How do you keep decimal hours separate from hours and minutes?

Minutes, decimal hours and the clock

Crabwise's fuel and time formulas take time in minutes and divide by 60 internally: fuel burned F = FF × t ÷ 60, endurance t = F ÷ FF × 60. That keeps the hour-and-minute ambiguity out of the input. The risk returns when a duration comes from a logbook or a clock written as 2:45 and is typed into a per-hour formula as 2.45.

In decimal hours, 0.45 is 27 minutes, not 45. The mismatch grows with the fraction: 1:30 typed as 1.30 loses 12 minutes, and 2:45 typed as 2.45 loses 18.

Worked example

Ninety elapsed minutes is 1.5 hours. An 8-unit-per-hour flow therefore gives 12 units. Treating 1:30 as the decimal 1.30 would instead imply 10.4 units.

Fuel used in 90 minutes (chosen units)

12

Convert before you enter

1. Write the duration as whole hours and minutes, for example 2 h 45 min.
2. Convert to minutes: 2 × 60 + 45 = 165.
3. Enter 165 in the minutes field; if another tool needs hours, divide by 60 to get 2.75.
4. Check the scale: the result should be a bit under three times the hourly flow.

The same leg two ways

With a flow of 9.5 units per hour and 165 minutes, fuel burned is 9.5 × 165 ÷ 60 = 26.125 units. If 2:45 were read as 2.45 hours, the result would be 9.5 × 2.45 = 23.275 units, which corresponds to only 147 minutes of burn.

Elapsed time (min)

165

Fuel burned from minutes (units)

26.125

Result from the mistaken 2.45 h (units)

23.275

Common mistakes

• Subtracting two clock times across midnight or a time-zone change and treating the result as a duration.
• Copying a decimal-hour entry from a logbook into the minutes field without multiplying by 60.
• Rounding 165 minutes to 2.7 hours before multiplying, which drops 3 minutes of burn.

What the conversion does not settle

Turning minutes into hours fixes the unit, not the duration itself. Whether 165 minutes is the right interval is a question for the record you took it from. The NIST conversion table below defines the hour and minute relation used here.

Primary sources and related work

• NIST SI conversion factors
• FAA Pilot’s Handbook: navigation and basic flight calculations