Convert man hours to man days or back, and estimate total man-days from number of workers, hours worked per worker, and hours per day.
| Man Hours to Man Days | Man Days to Man Hours |
|---|---|
| 1 man-hour = 0.125 man-days | 0.25 man-days = 2 man-hours |
| 2 man-hours = 0.25 man-days | 0.5 man-days = 4 man-hours |
| 4 man-hours = 0.5 man-days | 0.75 man-days = 6 man-hours |
| 6 man-hours = 0.75 man-days | 1 man-day = 8 man-hours |
| 8 man-hours = 1 man-day | 1.5 man-days = 12 man-hours |
| 10 man-hours = 1.25 man-days | 2 man-days = 16 man-hours |
| 12 man-hours = 1.5 man-days | 3 man-days = 24 man-hours |
| 16 man-hours = 2 man-days | 4 man-days = 32 man-hours |
| 20 man-hours = 2.5 man-days | 5 man-days = 40 man-hours |
| 24 man-hours = 3 man-days | 6 man-days = 48 man-hours |
| 32 man-hours = 4 man-days | 7 man-days = 56 man-hours |
| 40 man-hours = 5 man-days | 8 man-days = 64 man-hours |
| 48 man-hours = 6 man-days | 10 man-days = 80 man-hours |
| 56 man-hours = 7 man-days | 12 man-days = 96 man-hours |
| 60 man-hours = 7.5 man-days | 15 man-days = 120 man-hours |
| 80 man-hours = 10 man-days | 20 man-days = 160 man-hours |
| 100 man-hours = 12.5 man-days | 25 man-days = 200 man-hours |
| 120 man-hours = 15 man-days | 30 man-days = 240 man-hours |
| 160 man-hours = 20 man-days | 40 man-days = 320 man-hours |
| 200 man-hours = 25 man-days | 50 man-days = 400 man-hours |
| Formulas: man-days = man-hours ÷ 8; man-hours = man-days × 8 (assumes 1 man-day = 8 hours; 1 man-week = 40 hours). | |
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Man Hours To Man Days Formula
Variables:
- MD is the total Man Days
- MH is the total Man Hours
- 8 is the standard hours per man-day (adjust per your industry or country; see table below)
To convert man hours to man days, divide total man hours by the hours in a standard working day. The default of 8 hours reflects the international benchmark set by the ILO Hours of Work (Industry) Convention (No. 1) in 1919, the first binding international labor standard. The denominator is not always 8 and should match your project context.
What Are Man Hours and Man Days?
A man-hour, or person-hour, is one person’s hour of labor effort. A man-day, or person-day, expresses that effort using a stated number of hours per day. Eight hours is a convenient default, not a universal definition or a measure of productivity.
The 8-hour workday did not emerge arbitrarily. Welsh social reformer Robert Owen proposed the “8-8-8” division in 1817 (8 hours labour, 8 hours recreation, 8 hours rest). The ILO formalized 8 hours per day and 48 hours per week as the international labor standard in 1919. Ford Motor Company was among the first large industrial employers to adopt the 8-hour day commercially, in 1914, and found that output per worker increased rather than fell. The US Fair Labor Standards Act began with a 44-hour weekly threshold in 1938, phased down to 40 by 1940. Its overtime threshold does not define a universal person-day length.
A person-day must use the hours stated in the project basis, contract or labor agreement. The table illustrates possible conversion definitions; country labels do not establish a legal daily unit. Confirm applicable rules separately.
| Context | Hours per Man-Day | Basis |
|---|---|---|
| US / Canada (most industries) | 8.0 h | Example: 40-hour week divided into five equal days |
| UK | 7.5 h | Example: 37.5-hour week over five days; the regulations do not define a 7.5-hour day |
| France | 7.0 h | Example: 35-hour week divided into five equal days |
| Germany | 8.0 h | Example project day; check the actual contract and applicable rules |
| Australia | 7.6 h | Example: 38-hour week divided into five equal days |
| Japan | 8.0 h example | Example project day; no actual-average-hours assumption |
| Offshore oil and gas (global) | 12.0 h | Example 12-hour shift assumption; confirm the actual rotation |
| UAE construction (June to Sept) | Project-specific | The seasonal midday break does not define a six-hour person-day; check current local rules |
| Safety incidence-rate denominator | Actual employee hours worked | 200,000-hour normalization does not require conversion to eight-hour days |
Man-Hours in Safety Incident Reporting
Beyond project scheduling, man-hours serve as the exposure denominator in occupational safety metrics. OSHA’s Total Recordable Incident Rate (TRIR) uses the formula: TRIR = (Recordable incidents x 200,000) / Total man-hours worked.
The 200,000 baseline represents the annual man-hours of 100 full-time employees working 40 hours per week for 50 weeks (100 x 40 x 50 = 200,000 man-hours). In man-day terms, 200,000 man-hours equals 25,000 man-days, corresponding to 100 person-years of labor at a standard 8-hour day. This normalization allows companies of very different sizes to compare safety performance on a consistent basis.
Large construction and oil and gas contractors track cumulative man-hours specifically for safety milestone reporting. A widely cited benchmark is reaching 1,000,000 man-hours (125,000 man-days) without a lost-time injury. A project employing 500 workers for 250 working days at 8 hours per day accumulates exactly 1,000,000 man-hours. Contractors often publish these milestones in project communications and press releases as a measure of safety program performance.
Productive Hours vs. Contract Hours in Project Estimation
Project estimators distinguish paid or scheduled hours from productive work time. Any productivity or delay allowance must come from the project’s estimating basis; this converter does not assume an allowance. Applying an allowance requires consistently defining whether your input is paid time or productive effort.
In construction estimating, productivity rates depend on the scope and conditions. For a purely arithmetic example, 5,000 person-hours equals 625 person-days at eight hours per day. Dividing by eight equally available workers gives 78.125 working days only if the work is fully parallel; elapsed calendar duration also depends on the schedule and nonworking days.
How to Calculate Man Hours To Man Days
- Confirm the project’s person-day definition from its schedule basis or labor agreement. The calculator starts with an explicit eight-hour conversion definition; enter another value when appropriate.
- Calculate total man-hours. For a team: total man-hours = number of workers x average hours worked per worker. For a single worker over multiple days: total man-hours = hours per day x number of days.
- Divide total person-hours by the hours defining one person-day. Safety incidence-rate calculations instead use actual employee hours worked in their denominator; do not replace those hours with a fixed eight-hour conversion.
Example: A construction project uses 15 workers on 10-hour shifts for 12 days.
- Total man-hours: 15 x 10 x 12 = 1,800 man-hours
- Man-days for scheduling (at actual 10-hour shift): 1,800 / 10 = 180 man-days
- Eight-hour effort equivalents, if useful for comparison: 1,800 / 8 = 225 person-days. This is not a required OSHA conversion.
The 180-person-day figure uses ten-hour days, and the 225-person-day figure uses eight-hour days; both represent the same 1,800 labor hours. For a crew of 15 with equal availability, the idealized duration is 12 working days. An incidence-rate denominator retains the actual 1,800 employee hours worked.
