> ## Documentation Index
> Fetch the complete documentation index at: https://docs.fincept.in/llms.txt
> Use this file to discover all available pages before exploring further.

# Calculate I-Spread

> Calculate the I-spread (Interpolated spread) - the difference between a bond's yield to maturity and the swap rate of equivalent maturity. The I-spread uses the swap curve as the benchmark rather than government bonds, making it more appropriate for credit analysis since swap rates better reflect interbank lending rates. Used extensively in corporate bond markets and credit default swap pricing. More stable than G-spread as it removes sovereign risk component. [Tier: STANDARD, Credits: 2]



## OpenAPI

````yaml api-specs/solver.json post /quantlib/solver/finance/i-spread
openapi: 3.1.0
info:
  title: FinceptQuantLib API - Solver
  description: >-
    Solver module endpoints for FinceptQuantLib API. The Solver module (Basic
    Tier, 1 credit per request) provides advanced financial solvers including
    bond analytics (yield, duration, convexity), implied volatility
    calculations, spread computations (Z-spread, ASW, OAS), curve bootstrapping,
    interest rate conversions, and model calibration (Vasicek, CIR). Essential
    for fixed income analysis, derivatives pricing, and risk management.
  version: 3.0.0
  contact:
    name: Fincept API Support
    url: https://fincept.in
servers:
  - url: https://api.fincept.in
    description: Fincept API Production Server
security:
  - APIKeyHeader: []
tags:
  - name: quantlib-solver
    description: >-
      Advanced financial solvers for bond analytics, implied volatility, spread
      calculations, and model calibration
    x-displayName: Solver
paths:
  /quantlib/solver/finance/i-spread:
    post:
      tags:
        - quantlib-solver
      summary: Calculate I-Spread
      description: >-
        Calculate the I-spread (Interpolated spread) - the difference between a
        bond's yield to maturity and the swap rate of equivalent maturity. The
        I-spread uses the swap curve as the benchmark rather than government
        bonds, making it more appropriate for credit analysis since swap rates
        better reflect interbank lending rates. Used extensively in corporate
        bond markets and credit default swap pricing. More stable than G-spread
        as it removes sovereign risk component. [Tier: STANDARD, Credits: 2]
      operationId: i_spread
      requestBody:
        required: true
        content:
          application/json:
            schema:
              type: object
              required:
                - bond_yield
                - swap_rate
              properties:
                bond_yield:
                  type: number
                  description: Yield to maturity of the bond as a decimal
                  example: 0.055
                swap_rate:
                  type: number
                  description: Interpolated swap rate of equivalent maturity as a decimal
                  example: 0.04
            example:
              bond_yield: 0.055
              swap_rate: 0.04
      responses:
        '200':
          description: Successful Response
          content:
            application/json:
              schema:
                type: object
                properties:
                  success:
                    type: boolean
                    example: true
                  data:
                    type: object
                    properties:
                      i_spread:
                        type: number
                        description: >-
                          The I-spread as a decimal (multiply by 10000 for basis
                          points)
                        example: 0.015
              example:
                success: true
                data:
                  i_spread: 0.015
        '401':
          $ref: '#/components/responses/UnauthorizedError'
        '402':
          $ref: '#/components/responses/InsufficientTierError'
        '422':
          description: Validation Error
          content:
            application/json:
              schema:
                type: object
                properties:
                  detail:
                    type: array
                    items:
                      type: object
      security:
        - APIKeyHeader: []
components:
  responses:
    UnauthorizedError:
      description: Authentication information is missing or invalid
      content:
        application/json:
          schema:
            type: object
            properties:
              detail:
                type: string
                example: Invalid API key
    InsufficientTierError:
      description: API tier insufficient for this endpoint
      content:
        application/json:
          schema:
            type: object
            properties:
              detail:
                type: string
                example: Endpoint requires Basic tier or higher
  securitySchemes:
    APIKeyHeader:
      type: apiKey
      in: header
      name: X-API-Key
      description: >-
        API key for authentication. Get your key at
        https://api.fincept.in/auth/register

````