> For the complete documentation index, see [llms.txt](https://docs.altum.ai/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.altum.ai/english/financing/mortgage-calculation-api.md).

# Mortgage Calculation API

The Mortgage Calculation API performs real-time, data-driven mortgage calculations and Annual Percentage Rate (APR - Jaarlijks KostenPercentage JKP) for the Dutch housing market.

## Introduction

The Mortgage Calculation API returns a structured JSON response with a clear mortgage assessment. It is designed for mortgage advisors, banks, and developers who need reliable mortgage calculations, quickly and consistently.

## Key features

Key aspects of the Mortgage Calculation API includes:

* **Automatic property enrichment**: when you provide an address, the API retrieves the WOZ value and energy label. This removes the need to look up the property value manually.
* **Nibud 2026 compliant**: calculations follow the official Nibud 2026 debt-service-to-income (DSTI) standards.
* **Automatic interest rate:** if no interest rate is provided, the API fetches the current rate from DNB (De Netherlandsche Bank) automatically.
* **Energy label support:** providing an energy label applies the official Nibud 2026 extra mortgage amounts and adjust the NHG limit where applicable.
* **JKP/APR calculation:** calculates the real annual percentage rate, net credit amount, and monthly annuity payment taking into account initial costs.
* **Flexible calculation modes**: 
  * Standard mortgage calculation:
    * With an address: the API uses property data such as the WOZ value.
    * Without an address: the API uses the provided purchase price.
    * Without an address and purchase price: the API performs an income-based calculation and returns the maximum mortgage.
  * Standalone APR (JKP): calculates instantly APR by providing loan amount and nominal interest rate
  * Combined calculation: provide both income and APR parameters to get borrowing capacity and loan credit costs in one call


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation by asking a question.

Perform an HTTP GET request on the following URL with the `ask` and `goal` query parameters:

```
GET https://docs.altum.ai/english/financing/mortgage-calculation-api.md?ask=<question>&goal=<user_goal>
```

`ask` is the immediate question: it should be specific, self-contained, and written in natural language.
`goal` is what the user is ultimately trying to achieve, the reason they need the answer. Sharing it helps GitBook give you a better, more relevant answer. A goal is most helpful when it describes the outcome the user wants rather than restating the question. For example, with `ask=how do I create an API token`, a goal like `automate deployments from our CI pipeline` lets GitBook tailor the answer to that use case.

The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.