Calculator Documentation
Assumable Mortgage Calculator
Compare assuming a seller's low-rate loan against a new mortgage at today's rates
Schema version 1.0.0 · Category: mortgage
Inputs
Seller's Loan (To Assume)
The existing loan you would take over
| Field | Type | Default | Notes |
|---|---|---|---|
| Purchase Price | currency | $450,000 | Agreed price for the home |
| Seller's Loan Balance | currency | $320,000 | Remaining balance on the assumable loan |
| Seller's Interest Rate | percentage | 3.00% | The rate you inherit by assuming (FHA/VA/USDA loans are assumable) |
| Years Remaining | number | 25 | Years left on the seller's loan |
| Assumption Fee | currency | $900 | Lender processing fee (VA caps at 0.5% of balance; FHA up to ~$900) |
Your Financing
Down payment and the gap loan
| Field | Type | Default | Notes |
|---|---|---|---|
| Down Payment | currency | $45,000 | Cash you bring. The equity gap beyond this is financed separately. |
| Gap Loan Rate | percentage | 8.50% | Rate on the second mortgage/HELOC covering the equity gap |
| Gap Loan Term (Years) | number | 20 | Term of the gap financing |
Traditional Loan (For Comparison)
Today's market alternative
| Field | Type | Default | Notes |
|---|---|---|---|
| Today's Market Rate | percentage | 6.50% | Rate on a new conventional loan today |
| New Loan Term | select | 30 | Term of the comparison loan |
Calculations
Every figure this calculator produces, in evaluation order. Formulas reference input ids, parameters, and earlier calculations.
sellerMonthlyRateSeller's monthly rate
sellerRate / 12
sellerTermMonthsAssumed loan months remaining
sellerRemainingYears * 12
assumedPaymentMonthly P&I on the assumed balance at the inherited rate
(sellerLoanBalance * sellerMonthlyRate * pow(1 + sellerMonthlyRate, sellerTermMonths)) / (pow(1 + sellerMonthlyRate, sellerTermMonths) - 1)
gapNeededEquity gap to finance beyond the down payment
max(purchasePrice - sellerLoanBalance - downPayment, 0)
gapMonthlyRateGap loan monthly rate
gapRate / 12
gapTermMonthsGap loan months
gapTermYears * 12
gapPaymentMonthly payment on the gap loan
(gapNeeded * gapMonthlyRate * pow(1 + gapMonthlyRate, gapTermMonths)) / (pow(1 + gapMonthlyRate, gapTermMonths) - 1)
combinedPaymentTotal monthly P&I with the assumption structure
assumedPayment + gapPayment
marketMonthlyRateMarket monthly rate
marketRate / 12
marketTermMonthsComparison term months
marketTermYears * 12
traditionalLoanAmountNew-loan amount at the same down payment
purchasePrice - downPayment
traditionalPaymentMonthly P&I on a traditional loan today
(traditionalLoanAmount * marketMonthlyRate * pow(1 + marketMonthlyRate, marketTermMonths)) / (pow(1 + marketMonthlyRate, marketTermMonths) - 1)
monthlySavingsMonthly savings from assuming vs a new loan
traditionalPayment - combinedPayment
fiveYearSavingsFive-year savings net of the assumption fee
monthlySavings * 60 - assumptionFee
assumptionNotBeneficialInverted flag for the verdict (0 = assumption saves money, 1 = it does not)
if(monthlySavings > 0, 0, 1)
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