Formula used
This calculator estimates interest over a grace period using one of three methods.
Let B be the starting balance, r the annual rate (decimal),
d the number of days, and Y the days in year.
- Simple interest:
Interest = B * r * (d / Y) - Daily compounding:
End = B * (1 + r / Y)^d, thenInterest = End - B - Monthly compounding:
End = B * (1 + r / 12)^(d / (Y/12))(fractional periods allowed)
If a subsidy applies, borrower interest is reduced by the subsidy percent. Payment timing splits the period into segments when needed.
How to use this calculator
- Enter your starting balance and annual interest rate.
- Choose a compounding method that matches your agreement.
- Select dates for exact days, or enter a length instead.
- Add any subsidy percentage and an optional grace period payment.
- Pick when the payment happens and whether interest capitalizes.
- Click Calculate, then download the CSV or PDF report.
Example data table
| Scenario | Balance | APR | Grace days | Method | Subsidy | Payment | Estimated interest |
|---|---|---|---|---|---|---|---|
| Student loan (unsubsidized) | 10000 | 6.00% | 180 | Daily | 0% | 0 | About 300 |
| Subsidized program | 8500 | 5.25% | 90 | Simple | 50% | 0 | About 55 |
| Early payment reduces cost | 12000 | 9.90% | 60 | Daily | 0% | 1000 | About 145 |
Examples are rounded for readability. Your results may differ based on exact day counts, lender rounding, and capitalization rules.
What grace period interest represents
A grace period delays required payments, not always interest. If your balance is 10000 and APR is 7.50%, 30 days can add about 61.64 using a 365 day year. Some programs waive or subsidize this cost, which changes the borrower total for planning.
Inputs that change the estimate
The balance and day count drive most outcomes. A 180 day grace period is six times a 30 day period at the same rate. Date entry captures exact days, while length entry uses an approximation when you select months. Day count conventions matter too: using 360 instead of 365 raises the daily rate by roughly 1.39%, increasing interest on the same dates.
Comparing simple and compounding methods
Simple interest grows linearly: interest per day equals balance times APR divided by days in year. Compounding increases the base over time. With daily compounding, interest for later days is slightly higher because prior interest is included. Monthly compounding applies a monthly rate but is converted into an effective daily step for graphing, so longer periods show clearer differences.
How payments and subsidies shift results
A payment at the start lowers the balance that earns interest, producing the biggest reduction. A midpoint payment reduces the second half of the period and can materially lower compounding growth. A payment at the end lowers what you owe, but usually does not reduce accrued interest. Subsidy percent reduces borrower interest; 50% subsidy cuts the borrower portion in half. The calculator keeps gross interest and shows the subsidized share separately.
Using the report for budgeting decisions
Use the interest estimate to plan cash needs before your first scheduled payment. Compare capitalization on versus off to see whether interest becomes part of the balance, which can raise future interest charges. Export the CSV for spreadsheets, or share the PDF when discussing terms with a lender or servicer. Treat results as estimates and confirm rounding rules, dates, and capitalization triggers in your agreement.
FAQs
Does interest always accrue during a grace period?
Not always. Some loans pause interest, while others keep accruing daily. Your agreement may define a true no interest grace period, a deferred billing period, or subsidized interest that reduces what you pay.
Which day count should I use, 360 or 365?
Use the convention your lender applies to compute daily interest. Many consumer loans use 365, while some products use 360. Changing this value slightly changes the daily rate and the final interest estimate.
How do I choose the compounding method?
Match your statement or contract language. If it says interest compounds daily, choose daily. If it references monthly compounding, choose monthly. If you only have an annual rate with no compounding detail, simple interest is a conservative baseline.
What does capitalize interest mean here?
When enabled, accrued borrower interest is added to the balance, increasing the amount owed. When disabled, the calculator keeps principal separate and shows interest as an additional amount due, which is common when interest is billed but not added to principal.
How does a mid grace payment affect the graph?
The plot reduces the balance at the midpoint and recalculates remaining accrual from that lower base. Earlier payments generally reduce total interest more than later payments because fewer days accrue interest on the higher balance.
Are the CSV and PDF reports official documents?
No. They are planning summaries generated from your inputs. They can help you discuss scenarios, but lenders may calculate interest differently due to rounding, posting times, minimum interest rules, or policy specific grace definitions.