php 47 lines · 8 steps

Precise money math with PHP's BCMath

An invoice calculator that avoids floating-point errors by doing all arithmetic on decimal strings.

Explained by highlit

Intermediate bcmath arbitrary precision money rounding immutability

1final class InvoiceCalculator
2{
  private const SCALE = 4;
4 
  public function __construct(private readonly string $taxRate = '0.0825')
  {
  }
8 
  public function total(array $lineItems): array
  {
      $subtotal = '0.00';
12 
      foreach ($lineItems as $item) {
          $lineTotal = bcmul(
              (string) $item['unit_price'],
              (string) $item['quantity'],
              self::SCALE
          );
19 
          if (!empty($item['discount'])) {
              $factor = bcsub('1', (string) $item['discount'], self::SCALE);
              $lineTotal = bcmul($lineTotal, $factor, self::SCALE);
          }
24 
          $subtotal = bcadd($subtotal, $lineTotal, self::SCALE);
      }
27 
      $tax = bcmul($subtotal, $this->taxRate, self::SCALE);
      $grandTotal = bcadd($subtotal, $tax, self::SCALE);
30 
      return [
          'subtotal' => $this->round($subtotal),
          'tax'      => $this->round($tax),
          'total'    => $this->round($grandTotal),
      ];
  }
37 
  private function round(string $amount): string
  {
      $shifted = bcadd($amount, '0.005', self::SCALE);
      return bcmul(
          bcdiv($shifted, '0.01', 0),
          '0.01',
          2
      );
  }
47}

01 / 01

STEP 01

Walkthrough

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Three takeaways

1Represent monetary values as strings and use BCMath functions so binary floating-point rounding errors never enter your totals.
2Carrying extra internal precision (scale 4) and rounding only at the boundary keeps intermediate results accurate.
3Readonly constructor promotion gives you a configurable, immutable dependency like the tax rate with almost no boilerplate.

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