Hobbs Time and Decimal Flight Hours Explained

What is Hobbs time?

Hobbs time is an elapsed-time reading commonly used in aviation.

A Hobbs meter normally displays time in hours and tenths. For example, 1285.8 means 1,285 hours plus 0.8 of an hour.

0.8 × 60 = 48 minutes

So the equivalent is 1,285h48m.

For most day-to-day calculations, though, you normally do not need to convert the aircraft’s entire Hobbs reading into hours and minutes. You simply subtract the start reading from the end reading.

Hobbs time conversion chart

The decimal is a fraction of an hour. It is not a number of minutes written after a decimal point.

Why does 0.1 flight hour equal 6 minutes?

One hour contains 60 minutes. One tenth of an hour is:

60 ÷ 10 = 6 minutes

So 0.1 hour = 6 minutes. Other examples:

• 0.3 × 60 = 18 minutes
• 0.5 × 60 = 30 minutes
• 0.8 × 60 = 48 minutes

This is why aviation decimal time can look unfamiliar if you are used to normal clock notation.

1.3 Hobbs does not mean 1 hour 30 minutes

This is one of the easiest mistakes to make. The number after the decimal point represents a fraction of an hour.

So 1.3 is not 1h30m, but 1.5 = 1h30m.

How to read a Hobbs meter

Suppose the meter reads 4523.7. The whole-number part is 4,523 hours and the decimal part is 0.7 hour.

0.7 × 60 = 42 minutes

So 4523.7 = 4,523h42m. It does not mean 4,523 hours and 70 minutes.

How to calculate Hobbs time from start and end readings

Use: End Hobbs − Start Hobbs = Hobbs time used

Example 1

Start 4523.7, end 4525.2:

4525.2 − 4523.7 = 1.5

So the Hobbs time used is 1.5 hours, which equals 1h30m.

Example 2

Start 1268.4, end 1269.7:

1269.7 − 1268.4 = 1.3

So 1.3 Hobbs hours = 1h18m.

Student pilot Hobbs example

Suppose a training aircraft starts with 1284.6 and finishes with 1285.8:

1285.8 − 1284.6 = 1.2

So the Hobbs time used is 1.2 hours. Convert 0.2 hour into minutes:

0.2 × 60 = 12 minutes

Result: 1h12m

Convert Hobbs time to hours and minutes

To convert a decimal flight time into hours and minutes:

1. keep the whole number as the hours
2. multiply the decimal part by 60

For example, 2.7: whole hours 2h, decimal part 0.7.

0.7 × 60 = 42, so 2.7 hours = 2h42m

Another example, 3.4:

0.4 × 60 = 24, so 3.4 = 3h24m

Convert hours and minutes to decimal flight hours

To convert the other way: minutes ÷ 60 = decimal part

• 1h30m: 30 ÷ 60 = 0.5, so 1h30m = 1.5 hours
• 2h18m: 18 ÷ 60 = 0.3, so 2h18m = 2.3 hours
• 48 minutes: 48 ÷ 60 = 0.8, so 48m = 0.8 hours

What about flight times that do not land exactly on a tenth?

Not every number of minutes converts neatly into tenths.

• 45 minutes: 45 ÷ 60 = 0.75 hours
• 50 minutes: 50 ÷ 60 = 0.8333… hours

Neither value is an exact tenth.

If a logbook, operator or software system requires time to be recorded in tenths, follow the relevant rounding or recording convention rather than inventing your own.

The exact decimal value and a rounded tenth are not necessarily the same thing.

Hobbs time and aircraft rental charges

Hobbs time is often used as the basis for aircraft rental or flight-school charging.

For example, with 1.3 Hobbs used and a rental rate of £180 per hour:

1.3 × £180 = £234

So the aircraft-time charge would be £234.

The actual charging method depends on the operator or rental agreement.

Hobbs time vs tach time

Hobbs time and tach time are not the same thing.

Hobbs time

Hobbs is elapsed time according to whatever activates that particular meter installation.

Tach time

Tach time is tied to engine RPM, so it does not necessarily accumulate at the same rate as elapsed clock time.

At lower RPM, tach time may build more slowly than Hobbs time. The two readings should not be assumed to match.

What starts the Hobbs meter?

There is no single universal answer.

Depending on the aircraft and installation, a Hobbs meter may be triggered by:

• oil pressure
• the electrical or master system
• an airspeed switch
• another aircraft-specific signal

This means it is not always correct to say that Hobbs time simply runs from engine start to engine stop.

If the exact timing matters, check how the meter is configured in that aircraft.

Is Hobbs time the same as loggable flight time?

Not automatically.

A Hobbs meter is a meter reading. Loggable flight time depends on the applicable aviation rules, the type of operation and the way the flight is recorded.

So Hobbs time is not automatically loggable flight time.

The two may be similar in some operations, but they should not be treated as interchangeable without checking the relevant rules or procedures.

Decimal flight time vs hours and minutes

A flight duration may be written as 1.5 or 1h30m. Both can represent the same amount of time, but they are different formats.

For example, 1.3 = 1h18m, while 1h30m = 1.5.

Always check whether a logbook, school, operator or software system expects decimal hours or hours and minutes before entering the value.

UK, EASA and US-style flight-time notation

Decimal flight time is common in aviation, particularly where Hobbs readings are used. Hours-and-minutes notation is also used.

The correct format depends on the relevant:

• pilot logbook
• training organisation
• operator
• software
• licensing or regulatory requirements

The important thing is to use the required format consistently.

Common Hobbs-time mistakes

Reading 1.3 as 1h30m

It actually means 1h18m.

Thinking 0.1 means 10 minutes

It means 6 minutes.

Reading 4523.7 as 4,523h70m

The correct interpretation is 4,523h42m.

Confusing Hobbs with tach time

They measure operating time differently.

Assuming every Hobbs meter starts at engine start

The trigger depends on the installation.

Assuming Hobbs automatically equals loggable flight time

A Hobbs reading is not automatically the same as the time that should be recorded for licensing purposes.

Rounding awkward minute values without checking the required convention

For example, 45 minutes is exactly 0.75 hours. If the required format is tenths, the appropriate recording rule needs to be followed.

Subtracting the readings incorrectly

Always use End Hobbs − Start Hobbs.