Free calculators
Rent versus buy calculator documentation
Inputs, formulas, assumptions and API/MCP examples for the rent versus buy calculator. Reproduce a calculation and cite its sources.
Content updated . Assumptions reviewed .
Formula and assumptions
Buyer wealth = home sale proceeds - mortgage balance + investments; renter wealth = invested deposit, closing costs and monthly savings.
- Both options start with the same funds: the renter invests the buyer deposit and purchase costs.
- Each month both receive the larger housing cash outflow; the less expensive option invests the difference at month end.
- Property tax and maintenance track current property value. Insurance and service costs follow entered cost inflation.
- The buyer sells at the horizon using the entered selling-cost percentage; gains taxes and rent deposits are excluded.
- Break-even is the first modeled month when buying wealth reaches renting wealth; elapsed break-even years equal that month divided by 12. Both are null if no crossing occurs within the horizon, and later reversal remains possible.
- Return, appreciation and rent-growth assumptions have neutral zero defaults and are not forecasts.
Assumptions reviewed:
Sources
- KlarFort calculator methodology. KlarFort-authored arithmetic; free to use with credit.; vintage 1.
Parameters
| Name | Description | Type | Default | Minimum | Maximum |
|---|---|---|---|---|---|
| currency | Currency | string | "USD" | ||
| homePrice | Home purchase price | number | 400000 | 0 | 1,000,000,000 |
| downPayment | Cash deposit | number | 80000 | 0 | 1,000,000,000 |
| annualMortgageRate | Mortgage annual rate (%) | number | 5 | 0 | 100 |
| mortgageMonths | Mortgage term months | integer | 300 | 1 | 600 |
| horizonMonths | Comparison horizon months | integer | 120 | 1 | 600 |
| buyingCostsPercent | Purchase transaction costs (%) | number | 3 | 0 | 30 |
| sellingCostsPercent | Selling transaction costs (%) | number | 3 | 0 | 30 |
| homeAppreciation | Effective annual home appreciation (%) | number | 0 | -50 | 100 |
| monthlyRent | Starting monthly rent | number | 1800 | 0 | 1,000,000,000 |
| rentGrowth | Effective annual rent growth (%) | number | 0 | -50 | 100 |
| investmentReturn | Effective annual investment return (%) | number | 0 | -50 | 100 |
| annualPropertyTaxPercent | Annual property tax (% of current value) | number | 0 | 0 | 20 |
| annualMaintenancePercent | Annual maintenance (% of current value) | number | 1 | 0 | 20 |
| annualHomeInsurance | Starting annual home insurance | number | 1200 | 0 | 1,000,000,000 |
| monthlyServiceCharges | Starting monthly service charges | number | 150 | 0 | 1,000,000,000 |
| annualRentersInsurance | Starting annual renter insurance | number | 200 | 0 | 1,000,000,000 |
| costInflation | Effective annual insurance and service inflation (%) | number | 0 | -50 | 100 |
JSON API
GET accepts one input parameter containing URL-encoded JSON. POST sends the JSON input object. Prefer POST for personal figures. Inputs are limited to 16 KiB. POST results are not stored; GET results may be cached for up to 24 hours.
curl -X POST https://klarfort.com/tools/api/v1/rent-vs-buy-calculator -H "Content-Type: application/json" --data '{"currency":"USD","homePrice":400000,"downPayment":80000,"annualMortgageRate":5,"mortgageMonths":300,"horizonMonths":120,"buyingCostsPercent":3,"sellingCostsPercent":3,"homeAppreciation":0,"monthlyRent":1800,"rentGrowth":0,"investmentReturn":0,"annualPropertyTaxPercent":0,"annualMaintenancePercent":1,"annualHomeInsurance":1200,"monthlyServiceCharges":150,"annualRentersInsurance":200,"costInflation":0}'fetch("https://klarfort.com/tools/api/v1/rent-vs-buy-calculator", {method:"POST", headers:{"Content-Type":"application/json"}, body:JSON.stringify({"currency":"USD","homePrice":400000,"downPayment":80000,"annualMortgageRate":5,"mortgageMonths":300,"horizonMonths":120,"buyingCostsPercent":3,"sellingCostsPercent":3,"homeAppreciation":0,"monthlyRent":1800,"rentGrowth":0,"investmentReturn":0,"annualPropertyTaxPercent":0,"annualMaintenancePercent":1,"annualHomeInsurance":1200,"monthlyServiceCharges":150,"annualRentersInsurance":200,"costInflation":0})})import json, urllib.request
request = urllib.request.Request("https://klarfort.com/tools/api/v1/rent-vs-buy-calculator", data=json.dumps({"currency":"USD","homePrice":400000,"downPayment":80000,"annualMortgageRate":5,"mortgageMonths":300,"horizonMonths":120,"buyingCostsPercent":3,"sellingCostsPercent":3,"homeAppreciation":0,"monthlyRent":1800,"rentGrowth":0,"investmentReturn":0,"annualPropertyTaxPercent":0,"annualMaintenancePercent":1,"annualHomeInsurance":1200,"monthlyServiceCharges":150,"annualRentersInsurance":200,"costInflation":0}).encode(), headers={"Content-Type":"application/json"})
print(urllib.request.urlopen(request).read().decode())Connected assistants
Connect using https://klarfort.com/tools/mcp. Discover this calculator with tools/list, then call rent-vs-buy-calculator. Protocol versions 2026-07-28 and 2025-11-25 are supported. Modern requests carry protocol metadata and matching HTTP headers. Set Mcp-Name to params.name for tools/call and to params.uri for resources/read. Older clients initialize first.
Limits: 120 requests per minute per address and approximately 3,000 per minute per edge location. Simulations are capped at 1,000 paths, 600 months and 150,000 path-months (paths multiplied by months). Shorten the duration or reduce paths when that work limit is exceeded. A limit response returns 429 and Retry-After. No exact remaining-request count is available.
Usage and credit
Free for personal, commercial and any other lawful use with credit to KlarFort at https://klarfort.com/. Third-party data terms still apply.
Place a visible credit near the reused material or in the credits for your work or integration. One clear credit per work or integration is sufficient; no separate credit per API request is required. Links to individual calculators are optional.
Computed with KlarFort Calculators, https://klarfort.com/
The API returns this text in credit and the homepage in credit_url. The optional citation_url identifies the calculator for reference. See usage and credit for the full permission.
KlarFort is not financial, investment, tax, or legal advice. Verify outputs before making material decisions.