Why “taking turns” isn’t enough.
A fixed rotation works until the first holidays. Whoever was away “skips” their turn, someone covers, and after a few months the counters drift apart without anyone deciding it. A rotation has no memory; fairness needs one.
Each person’s share, and its margin.
Each month, divide the on-calls among the people who can do them that month (someone on leave for two weeks gets a smaller share). The result is rarely a whole number: 31 nights between 5 vets is 6.2 each. The fair share is therefore a band: 6 or 7 on-calls, never 4 or 9. Weekends are counted separately, with their own band.
Memory: count over twelve months, not one.
Someone who did 7 on-calls in March should tend towards 6 in April. Keep each person’s total over the previous twelve months and adjust this month’s share by their distance from the team average. Without that memory, small imbalances pile up month after month.
The volunteer and the person who can’t.
Some people ask for more weekends (for the allowance or for personal reasons): let them go above their band, and their extra eases the others. Others can’t: no on-call in their contract, fixed days, an absence. That person leaves the share of what they can’t do, rather than building up a debt they can never pay back.
Worked example: one month, five vets.
31 nights for 4.5 available people (Julie counts as half): about 6.9 each full-time, hence the 6 to 7 band. Emma, who has done the most on-call over twelve months, gets the low end; Julie, her reduced share.
Publish the counters, and the dates.
A visible number settles most arguments. Publish each person’s counters with the rota, and the on-call dates well ahead, so everyone can plan.
How Djed measures on-call fairness.
The above is the rule the Djed engine applies when it builds the month: a band per person worked out among those who can do on-call that month, weekends with their own band, and a twelve-month memory that adjusts each share. Fairness is a goal, not a blocker: if your hard rules prevent a perfect split, the month still comes out and the gap stays visible. Each person has a fairness index (1 = a perfect split), recalculated when you change the rota by hand. So that swaps don’t skew the counters, read our guide to on-call swaps.
Sources and date checked.
- No law sets a fairness rule between colleagues for on-call: the method above is a way of organising, not a legal obligation.
- Organisation of Working Time Act 1997, revised edition (sections 11 to 17 and 25).
- Working Time Regulations 1998 (SI 1998/1833), regulations 4 to 12. Northern Ireland has its own regulations, the Working Time Regulations (Northern Ireland) 2016, with the same limits.
Texts checked on 5 October 2026. This guide is not legal advice: for your own situation, speak to your HR or employment adviser.
Frequently asked questions.
How do you share night on-call fairly?
Work out each person’s share among those who can do on-call that month, with a tolerance band, and adjust it with the previous twelve months.
Does the law require on-call to be shared equally?
No: no law sets a fairness rule between colleagues. It’s a way of organising; the legal rest and hours limits still apply.