Every trade-off, explained.

How we compare three, four and five working days, and where a separate estimate is needed.

England · 2026/27 · Model 2026-09-06.1 · Reviewed 6 September 2026

A complete year for each alternative

Each scenario assumes the same working pattern throughout a full 2026/27 tax year in England, with regular equal monthly PAYE earnings. This is an annual comparison, not a forecast of the current transition year or an exact payslip. All values are nominal GBP. Two adults are taxed separately; household costs and support are combined. The partner can stay at one pattern or both adults can vary to produce nine combinations.

Hours and salary are separate from days

Base salary follows the ratio of actual weekly hours to the full-time reference unless an actual annual salary is entered for that pattern. A four-day compressed week can therefore retain full-time pay. Any entered bonus is either fixed or reduced with hours and is spread across twelve pay periods. Bonuses enter the qualifying-earnings pension basis; they are non-pensionable for the model’s other pension bases. Non-cash benefits are non-pensionable here. Leave does not reduce annual salary.

Tax and take-home income

For each adult, deduct pension salary sacrifice from gross cash pay before NI. Deduct net-pay pension contributions before Income Tax only. Relief-at-source pension contributions cost the employee 80% of the gross contribution; the provider’s basic-rate relief goes into the pension. Gross relief-at-source contributions and grossed-up Gift Aid reduce adjusted net income and extend tax bands. Gift Aid cash paid is also subtracted from spendable cash. Benefits in kind add taxable income without adding cash or employee NI. Income Tax is calculated annually; NI and loan repayments use monthly thresholds, with twelve equal months. Tax and NI are rounded to pennies; monthly loan deductions are rounded down to whole pounds. Annual tax relief may arrive later than the pay it relates to.

Pensions remain separate from current cash

Defined-contribution results show gross employee plus employer contributions for this year, without investment growth. The qualifying-earnings option uses the £6,240–£50,270 band on base pay plus bonus; otherwise all base pay is used. Employer contributions use reference pay before sacrifice and do not include employer NI savings. Salary sacrifice is assumed to reduce base pay, so the minimum-wage check uses base salary after sacrifice. The NHS 2015 option uses actual pensionable base pay, a whole-pay contribution tier and 1/54 annual pension accrual. Other career-average schemes use an entered contribution percentage and accrual denominator. Defined-benefit results are extra annual income earned for retirement; they are never added to a DC pot or treated as spendable today. An unknown denominator leaves accrual unknown.

Childcare uses real quotes

Each child has a monthly quote for each household working pattern, both with and without working-parent funded hours where relevant. Quotes already include universal funded hours if applicable. Add any extras outside Tax-Free Childcare separately. The model does not guess a day rate, nursery timetable, funded-hours discount or childcare availability. A blank quote is unknown; enter zero only when there is no paid bill. Quotes and child eligibility must remain constant throughout this comparison. For different terms, birthdays, school holidays or future arrangements, save separate comparisons.

Childcare eligibility and Tax-Free Childcare

The model tests every adult against the £100,000 adjusted net income ceiling and minimum earnings equivalent to sixteen hours a week at the selected wage rate (£12.71, £10.85 or £8). Under the constant-pay assumption the three-month earnings test is annualised. A confirmed exception may waive the minimum earnings test, but not the upper income limit. Child age, eligible term, provider registration, residency, application timing and any other conditions require the user’s confirmation for the schemes used. When working-parent eligibility fails, the unfunded-working-hours quote applies; universal entitlement stays in that quote. Tax-Free Childcare covers 20% of the eligible invoice subject to each child’s quarterly cap. The same bill in each quarter is an explicit assumption, so unused seasonal capacity is not redistributed. No top-up is applied with Universal Credit or another support route.

Benefits and the cash bridge

Child Benefit is annualised over 52 weeks. The High Income Child Benefit Charge uses the higher individual adjusted net income and is deducted once. Universal Credit and other means-tested support must be calculated externally for each exact working pattern, including any childcare reimbursement. A missing award is never copied from another scenario or replaced with zero. Monthly household cash equals annual net pay plus Child Benefit less its tax charge, plus entered benefits, Tax-Free Childcare and other tax-free income, less provider invoices, work costs and additional scenario costs, all divided by twelve. Incomplete inputs or unsupported tax circumstances prevent a complete cash result.

Time and the value of another working day

Annual committed time is actual weekly hours plus entered unpaid work plus return commuting time on office days, multiplied by 52 minus paid-leave weeks. Daily work, travel and homeworking costs use the same weeks; fixed work costs apply all year. Unpaid caring time is recorded separately across 52 weeks. Time released compares each pattern with the same household’s five-day main-adult pattern while keeping the partner fixed. The additional-day comparison shows the change in monthly cash, annual hours and each pension measure. Cash per additional hour divides the change in annual cash by the extra committed hours, and is omitted when no extra hours are required. It is a personal trade-off, not a market wage or a recommendation.

Affordability and uncertainty

If entered, ordinary living costs and the additional savings target are deducted from monthly cash to show a surplus or shortfall. The savings buffer covers an essential-living-cost shortfall only; missing a discretionary savings target does not consume the buffer automatically. Runway is a simple constant-cost estimate without returns or inflation. Test uncertain salaries, childcare quotes, office days and household costs by saving alternative comparisons. No probabilities, automatic winner or value for wellbeing are assigned. Confirm that reduced days really reduce workload and that the employer can offer the arrangement.

Limits that can change the decision

This version excludes Scotland, Wales and Northern Ireland; self-employment, directors’ NI, multiple jobs, non-standard tax codes and taxable savings, dividends, rent or pension income; non-pension salary sacrifice and childcare-voucher payroll arrangements; pension annual-allowance and access-related charges, final-salary and legacy NHS benefits, future revaluation and State Pension projections; uneven pay periods, one-off bonuses, loan payoff or write-off dates and annual loan refunds; mid-year changes, childcare grace periods, reconfirmation dates and actual benefit entitlement calculations. Mark a complex tax situation in the form if applicable: the tool will withhold complete cash results. Bank-holiday placement, paid-leave differences and term-time schedules require actual leave/quotes. Future career progression, promotion, parental leave and other employer benefits, wellbeing, household labour division and reversibility must be assessed separately.

Examples, privacy and reproducibility

The initial example is one adult earning £50,000 at 37.5 full-time hours, working 22.5/30/37.5 hours across three/four/five office days. It uses 5% net-pay employee and 3% employer pension contributions on base salary, 5.6 paid-leave weeks, a sixty-minute return commute and £8 travel per office day. No children, loans, extra support or ordinary living costs are assumed. Every example value can be inspected and changed. Browser inputs calculate locally and are stored in this browser only. Named comparisons change only on explicit Save or Load. Share links contain all assumptions in a URL fragment. Hosted REST and MCP calls run the same pure model only when deliberately invoked, with explicit complete inputs, no substituted defaults, versioned results and no intentional persistence.