Time Tracking Math: From Timesheets to Billable Hours

The Timesheet Mistake That Costs Lawyers $40,000 a Year

A law firm with 10 associates, each billing at $300 per hour, loses roughly $40,000 per year if every associate consistently under-reports their time by one 6-minute billing unit per day. That is one-tenth of an hour, six minutes, one phone call, one email review that doesn't make it onto the timesheet. Legal billing has tracked time in 6-minute increments (0.1 hours) since the 1960s specifically because round-number estimates ("about half an hour") systematically undercount actual time spent.

The difference between HH:MM and decimal hours trips up contractors, freelancers, and payroll processors every week. A shift from 9:15 AM to 5:45 PM is 8 hours and 30 minutes, which is 8.5 decimal hours, not 8.3 hours. Writing "8:30" on a timesheet and multiplying by the hourly rate produces the correct answer only by accident, because 8.30 × $50 = $415 while 8.5 × $50 = $425. The 12-minute difference in interpretation costs a $50/hour worker $10 per shift.

This guide covers the conversion between HH:MM and decimal hours, overtime calculation under US federal law (FLSA), the 6-minute and 15-minute billing increment systems, payroll math for salaried vs hourly workers, and the rounding rules that govern legal timekeeping.

HH:MM vs Decimal Hours: The Core Conversion

HH:MM format (hours and minutes) expresses time as most people read clocks: 1 hour 30 minutes appears as 1:30. Payroll calculations require decimal hours because multiplying "1:30" by a wage rate is ambiguous (does the :30 mean 30 minutes or 30 hundredths of an hour?). Decimal hours express time as a single number: 1.5 means 1 hour and 30 minutes.

Conversion formula: Decimal hours = Hours + (Minutes / 60)

Common conversions:

• 1:15 = 1 + 15/60 = 1 + 0.25 = 1.25 hours
• 1:30 = 1 + 30/60 = 1 + 0.50 = 1.50 hours
• 1:45 = 1 + 45/60 = 1 + 0.75 = 1.75 hours
• 7:20 = 7 + 20/60 = 7 + 0.333 = 7.33 hours
• 8:45 = 8 + 45/60 = 8 + 0.75 = 8.75 hours

To go the other direction: take the decimal portion, multiply by 60 to get minutes. 0.333 × 60 = 20 minutes.

Billing increments define the minimum unit of time a professional charges. The legal profession uses 0.1-hour (6-minute) increments. Accounting and consulting firms often use 0.25-hour (15-minute) increments. A task that takes 8 minutes rounds up to 0.1 hours (6 minutes) in the 6-minute system and to 0.25 hours (15 minutes) in the 15-minute system. The 15-minute system generates larger bills for short tasks but requires less precision in tracking.

Overtime, Payroll, and Weekly Hour Math

US federal overtime law (FLSA) requires non-exempt employees to receive 1.5× their regular hourly rate for all hours worked beyond 40 in a workweek. The workweek is any 7 consecutive days chosen by the employer; it does not have to align with the calendar week.

Weekly pay calculation with overtime:

Step 1: Count total hours worked in the week. Step 2: Identify hours at regular rate (first 40). Step 3: Identify overtime hours (above 40). Step 4: Calculate pay.

Formula: Pay = (Regular hours × Rate) + (Overtime hours × Rate × 1.5)

Example: an employee works 47 hours at $20/hour.

• Regular pay: 40 × $20 = $800
• Overtime pay: 7 × ($20 × 1.5) = 7 × $30 = $210
• Total: $800 + $210 = $1,010

The same 47 hours at $20/hour without overtime (incorrectly): 47 × $20 = $940. The employee loses $70 if the employer pays straight time only.

Salary vs hourly: Salaried exempt employees do not receive overtime under FLSA. As of 2024, the exempt salary threshold is $684 per week ($35,568 per year). Employees below that threshold qualify for overtime regardless of how they are classified. Salaried non-exempt employees do receive overtime; some employers classify workers as "salaried" incorrectly to avoid the obligation.

Common Misconceptions

• Myth: "Salaried" means no overtime. Under FLSA, salary status alone does not create exemption. The employee must also pass a duties test (executive, administrative, professional, or computer employee duties) and earn above the salary threshold. A "salaried" warehouse supervisor earning $600/week who works 50 hours qualifies for overtime.