How many people do you need for an on-call rotation?
Enter your team size, how many people are on call at once and how much time off you expect, and get the real load: days on call per person, how many weekends, how long between turns, and the smallest team that keeps everyone at or under the 25% limit in Google's SRE book. It runs in your browser and nothing you type is sent anywhere.
How the numbers are worked out
- Days on call per person = people on call at once x days you cover per year, divided by team size. 365 days if you cover every day, 260 for weekdays only (52 weeks x 5).
- Share of days around = people on call at once, divided by team size x the fraction of covered days each person is present. This is the conservative figure: someone away for 30 of 365 days is around for 92% of them, and the others absorb their turns. The SRE book's 25% limit is about time spent on call, so this is the number to compare with it.
- Smallest team within 25% = people on call at once divided by (0.25 x the fraction present), rounded up.
- Turns and gaps assume turns are shared equally and in a repeating cycle. A real rota with part-time staff, swaps and holidays will drift from this, which is what a points-weighted rota corrects.
What the 25% figure is, and what it is not
Google's SRE book recommends that no more than 25% of an engineer's time is spent on call, and works out that a 24/7 rota from a single site needs at least eight engineers to stay within that (Being On-Call, chapter 11). Its team sizes are for round-the-clock cover by engineers who also do other work. It is a guide for deciding how big a team needs to be, not a rule your team is bound by. A small team that cannot reach it can still be fair: keep the load even, compensate out-of-hours work, and say plainly that the load is high. This calculator does not give employment, pay or legal advice, and working-time rules differ between countries.
Making a small team's rota fair
If the result is above 25% and you cannot hire, the lever you have left is how the load is divided. The points method weights weekends and public holidays so the heavy turns are shared, and Turnkeep keeps the rota, the swaps and the running totals for a team, free for up to four people. Neither one pages anyone: keep whatever alerting you already use.