Hours to days
Convert hours to days by dividing by 24 — or by 8 if you mean working days. The distinction that ruins project estimates.
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The formula
days = hours ÷ 24Calendar days or working days
The formula above gives calendar days: 24 hours each, the kind that elapse whether anyone is working or not. That is what this converter produces.
Project estimates, contracts and service agreements usually mean something else:
| Kind of day | Hours | Used for |
|---|---|---|
| Calendar day | 24 | elapsed time, deadlines, rentals |
| Working day | 8 | effort estimates, person-days, billing |
| Shift | 12 | rota planning, healthcare, industrial |
The gap is a factor of three, and it is where estimates go wrong. “40 hours of work” is one working week — five working days — but only 1.67 calendar days of elapsed time. Neither number is wrong; quoting one when the reader expects the other is.
Quick reference
| Hours | Calendar days | Working days (8 h) |
|---|---|---|
| 8 h | 0.33 | 1 |
| 12 h | 0.5 | 1.5 |
| 24 h | 1 | 3 |
| 40 h | 1.67 | 5 |
| 48 h | 2 | 6 |
| 72 h | 3 | 9 |
| 168 h | 7 | 21 |
| 720 h | 30 | 90 |
Service level agreements
SLAs are where the distinction is written down explicitly, because the difference decides whether a breach happened:
| Commitment | If calendar hours | If business hours |
|---|---|---|
| 4-hour response | same day | up to next working day |
| 24-hour response | next day | three working days |
| 48-hour response | two days | six working days |
A “24-hour response” raised at 5pm on a Friday resolves on Saturday evening under one reading and on Wednesday afternoon under the other.
Uptime
Availability percentages convert to downtime allowances, and hours are the natural unit for the middle of the range:
| Availability | Downtime per year |
|---|---|
| 99% | 87.6 h — 3.65 days |
| 99.9% | 8.76 h |
| 99.95% | 4.38 h |
| 99.99% | 52.6 min |
| 99.999% | 5.26 min |
Each additional nine divides the allowance by ten, which is why the jump from three nines to four is an engineering programme rather than an adjustment.
Estimating in your head
Divide by 24, or halve three times and divide by three. For working days, divide by 8 — which is why an estimate given in hours is so easily misread as three times longer or shorter than intended.