A NOC coverage calculator, worked through.

Four people is 86 percent of the week. The 14 percent it misses is every weekend night.

A coverage calculator turns a headcount into a week of shifts and then tells you what that week does not do. Four people covers 144 of the 168 hours, which is 86 percent, and the 24 hours it misses are all at the weekend and all at night. This is that calculation worked through, shift by shift, with the tool that does it linked at the end.

Four people supply 160 hours. Shifts come in whole blocks on whole days, so the practical answer is lower than the supply. The pattern below is three eight hour shifts from Monday to Friday plus one twelve hour shift across Saturday and Sunday, which is 144 hours of the week.

Add the shift rows and you get 144 hours, which is 86 percent of the week. The 24 hours that are left are three stretches and every one of them is a night, Saturday from 00:00 to 08:00, Saturday to Sunday from 20:00 to 08:00 and Sunday from 20:00 to 00:00.

A percentage does not tell you where the hole is, and where it is decides how much it matters. This 14 percent is not spread thinly through the week. It is three unbroken stretches, every one of them a night, and all three fall on the two days with the fewest people anywhere near a laptop.

That is the difference between a calculator and a spreadsheet. A spreadsheet will tell you 86 percent. What you needed was which 14.

Five is the first headcount that reaches every hour, and the tool says so by changing the sentence at the top rather than by recommending a hire. At five the coverage line reads 100 percent of 168 hours. The line underneath reads that all 168 of them have one person on.

Both are true at the same time, and a calculator that showed only the first would be flattering you. The second one is what tells you whether a meal break, a training morning or one person's leave leaves the desk empty.

Every figure on this page is produced by the same code the calculator runs, over one fixed week, and it is checked against a pinned copy whenever the tests run. If the shift patterns change, this page fails a test until somebody regenerates it and reads the difference. That is a small thing, and it is the whole difference between a worked example and a screenshot of one.

Signal9 is alert and on call software for operations teams, and the calculator is free because the question it answers is one we had to answer for ourselves. Put your own headcount in and it draws the week, names the hours with one person on, and names the hours with nobody on. For those hours, automation acts on the alerts it has been told it can act on, and the alerts you marked most urgent keep escalating until somebody answers.

What is a NOC coverage calculator? A tool that turns a headcount into a week of shifts and then reports what that week covers and what it does not. The Signal9 one is free and needs no account. Choose a number of people and it draws the shifts, names the hours with nobody on, names the hours with one person on, and hands the week to a planner you can edit.

How much of the week can four people cover? 144 of 168 hours, which is 86 percent, on three weekday shifts plus a weekend day shift. The missing 24 hours are three stretches of weekend night. Four people supply 160 hours in total, so part of the shortfall is arithmetic and part of it is that shifts come in whole blocks on whole days.

Is the NOC coverage calculator free? Yes. No account, no signup, and nothing you type leaves your browser. The week it draws can be taken into the free NOC Schedule Planner, renamed, adjusted, and exported to calendar, spreadsheet or print.