Example data table
| Purpose | Annual income | Debts/mo | APR | Term | Extra costs/mo | Max new payment | Est. principal |
|---|---|---|---|---|---|---|---|
| Home / Mortgage | $95,000 | $650 | 6.50% | 30y | $450 | $1,450 | $156,900 |
| Auto | $72,000 | $500 | 9.50% | 5y | $120 | $900 | $42,300 |
| Personal | $60,000 | $350 | 14.99% | 3y | $0 | $500 | $15,800 |
Formula used
- Monthly income basis: annual income ÷ 12 (or net factor × gross).
- Total debt limit: income basis × (back-end DTI %).
- Room for new payment: total debt limit − existing monthly debts.
- Purpose cap: income basis × (purpose cap %).
- Max new payment: min(room, cap). For home loans, housing cap also applies.
- Principal from payment: PV = PMT × (1 − (1+r)−n) ÷ r, where r=APR/12 and n=term months.
- Purchase budget: principal + down payment.
How to use this calculator
- Select your loan purpose and your credit range.
- Enter annual income and your current monthly debt payments.
- Set APR and term, or keep the suggested defaults.
- Enter extra monthly costs for your selected purpose.
- Click “Calculate affordability” and review the results above.
- Download CSV or PDF to compare scenarios side-by-side.
Purpose-driven affordability logic
Affordability changes by intent because different risks are acceptable. This tool applies purpose caps such as 28% housing, 15% auto, 10% personal, and 8% education to keep payments aligned with priorities. Business and consolidation options allow higher caps up to 20%, reflecting cash-flow planning rather than lifestyle spending.
Income and ratio framework
Monthly income starts from annual income ÷ 12. If you choose a net basis, the calculator applies a net factor from 40% to 95% to reflect withholding and benefits. The back-end debt-to-income limit sets a ceiling for all monthly debts, commonly 36% for home loans and up to 45% for higher-risk purposes. Existing debts reduce the remaining room for a new payment.
Payment-to-principal conversion
After the maximum new payment is found, extra costs are handled realistically. For home loans, taxes, insurance, and HOA are subtracted so principal-and-interest is sized correctly. For other purposes, optional monthly costs like auto insurance reduce the payment available for principal. The principal estimate uses installment present value: PV = PMT × (1 − (1+r)^−n) ÷ r, where r is APR/12 and n is term months.
Scenario testing with key levers
Defaults suggest typical APR and term combinations, such as 6.50% for 30 years on housing or 9.50% for 5 years on auto. Credit selection adjusts APR by −1.00, −0.25, +1.00, or +3.00 points to stress the estimate. Try shorter terms, lower DTI, or tighter purpose caps to build margin for savings. The Plotly chart shows how principal capacity changes as APR shifts around your estimate.
Using results to set safe targets
Treat the maximum payment as an upper boundary, not a goal. If your implied DTI is near the limit, reduce the target payment by 5% to 10% to account for underwriting buffers, variable income, and future expenses. Use down payment to translate supported principal into a purchase budget, then compare offers across lenders. Re-run scenarios to align affordability with your timeline and risk tolerance. before committing to any contract.
FAQs
What does “purpose cap” mean?
It is the maximum share of monthly income you want the new loan payment to consume. The tool uses different default caps by purpose, but you can adjust it to match your comfort level.
Why are there two ratios for home loans?
Home loans often use a housing cap for total housing costs and a back-end DTI cap for all debts. The calculator applies both, then subtracts taxes, insurance, and HOA to estimate principal-and-interest.
How is the APR adjusted by credit range?
Your entered APR is adjusted by a simple offset: Excellent −1.00, Good −0.25, Fair +1.00, and Poor +3.00 percentage points. This helps stress-test affordability when rates vary by credit.
What does the Plotly chart show?
It plots estimated principal capacity across a small range of APR values around your current result, keeping payment and term constant. This highlights how sensitive your borrowing power is to interest rate changes.
Can I use net income instead of gross income?
Yes. Choose the net basis and set a net factor, such as 70% to 80%. The tool will size payments from that smaller income figure, which can be helpful for conservative planning.
Is this result a lender approval?
No. It is a planning estimate based on the ratios and inputs you provide. Lenders also evaluate credit history, assets, collateral, and documentation, and they may apply different underwriting rules.