🧯 Failures
Ten unit conversion mistakes that cost real money
Unit errors almost never come from bad arithmetic. They come from a value whose unit was assumed, a document that did not say, or a conversion done twice. These ten cases are the ones worth knowing, because every one of them is still being repeated somewhere today.
The ten mistakes at a glance
Every case below has the same shape: a value whose unit was assumed, written in one system and consumed in another. The table is the short version; the sections that follow explain what happened and what it cost.
| Case | Unit trap | Consequence |
|---|---|---|
| Mars Climate Orbiter (1999) | Pound-force seconds vs newton-seconds | Orbiter destroyed in the Martian atmosphere |
| Gimli Glider (1983) | Pounds vs kilograms of fuel | Fuel exhaustion; emergency landing, no fatalities |
| Troy vs avoirdupois ounce | 31.1035 g vs 28.3495 g | About 9.7% value error on precious metals |
| US vs imperial gallon | 3.785 L vs 4.546 L | About 20% error in fuel and volume comparisons |
| Mechanical vs metric horsepower | 745.7 W vs 735.5 W | About 1.4% error in power figures |
| Acre, hectare and bigha | 4,047 m² vs 10,000 m² vs local bigha | Land values wrong by large factors |
| Kilowatt vs kilowatt-hour | Power (a rate) vs energy (an amount) | Wrong charging times and electricity bills |
| Milligram vs microgram | 1,000× difference | Thousandfold medication dosing errors |
| Celsius vs Fahrenheit, fan ovens | Offset and 1.8× step; fan about 20 °C hotter | Failed bakes and unsafe temperatures |
| The 2.2 lb/kg shortcut | Rounded conversion factor | Small errors that compound in doses and manifests |
1. The lost Mars orbiter
In September 1999 NASA lost the Mars Climate Orbiter, a spacecraft that had travelled hundreds of millions of kilometres, because two teams were using different units for the same quantity. The spacecraft's thruster data was supplied in pound-force seconds; the navigation software that used it expected newton-seconds. Each small correction was misread by a factor of about 4.45, the trajectory drifted, and the orbiter entered the Martian atmosphere too low and was destroyed.
The lesson: the unit belongs in the interface between systems, not in a document that somebody may or may not read. It is a matter of record that both teams believed they were using the agreed units.
2. The airliner that glided into a drag strip
In 1983 an Air Canada Boeing 767 ran out of fuel at 41,000 feet over Manitoba and glided to a landing at a former air force base that had become a drag strip. The chain of events began with fuel figures: the aircraft was the first in the fleet to use metric measures, and the refuelling calculation produced a fuel load in pounds where kilograms were intended. The aircraft took on roughly half the fuel it needed.
The lesson: when a system changes units, the old habits outlive the old labels. Anything that was once expressed in one unit and is now expressed in another needs a physical check, not just a new form.
3. Troy ounces and ordinary ounces
Gold, silver and platinum are priced by the troy ounce, which is 31.1035 grams. The ounce on a kitchen scale is the avoirdupois ounce, 28.3495 grams. The troy ounce is about 9.7 percent heavier, so confusing the two moves value by nearly a tenth of the price — trivial on a necklace, enormous on a shipment.
The lesson: a shared name is not a shared unit. The weight converter lists both ounces explicitly; use the troy one for precious metals and the avoirdupois one for everything else.
4. The gallon that is not the gallon
A US gallon is 3.785 litres. An imperial gallon is 4.546 litres, about 20 percent more. Fuel consumption expressed in miles per gallon therefore looks around 20 percent better in Britain than in America for the same actual efficiency, which makes transatlantic comparisons meaningless unless the gallon is named.
The lesson: for fuel, use litres per 100 kilometres or miles per gallon with the country attached. The fuel economy converter handles US mpg, imperial mpg and L/100 km together.
5. Two kinds of horsepower
Mechanical horsepower is 745.7 watts. Metric horsepower — PS in German, cv in French, pk in Dutch — is 735.5 watts. The difference is about 1.4 percent, which is enough to make a car's specification sheet disagree with a magazine's dyno test and enough to matter in a homologation document.
The lesson: quote the standard, not just the number. "300 hp (PS)" is unambiguous; "300 hp" is not.
6. Acres, hectares and bigha
A hectare is 10,000 square metres; an acre is 4,047, so one hectare is about 2.47 acres. Both are in daily use, sometimes in the same country and the same conversation. Local units are worse: a bigha is a different area in Uttar Pradesh, Bihar, West Bengal and Gujarat, and a "guntha" or "cent" varies too.
The lesson: for land, write the unit and the locality into the contract, and convert to square metres for the record. The land converter covers the common regional units with their ranges.
7. Kilowatts and kilowatt-hours
A charger rated at 7 kW delivers 7 kWh in an hour, 21 kWh in three hours, and 3.5 kWh in half an hour. Confusing power with energy leads to confident wrong claims about charging times and electricity bills. The unit tells you which quantity you have: kW is a rate, kWh is an amount.
The lesson: if the number describes how fast something happens, it is a rate. If it describes what has accumulated, it is a quantity. The energy and power converters keep the two apart.
8. Milligrams and micrograms
A milligram is a thousand micrograms, and a prescription that says one when it means the other is a thousandfold error. This class of mistake is well documented in medication safety reporting, and the usual causes are handwritten abbreviations, the letter "u" used for micro, and decimal points that are easy to miss.
The lesson: write micrograms as "micrograms" or "µg", never "ug"; always put a leading zero before a decimal point; and check the unit and the basis — per kilogram or per dose — before calculating anything.
9. Celsius, Fahrenheit and fan ovens
Setting an oven to 180 °F instead of 180 °C gives you 82 °C, which will not bake bread. Switching from a conventional to a fan oven at the same dial setting gives you roughly 20 °C too much. And a recipe's "350" may be Fahrenheit, Celsius, gas mark 4 or a fan setting, depending on where it was written.
The lesson: the temperature converter handles the arithmetic, but the oven still needs an oven thermometer. Most domestic ovens are 10 to 15 degrees away from the number on the dial.
10. Pounds, kilograms and the 2.2 factor
One kilogram is 2.2046 pounds. Rounding that to 2.2 is fine for a suitcase and not fine for a veterinary dose, a parachute limit or a shipping manifest, where the error compounds across every item on the list. The reverse direction is the one people get wrong under pressure: dividing when they should multiply.
The lesson: convert once, in one direction, and sanity-check against something you know. A person who weighs 70 kg should come out around 154 lb; if the answer is 32 lb, the operation was inverted.
The six checks that prevent all of the above
- Write the unit next to every number. "3.5" is not a measurement. "3.5 kg" is.
- Convert once, at a defined boundary. Pick the point where one system becomes the other and never convert back and forth inside a calculation.
- Put units in column headers. In spreadsheets this is the single highest-value change you can make, because it survives copying and pasting.
- Name ambiguous units. US gallon or imperial gallon. Troy ounce or avoirdupois ounce. PS or mechanical horsepower.
- Run an order-of-magnitude check. If a car weighs two tonnes and your result says 200 kg, the arithmetic is not the problem.
- Use the exact factor and round once. 0.45359237 is not harder to type than 0.45, and it will not drift when the number is large.
Frequently asked questions
Sources
- NASA Mishap Investigation Board, Mars Climate Orbiter report, 1999.
- Transportation Safety Board of Canada, report on the 1983 Air Canada 767 fuel exhaustion incident.
- NIST Special Publication 811 and the BIPM SI Brochure — unit definitions used above.
- World Gold Council and LBMA — troy ounce conventions for precious metals.