How Many First Aiders Does a Team of 40 Need for Two on Site?
Four first aiders in a team of 40 keep two on site on all but about 1.9 working days a year. Three would leave it short on 11.5.
Four. If the rule is two first aiders present at all times, three holders are not enough: with three, the site drops below two on about 11.5 working days a year, because a single absence is all it takes. Four holders cut that to 1.9 days, and five to 0.3. The pattern is the same as it is for one required, one rung up: holding exactly the requirement is a coin flip, holding one more removes most of the risk, holding two more removes nearly all of it.
Each extra holder, and what it buys
The rule says 2 first aiders present on every working day. Each holder is absent on about 12.6 percent of rostered working days. This is how many working days a year fall short at each holder count.
Holding exactly 2 is a coin flipped every working day. The first spare holder removes most of the exposure; the second removes most of what is left. On these assumptions, 5 holders bring the expected shortfall to one working day a year or fewer, and 6 to one a decade.
How many are actually present
With 4 holders, the average working day has 3.49 present. Averages do not break rosters; the working days with fewer than 2 do. Across 261 working days a year, this is how the year is expected to distribute.
What is hard about this one
A requirement of two is where teams most often get the arithmetic wrong, because three feels like a spare. It is not. Three holders for two required means the site is short whenever any one of them is away, and each of them is away about 12.6 percent of working days, so the site is short on about 11.5 days a year. The spare in a three-for-two arrangement is consumed by the first absence, which is the same position as one-for-one with a better story attached. The real spare begins at four.
At 40 people the team is usually not in one place, and the two-present rule often exists because of that: one first aider for the office, one for the floor, or one per building. The moment the rule is really "one here and one there", it is two separate one-required problems, and each needs its own two holders, which is four in total but four in specific places. Four holders who all sit in the same open-plan office satisfy the model and fail the purpose. Assign holders to locations before counting them.
Forty is also where the holders stop being volunteers and start being a rota, and rotas have a failure the model cannot see: the same two people are always the ones present, because they are the ones who never take leave and never say no. The 1.9 short days assume all four holders are equally likely to be present. If two of them are really only holders on the certificate and never on the floor, the operation is running on two holders for two required, which is short on about 61.8 days a year. Audit who actually turns up, not who is on the list.
Move the requirement, move the answer
The holder count is usually the thing being argued about. The requirement itself is the other lever, and it is worth knowing what the same 4 holders buy if the rule were one looser or one stricter.
| Required present | Covered | Short working days a year |
|---|---|---|
| 1 | 100% | 0.1 |
| 2this page | 99.3% | 1.9 |
| 3 | 91.9% | 21 |
What this assumes, and where it is generous
These figures treat each holder as independently likely to be absent, which is the assumption most favourable to the roster. Holders are rarely independent: the two people who did the course together take the same school holidays, a certificate that lapses removes someone for the weeks it takes to rebook, and the person who is always asked to cover is the one most likely to be off sick. Treat every number here as the best case.
Only a valid certificate counts. A holder whose certificate has lapsed is a zero in this arithmetic for as long as the renewal takes, so the practical holder count is often one lower than the roster believes, and the row above the one you planned for is the one you are actually living on.
The absence share is 25 days of paid annual leave and 8 days of average sickness per person across 261 working days, which is 12.6 percent of rostered working days. Both inputs vary by country; the relief factor library carries sourced figures for 17 of them. Swap your own in and the shape of the ladder holds even as the counts move.
What to do with this
For two first aiders present, hold four, not three. Three leaves the site short on about 11.5 working days a year; four on 1.9.
If the requirement is really one per building, treat each building as its own one-required problem and give it two holders of its own.
Check which holders are actually on the floor. Four on the certificate list but two in the building is a two-for-two roster, which is short on about 61.8 days a year.
How long the certificate itself stays valid, and how it is renewed: First Aid at Work (FAW) certificate (United Kingdom, 3 years), Betrieblicher Ersthelfer (DGUV) (Germany, every 2 years).
The holder count on this page only works if it is the count of people whose certificate is valid today. Keeping that record current, who holds what, at which level, and until when, is what the Skills tab in Soon is for.
Common questions
- If we need two first aiders on site, is three enough?
- No. Three holders for two required is short whenever any one of the three is absent, which on the assumptions here is about 11.5 working days a year. Four holders bring that to 1.9 days, and five to 0.3. The first spare above the requirement is absorbed by a single absence; it is the second spare that buys cover.
- How many first aiders does a team of 40 need in total?
- Four, if two must be present and they work in one place. If the team is split across locations and the rule is really one per location, each location needs two of its own, so the total climbs with the number of places rather than with the number of people.
- What does adding a fifth holder buy?
- It takes the expected short days from 1.9 to 0.3 a year. That is worth having where the hazard is serious or where the holders are spread thin, but the step from three to four, from 11.5 days to 1.9, is the one that changes the character of the roster.
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