Calculator Inputs
Example Data Table
Use this sample to sanity-check your inputs and outputs.
| Item | Sample value | Notes |
|---|---|---|
| Credit limit | $50,000 | Maximum line available. |
| Starting balance | $35,000 | Current drawn amount. |
| APR | 9.25% | Interest rate used in the draw period. |
| Draw / Repay terms | 120 / 180 months | Interest-only draw, then amortizing repayment. |
| Extra payment | $100 / month | Applied on top of scheduled payment. |
| Output highlights | Months to payoff, total interest | See the full schedule after calculation. |
Practical Notes
Understanding HELOC phases
A home equity line typically starts with a draw phase followed by a repayment phase. During the draw phase, borrowers often pay interest only, which keeps required payments low while the balance stays largely unchanged. When the repayment phase begins, the remaining balance is amortized across the chosen term. This calculator models both phases and shows month-by-month payment allocation.
How monthly interest is calculated
Each month’s interest is computed from the current balance and the monthly rate. The monthly rate equals APR divided by twelve. If rates step up after the draw period, the repayment schedule uses the adjusted APR, optionally capped. The schedule table separates interest from principal so you can see how quickly the balance declines once amortization begins.
Payment structure and payoff timing
In repayment, the scheduled payment uses the standard amortization payment formula, then any extra payment is added. Extra payments reduce principal first, lowering future interest and shortening payoff time. The summary highlights months to payoff, total interest, fees, and total cost. Exporting CSV helps you compare scenarios side by side. For budgeting, compare the draw-period cash flow with the repayment payment jump and test rate caps or basis-point shifts to understand sensitivity before signing, renewing, or refinancing under extra-payment plans as well.
Fees, limits, and real-world inputs
Many lines include one-time closing costs and recurring annual fees. This tool tracks them separately from payments so you can evaluate true cost. It also checks that your starting balance does not exceed the credit limit. If you choose a non–interest-only draw payment, you can set a minimum payment to simulate aggressive paydown during the draw phase.
Using the chart for quick insights
The Plotly chart visualizes the balance curve alongside monthly interest and principal. A steepening balance decline usually indicates the repayment phase or meaningful extra payments. Rising interest bars can indicate a higher post-draw rate. Use the chart to spot inflection points, then confirm exact values in the amortization table or exports.
FAQs
1) Does this model new draws during the draw period?
No. It assumes a starting balance and simulates payments only. If you plan additional draws, increase the starting balance or run separate scenarios that reflect your expected utilization.
2) Why can repayment payments jump higher than draw payments?
Draw payments are often interest-only, so principal does not decline much. Repayment converts the remaining balance into an amortizing payment over a fixed term, increasing required monthly cash flow.
3) How are extra payments applied?
Extra payments are added to the scheduled amount and reduce principal after monthly interest is covered. This lowers future interest and can shorten the payoff time, sometimes by many months.
4) What does “rate change after draw” mean?
It adds the specified basis points to the APR for the repayment phase. For example, 75 bps increases the repayment APR by 0.75%. If you set an APR cap, the repayment APR is limited.
5) Are fees included in the amortization balance?
No. Closing costs and annual fees are tracked as fees, not added to the principal balance. They still affect total cost and are shown in the summary and schedule columns.
6) Why might the PDF be shorter than the CSV?
The PDF is a compact text export and may truncate long schedules for readability. Use the CSV when you need the full month-by-month schedule for spreadsheets or detailed comparisons.
Formula Used
- Monthly rate: r = APR ÷ 12.
- Interest each month: I = Balance × r.
- Principal paid: P = Payment − I (floored at 0).
- Repayment scheduled payment (amortizing): PMT = B×r ÷ (1 − (1+r)−n).
- Fees: closing costs add to total cost; annual fees apply in the selected month.
- Rate shift: optional basis-point change applies after draw (and is capped if set).
How to Use This Calculator
- Enter your credit limit, current balance, and APR.
- Set draw and repayment months to match your agreement.
- Choose interest-only during draw, or provide a draw minimum.
- Add extra payments and fees to see a realistic total cost.
- Click Calculate Amortization to view results above.
- Use Download CSV or Download PDF to save schedules.