change the result?
The convention dictates the denominator used in the daily rate equation. A smaller denominator, like 360, makes the daily fractional rate larger than a denominator of 365, resulting in more total accumulated interest over the same timeline.
Can I use a prorated interest calculator for loans and bonds?
Yes, provided the loan or bond calculates simple interest over a partial period without compounding during that specific timeframe. You just need to ensure you select the exact day-count basis required by your specific contract.
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