Take-home pay, minus childcare, minus any Child Benefit charge: the one number that actually answers it, plus the day count where it stops being worth it.
Rules current as of August 2026
Net household gain per month · 3 days/week
£1,511
After tax, National Insurance, student loan, childcare and any Child Benefit charge, this is what actually lands in the household budget each month.
Effective marginal rate: 14% of every extra gross pound is lost to tax, NI, student loan, childcare or the charge. You keep the rest.
Effective marginal rate 3% · 16.0 childcare hrs/week
Effective marginal rate 14% · 24.0 childcare hrs/week
Effective marginal rate 26% · 32.0 childcare hrs/week
Effective marginal rate 34% · 40.0 childcare hrs/week
“Best £/day” is the day count with the highest net household gain per day worked, not necessarily the most total money, but the most efficient trade.
It's tempting to assume that going from three days a week to four just means “a third more pay, a third more everything.” It doesn't, because tax, National Insurance and childcare don't scale in a straight line with your salary. They scale in steps, and the steps get steeper as you add days.
Income Tax and National Insurance genuinely are flat rates within each band, but extra days can tip part of your pay into the next band up, so the last day worked is taxed harder than the first. Student loan repayments work the same way once you cross the plan threshold. Each of these nudges the marginal rate up a little.
The bigger jump, in most households, is childcare. The government funds 1,140 hours of childcare a year, the figure usually quoted as “30 hours a week,” but that number only holds across the 38-week school-term pattern. A working parent typically needs cover across all 51 weeks of the year, holidays included, and spread that way the same entitlement works out closer to 22 hours a week. Two or three days of childcare usually fits inside that funded average. Four or five days usually doesn't. Every hour beyond it is paid close to full price, minus whatever Tax-Free Childcare tops up. That's why the jump from 3 to 4 days often costs far more, pound for pound, than the jump from 2 to 3.
The salary you enter is treated as a full-time-equivalent figure and scaled down proportionally by the days you enter: a £35,000 role at three days a week becomes £21,000 of gross pay for this calculation. If your actual part-time salary isn't a clean fraction of a full-time one, use the real part-time figure directly and set days to 5, so no further scaling is applied.
Childcare is assumed needed across all 51 weeks a working parent typically covers, not just term-time, and Tax, National Insurance and student loan are calculated on annual income rather than per payslip, close enough for a steady salary, though real payroll can differ slightly period to period, especially in the month you actually start back. Pension contributions aren't modelled here at all: both incomes are treated as full taxable salary throughout, which means this tool won't show you the pension lever that can reduce both the Child Benefit charge and the risk of losing childcare support near £100,000. The childcare and Child Benefit calculators linked below do model that.
The High Income Child Benefit Charge is assessed on whichever parent earns more, not on your combined household income. If your partner already earns more than your returning salary would be, going back to work changes nothing about this charge at all. The assessment simply keeps looking at your partner's income. But if your returning salary would make you the higher earner, and it's over the £60,000 threshold, returning to work can trigger a charge that wasn't there before, or push an existing one higher.
This calculator isolates that specific effect: it works out the charge with your partner's income alone, works it out again with your returning salary added, and reports only the difference: the part of the charge that returning to work actually causes, not your household's total charge, which might already exist regardless of this decision.
More days almost always means more total money in the household. The net gain figure tends to keep climbing from 2 to 5 days, even as the marginal rate does too. What changes is the efficiency of each additional day: the “best £ per day” marker highlights whichever day count returns the most net household gain for each day actually worked, which is a genuinely different question from “which day count pays the most in total.”
For some households that's 5 days, because a strong salary comfortably clears every cliff edge along the way. For others it's 2 or 3, because the funded childcare hours cover the days worked almost exactly, and the extra days beyond that mostly go toward paying for care rather than take-home pay. Neither answer is right or wrong. It depends entirely on your own numbers, which is exactly what this comparison is for.
This is a financial illustration, not the whole decision. It doesn't account for pension contributions building while you work, career progression that a gap could interrupt, or the state pension credits and professional relationships that come from staying in work even at a modest net gain. None of that has a clean £-per-month figure, but all of it is real. Treat the number this tool gives you as one important input among several, not the final word.