Crabwise · Library
How can a distance-unit mix-up change a route-time result?
Sixty statute miles is 96.56 km while sixty nautical miles is 111.12 km; a 150 SM leg typed as 150 NM adds almost 10 minutes at 120 kt.
Exact definitions behind each unit
Crabwise's unit screen converts through a base unit with exact factors: 1 NM = 1852 m, 1 statute mile = 1609.344 m, 1 ft = 0.3048 m, and for pressure 1 inHg = 33.8638866667 hPa. Speeds use the same distance bases, so knots, mph and km/h convert like their miles. Only units of the same quantity can be converted; distance to volume is refused.
The time–speed–distance screen works in nautical miles, knots and minutes. A statute-mile distance has to be converted before it enters that screen, or every time and fuel figure built on it is off by about 15 percent.
Worked example
Sixty nautical miles equals 111.12 kilometres using the stated unit definitions. Sixty statute miles is a different input, even though it begins with the same number.
- Converted distance (km)
- 111.12
Convert before you calculate
- Write the source unit beside the number, for example 150 SM.
- Convert it to nautical miles on the unit screen.
- Convert the result back to check the round trip.
- Enter the nautical-mile value in the time–speed–distance screen.
The cost of a missing conversion
Sixty statute miles is 60 × 1609.344 ÷ 1000 = 96.56064 km. A 150 SM leg is 150 × 1609.344 ÷ 1852 ≈ 130.346436 NM; at 120 kt it takes about 65.173218 minutes. Typed directly as 150 NM, the same leg would show 75 minutes, almost ten minutes too long.
- 60 SM in kilometres
- 96.56064
- 150 SM in nautical miles
- 130.346436
- Time for 150 SM at 120 kt (min)
- 65.173218
- Time if typed as 150 NM (min)
- 75
Pressure works the same way: 1013 hPa converts to about 29.913873 inHg.
Speeds follow the same factors. On the unit screen 120 kt is 120 × 1852 ÷ 1000 = 222.24 km/h and about 138.093534 mph, so a speed read in mph has to be converted before it is paired with a distance in nautical miles.
Common mistakes
- Reading a road-map distance in miles as nautical miles.
- Pairing a distance in kilometres with a speed in knots.
- Assuming an unlabelled gallon is the US gallon the converter uses.
- Converting the distance to nautical miles but leaving the ground speed in mph.
Scope
The factors are exact by definition, so rounding is the only source of difference. The NIST conversion appendix below lists the same definitions.
Educational calculation only. Check current aircraft limits, weather, performance and reserve requirements with approved information.