Thinking of taking a second job alongside your main one? A second job pro rata calculator shows you exactly what those extra hours are worth — and, just as importantly, how much of it you’ll actually keep after tax. This guide walks through the calculations, the tax implications, and the worked examples you need to decide whether the extra work is worth it.
Why a Second Job Needs Its Own Calculation
Your second job is almost always part-time, which means its pay is a pro rata slice of whatever full-time rate the employer offers. But unlike your main job, the tax position is different: your personal allowance is usually allocated entirely to your first job, so second-job earnings are often taxed from the first pound.
That makes the “what will I actually get?” question a two-step calculation: first the pro rata gross, then the tax. Let’s do both properly.
Step 1: The Pro Rata Gross
The gross calculation is the standard one. Say your second job is advertised at £24,000 FTE, full-time is 37.5 hours, and you’ll work 12 hours a week (two evenings).
- Proportion: 12 ÷ 37.5 = 0.32
- Annual gross: £24,000 × 0.32 = £7,680
- Monthly gross: £7,680 ÷ 12 = £640
- Hourly: £7,680 ÷ 52 ÷ 12 = £12.31
So the second job adds £640 a month before tax. You can verify this instantly with our free pro rata salary calculator — the gross figure should match exactly. Now the crucial part: the tax.
Step 2: The Tax Reality of Second Jobs
HMRC allocates your £12,570 personal allowance to one job — normally your main one. Your second employer will typically put you on a BR (basic rate) tax code, which taxes everything at 20% with no allowance, or a D0 code if you’re a higher-rate taxpayer.
On £7,680 a year at BR:
- Income tax: £7,680 × 20% = £1,536 a year (£128/month)
- National Insurance: second-job earnings above the primary threshold (£5,000 a year from April 2025) attract 8% employee NI. (£7,680 − £5,000) × 8% = £214.40 a year (~£17.87/month)
- Take-home: £7,680 − £1,536 − £214.40 = £5,929.60 a year (£494.13/month)
So your £640/month gross becomes roughly £494/month in your pocket. Still worthwhile for 12 hours a week — but it’s not £640, and you should know that before you sign up.
Worked Example: Weekend Retail Shifts
Let’s try a more typical scenario. You work full-time Monday–Friday (£32,000) and take weekend retail shifts: £12.60/hour, 10 hours a week.
- Annual gross: £12.60 × 10 × 52 = £6,552
- Monthly gross: £546
- Tax at BR (20%): £6,552 × 20% = £1,310.40/year
- NI: (£6,552 − £5,000) × 8% = £124.16/year
- Annual take-home: £6,552 − £1,310.40 − £124.16 = £5,117.44
- Monthly take-home: £426.45
- Effective hourly take-home: £5,117.44 ÷ 52 ÷ 10 = £9.84/hour
That £12.60 headline rate is really £9.84 per hour after tax and NI. Whether that’s worth your Saturdays is a personal call — but now it’s an informed one.
Second-Job Earnings Table
Here’s what various second-job patterns yield after basic-rate tax and NI (assuming the personal allowance is fully used by the main job):
| Hours/week | Hourly rate | Annual gross | Annual take-home | Monthly take-home |
|---|---|---|---|---|
| 8 | £12.50 | £5,200 | £4,744.00 | £395.33 |
| 10 | £12.50 | £6,500 | £5,680.00 | £473.33 |
| 12 | £13.00 | £8,112 | £6,275.04 | £522.92 |
| 15 | £13.50 | £10,530 | £7,887.60 | £657.30 |
| 16 | £14.00 | £11,648 | £8,584.16 | £715.35 |
| 20 | £14.50 | £15,080 | £10,599.20 | £883.27 |
The take-home column assumes BR tax on all earnings and 8% NI above £5,000. Your exact figures will vary with your tax code — check your payslip.
The Tax Code Trap
The single biggest second-job pitfall is the wrong tax code. If HMRC doesn’t know about the second job, you might be given a second personal allowance and underpay tax — then face a bill later. Conversely, an emergency code can overtax you for months.
What to do: when you start the second job, HMRC will usually issue a BR code automatically once the employer submits your starter checklist. Check your first payslip: if the tax code looks wrong (anything other than BR, D0, or a code you’ve agreed with HMRC), call HMRC or check your Personal Tax Account online. Don’t assume payroll will sort it out.
National Insurance: The Two-Job Wrinkle
NI works per job, not per person. Each employment has its own £5,000 annual threshold, which means:
- If your second job pays under £5,000 a year, you pay no employee NI on it at all.
- If it pays more, NI applies only to the amount above £5,000 in that job.
This is actually a small advantage of splitting work across two jobs rather than doing overtime in one — but the effect is modest (8% on the slice above £5,000), so don’t choose a job structure for NI reasons alone.
Pensions and the Second Job
Auto-enrolment applies per job too. If your second job pays over £10,000 a year (the trigger), that employer must enrol you in a pension — even if you’re already in one through your main job. You can opt out, but think carefully: employer contributions are effectively free money, and small pots add up.
If the second job pays under £10,000, you won’t be auto-enrolled, but you can ask to join the scheme voluntarily if you earn over £6,240.
Is It Worth It? The Decision Framework
Run through these questions with your calculated figures:
- What’s my effective hourly take-home? (Use the method above.) Compare it with your main job’s take-home hourly rate.
- What are the hidden costs? Travel, extra childcare, meals out because you’re too tired to cook — these eat into the gain.
- What’s the impact on rest? Working 50+ hour weeks affects health and your main-job performance. Be honest about sustainability.
- Could overtime in the main job pay better? Overtime is often at 1.5× and taxed the same way — sometimes the better deal without a second employer.
Student Loans and Second Jobs
If you’re repaying a student loan, a second job increases your repayments — and the calculation catches people out. Student loan repayments are based on total income across all jobs, at 9% above the threshold (£25,000 for Plan 2, £22,895 for Plan 4 Scottish, £21,000 for Plan 1).
Here’s the catch: each employer applies the threshold separately. So if your main job pays £24,000 (below the Plan 2 threshold, no repayment) and your second job pays £7,680, the second employer deducts 9% of the full £7,680 = £691.20 a year — even though your combined income of £31,680 is only £6,680 over the threshold, which should mean £601.20.
You’ve overpaid by £90. HMRC reconciles this at year-end and refunds the difference automatically, but it means your monthly take-home from the second job is lower than it should be until the refund arrives. Factor roughly 9% extra deduction into your planning if you’re over the threshold in total.
The Benefits Angle
If you claim Universal Credit, a second job’s earnings reduce your UC payment — but not pound for pound. The taper rate is 55p per £1 of net earnings above your work allowance (if you have one). So £494/month extra take-home might reduce UC by around £272, leaving you £222 better off overall.
Whether that’s “worth it” depends on your circumstances, but the key point is: the second job still leaves you better off in cash terms. The taper never takes more than you earn. What it does affect is the effective hourly rate — recalculate it after the UC reduction to see the true return on your time.
One more consideration: earnings from a second job can affect the benefit cap and childcare cost support. If you’re near any threshold, model the full picture before committing — a quick call to Citizens Advice or a benefits calculator is time well spent.
FAQs
Will a second job affect my main job’s tax?
No — but it can push your total income into a higher band. If your combined earnings exceed £50,270, the slice above that is taxed at 40%. HMRC may then adjust your main job’s code to collect the extra. Keep an eye on your total.
Do I need to tell my main employer about a second job?
Check your contract — many include a clause requiring disclosure or prohibiting work for competitors. Even without a clause, consider conflicts of interest and rest requirements. It’s rarely a secret worth keeping.
What if my second job pays cash in hand?
It still counts as taxable income and must be declared. “Cash in hand” doesn’t mean “tax-free” — it means the employer isn’t operating PAYE properly, which is their problem until HMRC comes asking, at which point it becomes yours too.
Can I have two personal allowances?
No. The £12,570 personal allowance applies once across all your income. If it’s accidentally allocated to both jobs, you’ll underpay and owe HMRC the difference.
Calculate Your Extra Earnings
Before you commit your evenings or weekends, know the real numbers. Work out the pro rata gross for the hours on offer, then apply the tax reality — or run the gross through our free pro rata salary calculator and do the take-home maths with the method above. An informed decision beats a hopeful one every time.