Part-Time vs Full-Time: Calculating the Pro-Rata Salary
When you switch from full-time to part-time work, understanding how calculating your salary changes is crucial — and it's simpler than you might think. You don't just "lose money proportionally." Instead, your salary is calculated pro-rata, which means it scales with your hours. If you work 60% of full-time hours, you earn roughly 60% of your full-time salary. But that's the start of the calculation, not the end. Your tax, National Insurance, pension contributions, and holiday entitlement all adjust too. This guide walks you through exactly how to work out your new take-home pay when moving to reduced hours.
How Pro-Rata Salary Works
Pro-rata means "in proportion." If your full-time salary is £35,000 and you drop from 37.5 hours to 22.5 hours per week (60% of full-time hours), your pro-rata salary becomes £21,000. The formula is simple:
Pro-rata salary = Full-time salary × (New hours ÷ Original hours)
So if you earned £35,000 at 37.5 hours/week and move to 30 hours/week, your salary becomes: £35,000 × (30 ÷ 37.5) = £28,000
Here's where people often get confused: your take-home pay doesn't drop by exactly the same percentage as your hours. Tax bands and National Insurance thresholds don't scale with your hours — they're fixed. So a part-time salary of £28,000 doesn't trigger the higher rate band (£50,270), and you still get the full personal allowance (£12,570). That can actually work in your favour. Use our salary calculator to see your exact take-home figure for any pro-rata arrangement.
Common Misconceptions About Part-Time Pay
"I'll just lose 40% of my pay if I drop to 60% hours."
Not quite. You lose 40% of your gross pay, but your tax bill doesn't drop by 40%. A 40% reduction in salary might only reduce your tax by 30–35%, because you're still within the basic rate band. This is actually good news — part-time work is often less financially painful than it first appears.
"Part-time workers get less holiday."
No. You get the same legal minimum (5.6 weeks, or 28 days, per Working Time Regulations), but calculated pro-rata. If full-time staff get 20 days of paid holiday per year and you work 60% hours, you get 12 days. More on that below.
"I'll miss out on my pension."
You don't lose pension contributions — they scale with your pay. If your employer contributes 5% and you work part-time at £28,000, they contribute 5% of £28,000, not 5% of £35,000. But your pension entitlement will be smaller at retirement because you've contributed less total. That's factual, not a surprise — it's one of the real trade-offs of part-time work.
Calculating Your Reduced Hours
Let's work through a realistic example. Imagine you earn a full-time salary of £40,000 at 37.5 hours per week and are switching to 30 hours per week.
Step 1: Calculate pro-rata salary £40,000 × (30 ÷ 37.5) = £32,000
Step 2: Check what you actually take home
- Gross: £32,000
- Income tax (20% on earnings above £12,570): £3,886
- National Insurance (8% on earnings above £12,570): £1,542
- Take-home: £26,572 (£2,214/month)
Compare that to your full-time take-home on £40,000:
- Gross: £40,000
- Income tax: £5,486
- National Insurance: £2,194
- Take-home: £32,320 (£2,693/month)
The difference is £5,748/year, or £479/month. You've reduced your hours by 20% (from 37.5 to 30), but your take-home has only dropped by 17.8%. That's the tax system working slightly in your favour.
If you're moving from full-time to part-time and want to compare different scenarios, plug your numbers into our UK salary calculator — it handles the tax bands, thresholds, and any pension contributions automatically.
Holiday, Pension, and Benefits (Pro-Rata)
Holiday entitlement
Under UK law, all workers get a minimum of 5.6 weeks' paid holiday per year. For a part-time employee, this is calculated pro-rata based on hours worked.
- Full-time (37.5 hours/week) = 28 days/year = 5.6 weeks
- Part-time (30 hours/week) = 22.4 days/year
To calculate your holiday days: (Your weekly hours ÷ Full-time hours) × 28 days.
If you work 30 hours/week and the full-time standard is 37.5, you get (30 ÷ 37.5) × 28 = 22.4 days. Your employer can round this, so you'd typically get either 22 or 23 days. Check your contract — many employers offer more than the legal minimum. When you calculate holiday pay for part-time work, multiply your daily rate by your entitled days.
Pension contributions
Workplace pensions scale with your pay. If the minimum auto-enrolment contribution is 8% total (3% employer, 5% employee) and you earn £32,000, the contributions are based on that figure, not your original full-time salary.
- Full-time (£40,000): 8% = £3,200/year
- Part-time (£32,000): 8% = £2,560/year
The pension contribution is smaller because you earn less, not because part-time staff are treated differently. Pension contributions also reduce your taxable income, so the real cost to you is less than it appears. If you contribute 5% of £32,000 (£1,600), tax relief at 20% means the actual cost from your net pay is £1,280. The system gives you a 20% discount via tax relief.
Benefits in kind
Private health insurance, gym memberships, or other benefits are usually not pro-rated — you either get them or you don't. Check your contract or ask HR. Some companies offer reduced benefits for part-time staff; others treat part-timers the same as full-time. It's worth negotiating, especially if you're moving to part-time for family reasons.
Part-Time vs Full-Time: What You Actually Earn
Here's the thing people often overlook: moving to part-time doesn't always cost as much as you'd expect because tax scales more generously than hours do.
Scenario: Moving from full-time to 75% hours
Full-time (37.5 hours): £40,000
- Take-home: £32,320/year
Part-time (28.1 hours): £30,000
- Take-home: £25,486/year
You've cut your hours by 25%, but your take-home has only dropped by 21%. You're "losing" less financially because you've exited higher-rate taxation entirely.
Scenario: Comparing job offers
Here's where comparing job offers gets tricky. You're offered:
- Job A: Full-time, £36,000/year
- Job B: Part-time (30 hours vs 37.5), unspecified salary
Many candidates assume Job A is the winner. But if Job B offers £30,000 pro-rata (which equals £36,000 at full-time rates), the take-home is nearly identical — and you've gained 7.5 hours per week of personal time. Run both through the calculator and compare net pay, not gross.
Hourly rate vs salary
If you're moving from salaried to hourly part-time work, convert your salary to an hourly rate first. A £36,000 salary at 37.5 hours/week = £17.31/hour (gross). If a part-time role offers £18/hour, that's a modest pay rise — but only if the hours are guaranteed. If the part-time role offers 20 hours/week at £18/hour, that's £46,800/year in theory, but you'll earn only £18,720 if you work exactly 20 hours consistently.
Negotiating Part-Time Arrangements
When you switch to part-time, your negotiating position is often stronger than you think. Your employer benefits from retaining experienced staff — recruitment and training costs are real. Here are the levers:
Salary
The pro-rata calculation is standard, but you can negotiate whether the offer is fair. If you were earning £40,000 and you move to 30 hours from 37.5, the standard calculation is £32,000. But if you're doing specialist work or were high-performing, you might negotiate £33,000 or £34,000 to retain you. It's worth asking.
Benefits
Some benefits don't pro-rata. If full-time staff get 25 days of holiday, try negotiating 20 days instead of the pro-rata 20 (which might be 16 if you drop to 30 hours). A modest increase in holiday, pension match, or flexible working adds value without changing salary bands.
Review schedule
If you're moving to part-time temporarily (e.g. to care for a young child), negotiate a review date. After 2 years, you might be ready to return to full-time. Having a clear review point stops you getting stuck in a lower-paying tier.
Frequently Asked Questions
Does my National Insurance change when I go part-time?
Yes. National Insurance is calculated on your actual earnings, not your hours. On a pro-rata salary of £28,000, you pay 8% on earnings above £12,570 — that's £1,229/year. On a full-time salary of £35,000, you'd pay £1,794/year. The reduction is proportional to your salary drop.
If I work part-time, do I still get my full personal allowance?
Yes. The personal allowance (£12,570 in 2026) is the same for everyone, regardless of hours worked. It's not reduced for part-time staff. This is why dropping to part-time can be surprisingly affordable — you're still getting the full tax-free threshold even though you earn less.
Can my employer reduce my benefits when I go part-time?
Not your statutory benefits (holiday, pension, sick pay). These must be pro-rated in line with hours. However, discretionary benefits like private health, gym memberships, or enhanced maternity pay can sometimes be withdrawn. Check your contract and discuss with your employer before accepting part-time terms.
How is my bonus or overtime taxed if I go part-time?
The same way as for full-time staff — at your marginal rate. If you're a basic rate taxpayer and you earn a bonus, 20% goes to income tax (plus National Insurance). There's no special "part-time tax rate" on bonuses. Just make sure you don't accidentally push yourself into the higher rate band (£50,270).
If I'm part-time, do I pay less National Insurance?
Yes, you pay NI on your actual earnings. But you also accrue less towards your state pension unless you've paid enough voluntary contributions. If you drop to part-time for a few years, you might miss some national insurance contributions, which could slightly reduce your eventual state pension. It's a small effect (usually a few pounds/week), but it's real. You can pay voluntary contributions to fill the gaps.
What happens if I go part-time and my new salary is below the personal allowance?
If you earn less than £12,570/year, you pay no income tax. You'd still pay National Insurance if you earn more than £12,570, though — yes, it's confusing. The thresholds are different. On £10,000/year, you'd pay no income tax but you'd still owe NI if you'd earned more than the NI threshold (currently the same as the personal allowance, but they can diverge). You'd get a P45 from your old employer and a new P45 from your new employer, and HMRC would sort out your annual tax position.
Can I go part-time for a year and then return to full-time at the same salary level?
There's no legal obligation for your employer to restore your full-time salary when you return. That said, if you're valuable and the business needs you, they'll likely offer competitive terms. Before you go part-time, ask what the reversion process looks like. Some employers guarantee a return to your original salary or better; others treat it as a new negotiation.
When you're calculating what part-time work actually costs, use our UK salary calculator to see your exact take-home figure. Plug in your pro-rata salary and any other adjustments — pension, student loan, tax codes — and you'll see the real number you'll earn. It's often kinder to your bank account than you expect. And if you're negotiating a part-time arrangement, remember: your gross salary is the starting point, not the final offer. Holiday, pension, flexibility, and benefits all have value. Factor them in when you compare job offers or discuss a switch with your employer.