Chargeable weight: the pricing bug that is not a bug

If you have ever built a shipping quote engine, you have met chargeable weight. It trips up developers because the weight you bill is not the weight on the scale.

The rule: chargeable weight is the greater of actual weight and volumetric weight.

volumetricWeight = (L x W x H in cm) / divisor

The divisor is usually 5000 (air/express) or 6000 (some economy lanes). A small, honest implementation:

function chargeableWeight({ length, width, height, actualKg, divisor = 5000 }) {
  const volumetricKg = (length * width * height) / divisor;
  return Math.max(actualKg, volumetricKg);
}

// 50x40x30 cm box that actually weighs 8 kg
chargeableWeight({ length: 50, width: 40, height: 30, actualKg: 8, divisor: 5000 });
// -> 12  (volumetric wins)

chargeableWeight({ length: 50, width: 40, height: 30, actualKg: 8, divisor: 6000 });
// -> 10  (same box, cheaper lane)

Two things this gets wrong in production if you are not careful:

1. Rounding. Carriers round up to the next 0.5 kg or 1 kg. Bill before rounding and you undercharge; round the wrong way and you overquote.

2. The divisor is per-lane, not global. Hardcoding 5000 silently overcharges every customer who qualifies for a 6000 economy lane. Model the divisor as a property of the lane, not a constant.

The "why am I paying for 12 kg when my box is 8 kg" tickets are almost never bugs. They are volumetric weight. Surface both numbers in the UI so the customer sees actual vs chargeable side by side, and the tickets stop.