The four-day week: what the pilots measured and what it changes

The four-day week is not “working less”. The model the pilots tested is called 100-80-100: one hundred per cent of pay, eighty per cent of the time, one hundred per cent of the output. Whether that trade holds is an empirical question - and by now there is real data.

What the pilots measured

Iceland went first: large public-sector trials between 2015 and 2019 covered about 2,500 workers, were judged an “overwhelming success” by the researchers - service levels held, wellbeing rose - and led unions to negotiate shorter hours for most of the country’s workforce.

The largest business trial ran in the UK in 2022: 61 companies, about 2,900 people, six months on 100-80-100. At the end, 56 of the 61 kept the four-day week, and 18 made it permanent on the spot. Revenue on average held steady against the previous year, while resignations and reported burnout dropped sharply.

Belgium took a different road: since 2022 employees may request to compress a full-time week into four days - the same hours, longer days. That is a legal right to a schedule, not a reduction of time.

Two very different models

Fewer hours (4×8): the working week genuinely shrinks. This is what the UK and Icelandic pilots tested - and it is a change to pay-per-hour arithmetic, norms and entitlements;
Compressed hours (4×9.5-10): the same weekly total packed into four longer days. Pay and yearly entitlements stay put - what changes is the schedule and everything that depends on it.

Companies regularly announce “a four-day week” meaning either of the two. Before copying anyone’s experience, check which model they actually ran.

What it changes in the books

Whichever model you pick, the working week is the unit everything else is counted in - so several rules have to be rewritten on day one:

Leave entitlements. A yearly norm written in days quietly changes meaning when a day is 9.5 hours or when the week has four of them. We unpacked how norms are built in the entitlement article - after the switch, that arithmetic runs on new inputs;
The free day is a schedule, not an absence. An off-Friday must not eat a day of leave or look like sick leave in reports - it is part of the person’s working pattern;
Public holidays. What happens when a holiday lands on somebody’s free day - is it lost, moved or compensated? The rule must be written before the first such Monday;
Coverage. When half the team is off on Fridays and half on Mondays, approvals need a calendar that shows coverage, not rows in a file.

Labour law on working time, overtime and compressed schedules differs by country - Belgium’s route is legal by statute, elsewhere it may need contract changes. Check the law of every country where your people are employed before switching.

If you want to try it

The pilots that worked share a shape: a fixed trial period (a quarter or two), explicit metrics agreed up front - output, revenue, sick days, attrition - and a written schedule rule for every person. Run it as an experiment with numbers, not as a perk with vibes: that is exactly what made the UK results credible enough for 56 companies to keep going.