Pro rata pay, and the parts of it that go wrong

These are listed in the order worth acting on, which with pro rata calculations is not the order they are usually presented in.

The fraction is hours, not days

Pro rata pay is normally the full-time salary multiplied by contracted hours divided by full-time hours. Counting in days goes wrong as soon as days are of unequal length, which is common in part-time patterns. A pattern of three long days and a pattern of five short ones can produce the same fraction and very different working lives.

Check the contract states hours, not just days, because the hours figure is what everything else is calculated from.

Public holidays are where it breaks

A part-timer who never works Mondays gains nothing from public holidays that fall on Mondays, while a colleague working Mondays gains all of them. The usual correction is to convert the full-time public holiday entitlement into hours and give the part-timer their proportion to take when they choose. Many employers do not do this, and in several jurisdictions it can amount to less favourable treatment of part-time workers.

In practice, the rules differ by country, so check with your labour authority before concluding either way.

Leave in hours avoids most disputes

Expressing annual leave in hours rather than days removes the ambiguity of what a day means for someone working uneven shifts. It also makes it obvious when a change of hours mid-year requires a recalculation. A change of pattern part way through a leave year should trigger one, and often does not.

Ask for the leave balance in hours if it is currently given in days.

Advertised salaries are full-time equivalents

A part-time role advertised with a salary figure is almost always quoting the full-time equivalent unless it says otherwise. Confirming which is being quoted before an offer is accepted avoids a genuinely painful misunderstanding.

In practice, the same applies to bonuses, which are usually prorated but occasionally not. One clarifying sentence at offer stage settles it.

Benefits scale unevenly

Pension contributions and leave scale naturally with hours; health cover, equipment and season ticket support often do not scale at all. That means a part-time package can be worth proportionally more or less than the fraction suggests, depending on what is in it. Listing the package rather than the salary is the only way to compare two part-time roles honestly.

It matters more at lower salaries, where fixed-value benefits are a larger share of the total.

Adjust the size of it until it is something you would actually do tired.

Changing hours later

Reducing hours changes pension accrual, any service-related sick pay, and in some countries the calculation of statutory payments based on recent earnings. Those effects can persist beyond the period of reduced hours, depending on the reference periods used. This is worth checking before agreeing a reduction, particularly if a statutory claim is likely in the following year.

On an ordinary week, the specifics are jurisdictional and belong with your labour authority or a union.

Everything above, in order of what to do first

1. The fraction is hours, not days. Pro rata pay is normally the full-time salary multiplied by contracted hours divided by full-time hours.
2. Public holidays are where it breaks. A part-timer who never works Mondays gains nothing from public holidays that fall on Mondays, while a colleague working Mondays gains all of them.
3. Leave in hours avoids most disputes. Expressing annual leave in hours rather than days removes the ambiguity of what a day means for someone working uneven shifts.
4. Advertised salaries are full-time equivalents. A part-time role advertised with a salary figure is almost always quoting the full-time equivalent unless it says otherwise.
5. Benefits scale unevenly. Pension contributions and leave scale naturally with hours; health cover, equipment and season ticket support often do not scale at all.
6. Changing hours later. Reducing hours changes pension accrual, any service-related sick pay, and in some countries the calculation of statutory payments based on recent earnings.

The takeaway

Get leave, hours and public holiday entitlement stated in hours. Days hide the errors.

Small and repeatable beats ambitious and abandoned, almost every time.