How the calculator works

  1. Surface: inner walls, roof and floor, × 1.25 to approximate the mean surface through the insulation.
  2. Wall heat = K × surface × (outside − box temperature).
  3. Margin: wall heat × 1.75, as used in ATP type-testing.
  4. Door heat ≈ 0.29 W × drops × volume0.8 × ΔT, reduced by 40% with strip curtains.
  5. Rating at 0 °C: units are catalogued at 0 °C box temperature; capacity falls about 3.3% per degree below 0 °C, so we convert the requirement to an equivalent 0 °C rating.

Full explanation in our truck refrigeration sizing guide.