Time Card Calculator: How to Add Up Hours and Minutes

A time card calculator exists to solve one deceptively tricky problem: adding up hours and minutes correctly. The reason it's tricky is that time isn't base-10. You can't just add 7:45 and 8:30 the way you'd add 7.45 and 8.30 — do that and every total is wrong. This guide explains how the calculation actually works, so whether you use a tool or do it by hand, your numbers hold up.

Why you can't just add the numbers

Minutes run 0 to 59, then roll over to a new hour — they're base-60, not base-10. So "7 hours 45 minutes plus 8 hours 30 minutes" isn't 15.75 by simple decimal addition of 7.45 + 8.30. You have to add the minutes as minutes: 45 + 30 = 75 minutes = 1 hour 15 minutes, giving 16 hours 15 minutes. The clean way to avoid this trap entirely is to convert to decimals first.

Step 1: Convert each entry to decimal hours

Divide minutes by 60:

• 45 minutes ÷ 60 = 0.75
• 30 minutes ÷ 60 = 0.50

So 7:45 becomes 7.75 and 8:30 becomes 8.50. Now they add like normal numbers: 7.75 + 8.50 = 16.25 hours. Common conversions worth memorizing: 15 min = 0.25, 20 min = 0.33, 40 min = 0.67, 50 min = 0.83.

Step 2: Subtract unpaid breaks

If a shift included a 30-minute unpaid lunch, subtract 0.5 from that day's total. Only subtract genuinely unpaid breaks; leave paid rest breaks in.

Step 3: Total each day, then the week

Add the decimal totals per day, then sum the days for a weekly figure. Keeping everything in decimals the whole way avoids re-introducing the base-60 problem at the end.

Step 4: Split regular and overtime

If the weekly total exceeds the threshold (40 hours in a standard US workweek), the excess is overtime. Total each week separately, since overtime is calculated per workweek, and confirm the rules that apply to you.

A quick worked example

Monday to Friday:

• Mon 8:15 → 8.25
• Tue 7:50 → 7.83
• Wed 8:30 → 8.50
• Thu 9:10 → 9.17
• Fri 6:45 → 6.75

Total = 40.50 hours → 40 regular and 0.5 overtime. Try adding those clock times without converting and you'd get a different, wrong answer.

Common time card mistakes

• Adding clock time as decimals — 7:45 is 7.75, not 7.45.
• Forgetting to subtract unpaid breaks.
• Rounding inconsistently — pick one rule and apply it every time.
• Totaling a pay period instead of each week, hiding overtime.
• Transcription slips when copying times between sheets.

Let it total automatically

Every step here is arithmetic a computer never gets wrong. Timesheet Maker records clock-in and clock-out times, converts to decimals, subtracts breaks, and totals each week — regular and overtime split out — so you never add hours and minutes by hand again.

Frequently asked questions

How do I add up hours and minutes? Convert each time to decimal hours (minutes ÷ 60), then add the decimals. 7:45 + 8:30 becomes 7.75 + 8.50 = 16.25 hours.

What is 45 minutes as a decimal? 0.75 hours. Thirty minutes is 0.5, and 15 minutes is 0.25.

Do I subtract lunch from a time card? Yes, if the break is unpaid. Subtract it in decimal hours from the day's total. Paid breaks stay in.

Why does my time card total look wrong? Usually because clock times were added as decimals (7:45 treated as 7.45). Convert to decimal hours first, then add.

Get the total right every time

Convert to decimals, subtract unpaid breaks, and total each week. When you'd rather skip the math entirely, try Timesheet Maker free.