erp_calculator

erp_calculator is a pure Dart billing engine for ERP, POS, checkout, and invoicing flows. It calculates taxes, discounts, and final totals with a step-by-step audit trail so the result is both accurate and explainable.

Built for Flutter and Dart apps that need deterministic invoicing logic.

Features

• Supports TaxMode.none, TaxMode.exclusive, and TaxMode.inclusive
• Handles item-level and bill-level discounts
• Supports percentage and fixed discounts
• Recalculates net, tax, and gross totals automatically
• Returns a detailed audit trail for every calculation
• Works in Flutter, backend, CLI, and web apps

Installation

Add the package to your pubspec.yaml:

dependencies:
  erp_calculator: ^1.0.0

Then run:

dart pub get

Quick start

import 'package:erp_calculator/erp_calculator.dart';

void main() {
  final result = BillingEngine.calculate(
    items: const [
      LineItem(
        id: 'sku-1',
        name: 'Laptop',
        unitPrice: 5000,
        qty: 1,
        taxMode: TaxMode.exclusive,
        taxRate: 14,
      ),
    ],
    billDiscounts: const [],
  );

  print(result.totalGross); // 5700.0
}

Core concepts

Every line item has three money values:

Term	Meaning
net	Price before tax
tax	Tax amount
gross	Final price the customer pays

Rule:

gross = net + tax

Tax modes

1. Exclusive tax

Tax is added on top of the stated price.

net   = price
tax   = net * rate
gross = net + tax

2. Inclusive tax

Tax is already included in the stated price.

gross = price
net   = gross / (1 + rate)
tax   = gross - net

3. No tax

net = gross = price
tax = 0

Discounts

Item-level discounts

Apply directly to one product or service line:

const item = LineItem(
  id: 'chair-1',
  name: 'Chair',
  unitPrice: 200,
  qty: 3,
  taxMode: TaxMode.exclusive,
  taxRate: 15,
  discountValue: 10,
  discountType: DiscountType.percentage,
  discountApplyOn: DiscountApplyOn.gross,
);

Supported options:

• Percentage or fixed amount
• Applied on gross or net

Bill-level discounts

Applied after all line items are calculated:

const billDiscount = BillDiscount(
  id: 'promo-5',
  value: 5,
  type: DiscountType.percentage,
  applyOn: DiscountApplyOn.gross,
);

Bill discounts are distributed proportionally across lines and can be stacked sequentially.

Full example

import 'package:erp_calculator/erp_calculator.dart';

void main() {
  final result = BillingEngine.calculate(
    items: const [
      LineItem(
        id: 'chair-1',
        name: 'Chair',
        unitPrice: 200,
        qty: 3,
        taxMode: TaxMode.exclusive,
        taxRate: 15,
        discountValue: 10,
        discountType: DiscountType.percentage,
        discountApplyOn: DiscountApplyOn.gross,
      ),
    ],
    billDiscounts: const [
      BillDiscount(
        id: 'promo-5',
        value: 5,
        type: DiscountType.percentage,
        applyOn: DiscountApplyOn.gross,
      ),
    ],
  );

  print(result.totalGross);
  print(result.totalSaved);
}

Output

BillResult includes the final aggregated totals:

Field	Description
subtotalNet	Net total after item discounts, before bill discounts
subtotalTax	Tax total after item discounts, before bill discounts
subtotalGross	Gross total after item discounts, before bill discounts
billDiscountAmount	Total saved from bill-level discounts
totalNet	Final net amount
totalTax	Final tax amount
totalGross	Final amount the customer pays
totalSaved	Total savings from item and bill discounts
lineResults	Per-item breakdown
billSteps	Audit trail for bill-level discounts

Each LineItemResult includes:

Field	Description
basePrice	unitPrice * qty
net	Final net amount for the line
tax	Tax amount for the line
gross	Final line total
discountAmount	Saved amount on the line
steps	Full audit trail

Audit trail

Each line result includes a steps list with:

• Calculation labels
• Exact formulas
• Final values

Example:

100.00 * 15% = 15.00

This is useful for:

• Receipts
• Debugging
• Financial transparency

When to use it

Use this package when you need deterministic pricing logic for:

• POS systems
• E-commerce checkouts
• ERP systems
• Invoice generators
• Accounting tools
• Quotation builders

Because the engine is stateless and side-effect free, it is safe to call from UI code, services, isolates, and tests.

Example app

See example/main.dart for a runnable Dart example and example/lib/main.dart for the Flutter demo app.