PHP Floating Point Issues and Safe Progress Calculation

Problem

We got a bug report: a student completed all lessons but progress showed 99%. The actual data was 73/73, but the result was 99%. After checking, we found floating point was the cause.

What is Floating Point Problem?

Computers use binary to store decimal numbers, which can cause small errors.

var_dump(0.1 + 0.2); // float(0.30000000000000004)

Why the Original Code Has Errors

  1. Division (/) - 73 / 73 = 1.0 should be exact, but internally it might become 0.9999999999999999

  2. Multiplication (* 100) - Multiplying an already wrong number by 100 makes the error bigger (0.9999999999999999 * 100 = 99.99999999999999)

  3. Operation Order - (divide → multiply) makes errors worse. A small division error becomes 100x bigger after multiplication

  4. round() Function Limit - round(99.99999999999999) → 99. Rounding an already wrong number gives wrong results

Original Code

$progressRate = round(($processedItemQty / $totalItemQty) * 100);
  • $processedItemQty = 73, $totalItemQty = 73
  • Should be 100% but got 99%

Fixed Safe Code

// Multiply first, divide later to reduce errors
$progressRate = ($totalItemQty > 0) ? round(($processedItemQty * 100) / $totalItemQty) : 0;

// Keep result between 0-100%
return max(0, min(100, $progressRate));

How is it Safe?

1. Multiply First, Divide Later

// Original: (73 / 73) * 100
// Fixed: (73 * 100) / 73
  • Original: 73 ÷ 73 = 0.9999999999999999 → * 100 = 99.99999999999999 → round() = 99
  • Fixed: 73 × 100 = 7300 (exact integer) → ÷ 73 = 100.0 → round() = 100

2. Keep 0-100% Range

  • max(0, …): Prevents negative numbers (e.g., -1% → 0%)
  • min(100, …): Prevents over 100% (e.g., 101% → 100%)

3. Prevent Division by Zero

  • ($totalItemQty > 0) ? ... : 0 → Returns 0% instead of error when dividing by zero

Summary

  1. Multiply first, divide later → Reduces floating point errors
  2. Use max/min for range limit → Guarantees 0-100%

This method fixes the 99% problem and ensures 73/73 correctly shows 100%.