How to Add, Subtract, and Calculate Work Hours

Time arithmetic uses 60 minutes per hour and 60 seconds per minute. You can work in h:mm (or h:mm:ss), or convert everything to total minutes, then convert back.

Add two durations

1. Add minutes (and seconds if present).
2. If minutes ≥ 60, carry full hours (90 min → 1 h 30 min).
3. Add the hours, including any carry.

Example: 3:45 + 2:50 → minutes 45 + 50 = 95 → 1 hour + 35 minutes; hours 3 + 2 + 1 = 6:35.

Subtract durations

If minutes go negative, borrow 1 hour (60 minutes). Example: 5:10 − 2:40 → borrow → 4 hours and 70 minutes − 40 = 30 minutes → 2:30.

Decimal hours vs clock time

Payroll sometimes shows 7.5 hours (decimal) instead of 7:30. Convert: decimal hours × 60 = minutes past the hour (0.25 × 60 = 15 → :15). Reverse: minutes ÷ 60 (45 ÷ 60 = 0.75 → 0.75 hours). Mixing 7.5 with 7:50 is a common timesheet error.

Need a quick percent of a shift completed? The free percentage calculator pairs well once you have total and elapsed minutes.

A timecard (timesheet) is a list of clock-in and clock-out pairs. For each day or segment, elapsed work time = out − in, then you sum segments for the pay period.

Single day

In 9:00, out 12:00, in 12:30, out 17:00 → morning 3:00 + afternoon 4:30 = 7:30 (assuming 30 minutes unpaid lunch).

Multi-day total

Add each day’s net hours. Example week: 8:00 + 7:45 + 8:15 + 8:00 + 6:30 = minutes first if needed → 38:30 for the week (38.5 decimal hours).

Practical tips

• Record AM/PM or use 24-hour time so 7:00 is never ambiguous
• Log unpaid breaks explicitly if your policy requires them
• Keep the same rounding rule every day (some employers round to 6 or 15 minutes)
• Sum to minutes, then convert to h:mm for the pay period total

When you have many rows, enter them in the free time calculator timecard mode instead of chaining phone calculator mistakes.

Overnight shifts cross midnight. Clock math that only looks at the clock face without the date will undercount or go negative.

Rule that always works

Treat end time as next calendar day when out is “earlier” on the clock than in and you know the shift spans midnight.

Example: In 22:00 (10:00 PM), out 06:00 (6:00 AM next day). Elapsed = 8 hours. Mentally: from 22:00 to 24:00 = 2:00, plus 00:00 to 06:00 = 6:00 → 8:00.

Split across two dates on a timesheet

Some systems want Day 1: 22:00–24:00 (2 h) and Day 2: 00:00–06:00 (6 h). Your total pay should still reflect 8 hours unless policy splits premiums by calendar day (night differential rules vary).

Long shifts

A 14-hour turn that starts at 18:00 and ends at 08:00 next day is 14 hours, not “8 − 18.” If your tool supports full date-times, prefer those fields so the date change is explicit — related date span tools and the free date calculator help when the question is “how many midnights passed?” rather than pure clock duration.

In the U.S., many non-exempt employees earn overtime under the FLSA for hours over 40 in a workweek at not less than 1.5× the regular rate. Some states or contracts also use daily OT (for example, over 8 hours in a day). Always follow your jurisdiction and agreement.

Simple weekly split

Worked 46:00 in one workweek at $20/hour regular rate:

• Straight time: 40 × $20 = $800
• OT hours: 6 × $30 = $180
• Gross wages from hours: $980 (before taxes)

Finding OT hours from a total

If total hours ≤ 40, OT = 0. If total = 43:30, OT = 3:30 (3.5 hours). Convert minutes carefully: 43:30 − 40:00 = 3:30, not 3.30 decimal.

Regular rate caveats

Bonuses, shift differentials, and multiple rates can change the “regular rate” used for the 1.5× multiplier. A basic hours tool totals time; payroll still applies rate rules. For converting OT pay into annual or hourly equivalents, the free salary calculator and salary conversion guide help on the money side once hours are solid.

12-hour time uses AM/PM (2:30 PM). 24-hour time runs 00:00–23:59 (14:30). Both represent the same instants; errors come from missing AM/PM or misreading 12:00.

Conversion shortcuts

• 1:00 PM–11:59 PM → add 12 for 24-hour (1:00 PM → 13:00; 11:00 PM → 23:00)
• 12:00 AM (midnight) → 00:00; 12:00 PM (noon) → 12:00
• 00:00–11:59 in 24-hour → same clock with AM (09:15 → 9:15 AM)

Which to pick

Use 24-hour for logs, aviation-style ops, hospitals, and any multi-person sheet where AM/PM typos are costly. Use 12-hour when your workplace forms and culture expect it — but write AM/PM every time.

12:00 traps: “12 AM” is midnight, “12 PM” is noon. Saying only “12:00” without a marker is ambiguous on mixed teams.

Sleep schedules and wake times often mix both formats; the free sleep calculator and sleep cycle guide use clock times you can align with the same 12h/24h discipline.

Pick the mode that matches your question: pure arithmetic, multi-entry timecard, or duration between two clock times (optionally with dates).

1. Open the free time calculator.
2. Choose add/subtract for duration math, or timecard mode for in/out rows.
3. Enter times in a consistent format (stick to 24-hour if AM/PM confuses you).
4. Mark overnight segments so end times after midnight are handled correctly.
5. Subtract unpaid breaks if your policy requires them and the tool does not auto-deduct.
6. Read total hours in h:mm and decimal hours if both are shown.
7. Compare weekly totals to 40:00 (or your OT threshold) to see overtime hours.
8. Save or screenshot period totals before you submit a manual timesheet.

Unit converter mode

Some calculators also convert among hours, minutes, seconds, and days. Handy when a ticket says “9,000 seconds” and you want hours, or when you need total minutes for a formula elsewhere.

Most errors are format and midnight issues, not hard math.

• Treating minutes as decimals — 1:45 is not 1.45 hours (1:45 = 1.75 hours)
• Forgetting AM/PM on 12-hour sheets
• Ignoring overnight so 10 PM–6 AM becomes negative or 4 hours
• Double-counting or skipping unpaid lunch
• Mixing biweekly pay periods with calendar weeks when checking OT
• Rounding each day differently than payroll software

Age and date questions (“how many full days since hire?”) use calendar rules, not clock addition — see the free age calculator and exact age guide when the unit is calendar age rather than shift length.

This guide is educational, not legal or payroll advice. Confirm overtime and rounding with your employer’s written policy.