Example 2: Determine the APR implicit in a repayment schedule, including charges

This example illustrates the use of the one of three supported legally defined APR Day Count Conventions

Overview

This example demonstrates how to calculate the Annual Percentage Rate (APR) for a consumer loan, in accordance with the EU 2008/48 APR. The same approach works for any supported APR convention (e.g., US Appendix J APR or UK Conc App APR).

Code

This example solves for an unknown APR for a €10,000 loan over six months, including a €50 fee due with the first instalment.

First the unknown instalment payment is solved using US30U360, a standard convention, and then we show how to confirm the APR thereafter. If all values are known beforehand, skip the solveValue step and go straight to solving the implicit APR.

Notes:

• Use date input to correctly align end of contract fees or charges.

• Dates are optional and default to the current system date. Here, a fixed startDate is provided solely to ensure reproducible test results.

import 'package:curo/curo.dart';

void main() async {

  // Create a calculator instance and define cash flow series
  final calculator = Calculator()
    ..add(SeriesAdvance(
      label: 'Loan',
      amount: 10000.0,
    ))
    ..add(SeriesPayment(
      numberOf: 6,
      label: 'Instalment',
      amount: null,
      mode: Mode.arrear,
    ))
    ..add(SeriesCharge(
      label: 'Fee',
      amount: 50.0,
      mode: Mode.arrear,
    ));

  // Solve for the unknown and validate rate
  final payment = await calculator.solveValue(
    convention: US30U360(),
    interestRate: 0.0825,
    startDate: DateTime.utc(2026, 1, 5),
  );

  final convention = EU200848EC();

  final apr = await calculator.solveRate(convention: convention);

  // Optionally create a, APR proof schedule and display results
  final schedule = calculator.buildSchedule(
    convention: convention,
    interestRate: apr,
  );

  print('Monthly instalment: €${payment.toStringAsFixed(2)}');
  print('Annual percentage rate: ${(apr * 100).toStringAsFixed(2)}%\n');
  schedule.prettyPrint(convention: convention);

Console output:

Monthly instalment: €1707.00
Annual percentage rate: 10.45%

post_date    label               amount discount_log                  amount_disc     disc_balance
--------------------------------------------------------------------------------------------------
2026-01-05   Loan            -10,000.00 t = 0 = 0.00000000             -10,000.00       -10,000.00
2026-02-05   Instalment        1,707.00 t = 1/12 = 0.08333333            1,692.92        -8,307.08
2026-02-05   Fee                  50.00 t = 1/12 = 0.08333333               49.59        -8,257.49
2026-03-05   Instalment        1,707.00 t = 2/12 = 0.16666667            1,678.96        -6,578.53
2026-04-05   Instalment        1,707.00 t = 3/12 = 0.25000000            1,665.12        -4,913.41
2026-05-05   Instalment        1,707.00 t = 4/12 = 0.33333333            1,651.38        -3,262.03
2026-06-05   Instalment        1,707.00 t = 5/12 = 0.41666667            1,637.77        -1,624.26
2026-07-05   Instalment        1,707.00 t = 6/12 = 0.50000000            1,624.26             0.00

Cash Flow Diagram

The diagram below visualizes the cash flow dynamics of a €10,000 loan with six instalment payments, including a €50 fee, as implemented in the example code.

• Advance: Depicted by a blue downward arrow at the start of the timeline, this represents the full cash price or loan value, known from the outset.

• Payments: The red upward arrows, signifying they are unknown, are the regular payments made in arrears, meaning at the end of each payment period.

• Charges: Represented by a green upward arrow (e.g., origination or setup fees).