Six Unit Conversion Mistakes That Cost Millions
A spacecraft lost, an airliner out of fuel at 41,000 feet, and a hospital dosing error — all traced to a unit that was never written down.
25 May 2026 · 7 min read · Reference
Unit errors are not a schoolroom problem. They are a recurring failure mode in engineering, aviation and medicine.
1. The Mars Climate Orbiter (1999)
NASA's $327 million orbiter arrived at Mars and was never heard from again. Lockheed Martin's ground software produced thruster impulse in pound-force seconds; NASA's navigation software expected newton-seconds. Nobody converted.
The 4.45× discrepancy accumulated over nine months of small trajectory corrections. The spacecraft entered the atmosphere about 170 km lower than planned and broke up. The board's finding was blunt: the numbers were passed between teams without units attached.
2. The Gimli Glider (1983)
Air Canada Flight 143 ran out of fuel at 41,000 feet over Manitoba. Canada was switching to metric, and the Boeing 767 was the airline's first metric aircraft. Ground crew calculated the fuel load using 1.77 lb/L — the old pounds figure — instead of 0.803 kg/L.
The aircraft departed with roughly half the fuel it needed. Both engines flamed out. The captain, a glider pilot, dead-sticked the airliner onto a decommissioned airbase runway that had been converted into a drag strip. Nobody died — a result of exceptional airmanship, not of the system working.
3. The Vasa (1628)
The Swedish warship Vasa capsized twenty minutes into her maiden voyage. Archaeologists examining the wreck found four rulers used by the shipwrights: two in Swedish feet (12 inches) and two in Amsterdam feet (11 inches). The ship was measurably thicker on one side than the other, contributing to the instability that sank her.
4. Radiotherapy overdoses (Panama, 2000)
A treatment planning system's shielding data was entered in a different arrangement than the software expected, causing miscalculated exposure times. At least eight patients died and dozens more were injured. The root cause was a mismatch between how a human entered data and how the machine interpreted it.
5. Disneyland Tokyo's Space Mountain (2003)
An axle broke on a roller coaster mid-ride. The cause: a 2002 parts order specified the axle in millimetres, but an older imperial-based drawing was used to manufacture it. The axle was 0.86 mm undersized — enough for vibration to fracture it. No one was injured; the ride was closed for months.
6. Everyday medication errors
The commonest clinical version: mg confused with mL, or a paediatric dose in mg/kg applied to the total body weight in pounds. A child weighing 22 lb (10 kg) given a dose calculated on 22 kg receives 2.2× the intended amount. Hospitals now mandate weight in kilograms on paediatric charts for exactly this reason.
The common thread
None of these were arithmetic failures. In every case someone could do the conversion — the number simply travelled without its unit attached, and the receiving system assumed.
The defences are unglamorous and effective:
- Never write a bare number. Units go in the variable name, the column header, the API field, the label.
- Convert at the boundary. One place, tested, not scattered through the codebase.
- Sanity-check magnitude. If a fuel figure is half what it usually is, stop.
- Make the machine check. Type systems and unit libraries catch what review misses.
Related reading
This article is for general information. Please verify figures against the relevant standard before using them in critical work. About accuracy