Adding hours and minutes

Carry at sixty, not at one hundred

Add the minutes first; every full 60 carries into the hours column. 2 h 45 min plus 1 h 30 min is 4 h 15 min because 75 minutes carries one hour.

Decimal hours divide minutes by 60

Payroll and billing systems multiply hours by a rate, so minutes must become sixtieths. The common shortcuts are worth memorizing.

When in doubt, work entirely in minutes

Converting every entry to minutes, adding once, and converting back removes carrying mistakes. That is exactly what the calculator does internally.

Check your total: a result whose minutes are 60 or more was carried incorrectly somewhere.

Subtracting time: borrow sixty

Subtraction has the mirror-image mistake. When the minutes would go negative, borrow one hour as sixty minutes, not one hundred.

5 h 10 min minus 2 h 45 min: ten minutes cannot lose 45, so borrow an hour. 5 h 10 min becomes 4 h 70 min, and 4 h 70 min − 2 h 45 min = 2 h 25 min.

Treating the times as decimals gives 5.10 − 2.45 = 2.65, which reads as 2 hours 65 minutes — an impossible value, and a sure sign the arithmetic was done in base ten.

From a total to an invoice

Billing multiplies decimal hours by a rate, so convert once at the end and round only the money.

Entries totalling 23 h 40 min are 23 + 40 ÷ 60 = 23.667 hours. At $45 an hour that is $1,065.00. Rounding the hours to 23.67 first gives $1,065.15; rounding to 23.7 gives $1,066.50.

The difference is small on one invoice and noticeable across a year of them. If a client or employer uses a rounding convention — tenths of an hour are common in billing — apply it deliberately and state it on the invoice rather than letting it happen by accident.