Useful calculations.
Clear assumptions.

curl https://losttofound.org/api/v1/calculators/mortgage \
  -H 'Content-Type: application/json' \
  --data '{
  "price": 350000,
  "downPercent": 10,
  "rate": 6.71,
  "years": 30,
  "loanType": "conventional",
  "vaUse": "first",
  "financeFee": true,
  "taxPercent": 1.1,
  "annualInsurance": 1800,
  "monthlyHoa": 0,
  "pmiPercent": 0.5,
  "maintenancePercent": 1,
  "closingPercent": 3,
  "extraCash": 1500,
  "reserveMonths": 3,
  "savings": 0,
  "grossIncome": 0,
  "monthlyDebts": 0,
  "customUpfrontFee": 0,
  "customMonthlyInsurance": 0
}'

Use the units listed for each field. Every field is required, including fields unused by the selected loan program.

{
  "apiVersion": "v1",
  "calculationVersion": "2026-09-17.1",
  "model": "mortgage",
  "currency": "USD",
  "result": {
    "down": 35000,
    "baseLoan": 315000,
    "feePercent": 0,
    "fundingFee": 0,
    "loan": 315000,
    "principalInterest": 2034.7155423173795,
    "tax": 320.83333333333337,
    "insurance": 150,
    "pmi": 131.25,
    "monthlyInsurance": 131.25,
    "annualInsurancePercent": 0,
    "insuranceLabel": "Mortgage insurance (PMI)",
    "upfrontFeeLabel": "VA funding fee",
    "insuranceDuration": "PMI is an initial estimate; cancellation rules and lender terms determine when it ends.",
    "housing": 2636.798875650713,
    "maintenance": 291.6666666666667,
    "monthly": 2928.4655423173795,
    "closing": 10500,
    "cashFee": 0,
    "upfront": 47000,
    "reserves": 8785.396626952139,
    "cashTarget": 55785.39662695214,
    "gap": 55785.39662695214,
    "debtToIncome": null,
    "totalInterest": 417497.5952342566
  },
  "warnings": [
    {
      "field": "pmiPercent",
      "code": "check_percentage_units",
      "message": "pmiPercent is interpreted as 0.5%. Percentage fields use percentage points: 7 means 7%. If a value below 1% is intentional, keep this input."
    }
  ],
  "assumptions": [
    "User-supplied rate; no live quote or eligibility check.",
    "All dollar amounts are USD; rate and percentage fields use percentage points, not fractions.",
    "Fixed fully amortizing loan; mortgage insurance is an initial estimate.",
    "Taxes, insurance, closing costs and upkeep are supplied assumptions."
  ],
  "sources": [
    "https://www.consumerfinance.gov/owning-a-home/",
    "https://www.hud.gov/buying/loans",
    "https://www.va.gov/housing-assistance/home-loans/"
  ],
  "disclaimer": "Educational estimate, not a quote, appraisal, approval or personalized financial advice."
}

curl https://losttofound.org/api/v1/calculators/rental-property \
  -H 'Content-Type: application/json' \
  --data '{
  "price": 350000,
  "down": 20,
  "rate": 7,
  "term": 30,
  "closing": 3,
  "rehab": 10000,
  "reserves": 10000,
  "rent": 2800,
  "other": 0,
  "vacancy": 5,
  "tax": 4200,
  "insurance": 1800,
  "hoa": 0,
  "utilities": 100,
  "repairs": 2000,
  "capex": 2000,
  "management": 8
}'

Use the units listed for each field. Every field is required, including fields unused by the selected loan program.

{
  "apiVersion": "v1",
  "calculationVersion": "2026-09-17.1",
  "model": "rental-property",
  "currency": "USD",
  "result": {
    "headline": "This property needs cash each month",
    "metrics": [
      {
        "label": "Monthly cash flow",
        "value": -348.98031983504615,
        "format": "money"
      },
      {
        "label": "Cash needed to buy",
        "value": 100500,
        "format": "money"
      },
      {
        "label": "Cap rate",
        "value": 5.7618285714285715,
        "format": "percent"
      },
      {
        "label": "Cash-on-cash return",
        "value": -4.166929192060252,
        "format": "percent"
      }
    ],
    "rows": [
      {
        "label": "Scheduled annual income",
        "value": 33600,
        "format": "money"
      },
      {
        "label": "Vacancy allowance",
        "value": 1680,
        "format": "money"
      },
      {
        "label": "Operating expenses",
        "value": 11753.6,
        "format": "money"
      },
      {
        "label": "Net operating income (NOI)",
        "value": 20166.4,
        "format": "money"
      },
      {
        "label": "Capital replacement reserve",
        "value": 2000,
        "format": "money"
      },
      {
        "label": "Annual mortgage payments",
        "value": 22354.163838020555,
        "format": "money"
      },
      {
        "label": "NOI / debt service",
        "value": 0.9021317078163511,
        "format": "ratio"
      },
      {
        "label": "Monthly rent needed to break even",
        "value": 3199.2909837929587,
        "format": "money"
      },
      {
        "label": "Downside monthly cash flow",
        "value": -709.6203198350462,
        "format": "money"
      }
    ],
    "notes": [
      "NOI excludes financing, income taxes and capital replacements. Cash flow includes your capital reserve. No appreciation or tax benefits are assumed.",
      "Downside: scheduled rent and other income fall 10%, vacancy rises 5 percentage points, and fixed expenses remain unchanged.",
      "Verify actual rent rolls, property taxes after sale, insurance quotes, local rules, repairs and vacancy. Rates and costs are editable assumptions, not investment-loan quotes."
    ]
  },
  "warnings": [],
  "assumptions": [
    "USD; rent, other, HOA and utilities are monthly. Tax, insurance, repairs and capex are annual.",
    "Vacancy and management are percentages; down and closing are percentages of price.",
    "Before income taxes, appreciation and depreciation benefits.",
    "Downside reduces scheduled income 10% and raises vacancy 5 percentage points."
  ],
  "sources": [
    "https://www.consumerfinance.gov/owning-a-home/prepare/figure-out-how-much-you-want-to-spend/",
    "https://www.irs.gov/publications/p527"
  ],
  "disclaimer": "Educational estimate, not a quote, appraisal, approval or personalized financial advice."
}

curl https://losttofound.org/api/v1/calculators/business \
  -H 'Content-Type: application/json' \
  --data '{
  "price": 450000,
  "revenue": 900000,
  "earnings": 180000,
  "basis": "sde",
  "unsupported": 15000,
  "labor": 0,
  "reinvestment": 15000,
  "ownerPay": 60000,
  "debt": 0,
  "acquiredCash": 0,
  "annualDebt": 0,
  "down": 20,
  "rate": 10,
  "years": 10,
  "costs": 15000,
  "workingCapital": 30000,
  "low": 2,
  "high": 3,
  "decline": 20,
  "target": 1.25,
  "assets": 0
}'

Use the units listed for each field. Every field is required, including fields unused by the selected loan program.

{
  "apiVersion": "v1",
  "calculationVersion": "2026-09-17.1",
  "model": "business",
  "currency": "USD",
  "result": {
    "normalized": 165000,
    "operatingPrice": 450000,
    "lowValue": 330000,
    "highValue": 495000,
    "loan": 360000,
    "payment": 4757.42652774342,
    "debtService": 57089.11833292103,
    "available": 150000,
    "residual": 92910.88166707897,
    "surplus": 32910.88166707897,
    "coverage": 2.6274709503352227,
    "stressCoverage": 2.0494273412614734,
    "stressedResidual": -89.11833292103256,
    "cashNeeded": 135000,
    "earningsMultiple": 2.727272727272727,
    "revenueMultiple": 0.5,
    "margin": 18.333333333333332,
    "leverage": 2.1818181818181817,
    "cashReturn": 24.37843086450294,
    "assetEquity": 0,
    "pricePosition": "Within your selected range"
  },
  "warnings": [],
  "assumptions": [
    "All dollars are USD and cash-flow inputs are annual unless specified.",
    "User-selected SDE or EBITDA multiples; no appraisal or automatic comparable-deal lookup.",
    "One fully amortizing acquisition loan; transferred debt payments are supplied separately.",
    "No tax, balloon, refinancing or lender eligibility model."
  ],
  "sources": [
    "https://www.bizbuysell.com/insight-report-data-tables/",
    "https://www.sba.gov/loans/7a-loans/"
  ],
  "disclaimer": "Educational estimate, not a quote, appraisal, approval or personalized financial advice."
}

[
  {
    "field": "rate",
    "code": "check_percentage_units",
    "message": "rate is interpreted as 0.0671%. Percentage fields use percentage points: 7 means 7%. If a value below 1% is intentional, keep this input."
  },
  {
    "field": "pmiPercent",
    "code": "check_percentage_units",
    "message": "pmiPercent is interpreted as 0.5%. Percentage fields use percentage points: 7 means 7%. If a value below 1% is intentional, keep this input."
  }
]

This response comes from the mortgage example with price: 0 and an added unexpected: true field.

{
  "error": {
    "code": "unknown_field",
    "message": "Body contains an undocumented field.",
    "details": [
      {
        "field": "unexpected",
        "code": "unknown_field",
        "message": "This field is not documented for this calculator."
      },
      {
        "field": "price",
        "code": "out_of_range",
        "message": "Must be at least 1."
      }
    ]
  }
}