IR35 Calculator Methodology — Full Transparency on Every Calculation

Income tax is calculated on taxable income after deducting the personal allowance. Our calculators apply progressive marginal rate bands: each portion of income is taxed at the rate for that band, not the entire income at the highest rate.

England, Wales, and Northern Ireland

• Personal allowance: £0 tax on the first £12,570 of income
• Basic rate (20%): £12,571 to £50,270
• Higher rate (40%): £50,271 to £125,140
• Additional rate (45%): Over £125,140

The personal allowance begins to taper by £1 for every £2 of income above £100,000, disappearing entirely at £125,140. For simplicity, our calculators use the standard £12,570 allowance and do not model high-income tapering — a limitation noted in the assumptions section below. Source: gov.uk/income-tax-rates

Scotland

Scottish income tax rates differ from the rest of the UK. The Scottish Government sets its own bands, which are applied by HMRC through a separate Scottish rate of income tax. Our calculators apply the following five bands when the location is set to Scotland:

• Starter rate (19%): £0 to £2,306
• Basic rate (20%): £2,307 to £13,991
• Intermediate rate (21%): £13,992 to £31,091
• Higher rate (42%): £31,092 to £62,570
• Top rate (48%): Over £62,570

Note that the Scottish higher and top rates (42% and 48%) are significantly higher than their England and Wales equivalents (40% and 45%), which means Scottish contractors inside IR35 or umbrella arrangements typically see a materially lower net take-home. The personal allowance (£12,570) is the same across all UK nations. Source: gov.uk/scottish-income-tax

National Insurance contributions (NICs) are calculated separately for employees (deducted from pay) and employers (paid on top of earnings). Inside IR35 and umbrella company calculations treat both as deductions from the total contract value, since the fee-payer must account for employer NI before paying the contractor.

Employee NI (Class 1)

• 8% on earnings between £12,570 and £50,270 per year (the "primary threshold")
• 2% on earnings above £50,270 per year

No employee NI is due on earnings below £12,570. These rates apply across the whole UK including Scotland.

Employer NI (Class 1 Secondary)

• 15% on earnings above £5,000 per year

The employer NI rate was increased from 13.8% to 15% in the Autumn 2024 Budget, effective from April 2025. The secondary threshold was also reduced from £9,100 to £5,000. This change has a significant impact on inside IR35 take-home pay because employer NI is paid from the contract value before the contractor receives their net pay. For a contractor earning £149,500 gross, the employer NI charge is approximately £21,675 — nearly 14.5% of gross income. Source: gov.uk/national-insurance-rates

Corporation tax applies only to the outside IR35 (limited company) scenario. It is charged on the company's taxable profits — gross income minus allowable expenses (in our model, the pension contribution is the only deduction).

• 19% on profits up to £50,000 (small profits rate)
• 25% on profits over £250,000 (main rate)
• Marginal relief: Profits between £50,000 and £250,000 attract a tapered rate between 19% and 25%

The marginal relief calculation follows the formula set out in CTA 2010, Section 18D. Our calculator computes the exact tapered rate using the fraction 1/200 (the standard marginal relief fraction for financial year 2026), applied to the difference between the upper limit and the company's profits, reduced by the number of associated companies (assumed to be one). Source: gov.uk/corporation-tax-rates

Dividend tax applies only to the outside IR35 (limited company) scenario, where post-corporation-tax profits are distributed to the shareholder. Dividends are taxed at lower rates than employment income to compensate for the fact they are paid from post-tax company profits.

• Dividend allowance: £500 (reduced from £1,000 in 2024/25, and from £2,000 in previous years)
• Basic rate band: 8.75% on dividend income within the basic rate band
• Higher rate band: 33.75% on dividend income within the higher rate band
• Additional rate band: 39.35% on dividend income within the additional rate band

The dividend allowance is not a true nil-rate band — it sits within the £500 at 0% and consumes basic-rate band headroom. Dividends above the allowance are taxed at the rate corresponding to the income tax band they fall into, determined by adding the dividend income to all other taxable income. Our calculator assumes the limited company director takes no salary (all profit is extracted as dividends), which is the most common arrangement for single-director contractor companies. Source: gov.uk/tax-on-dividends

Student loan repayments are calculated as 9% of gross employment income above the annual threshold for the applicable plan. The calculators support all four current repayment plans:

• Plan 1: 9% above £24,375/year (for students who started their course before 1 September 2012 in England or Wales)
• Plan 2: 9% above £27,295/year (for students who started between 1 September 2012 and 31 July 2023)
• Plan 4: 9% above £31,395/year (for Scottish students, regardless of when they started)
• Plan 5: 9% above £25,000/year (for English students who started after 1 August 2023)

Repayments are calculated on gross employment income before pension contributions. For inside IR35 and umbrella scenarios, the student loan deduction is applied after income tax and employee NI. This matches HMRC's collection methodology for PAYE. For the outside IR35 (limited company) scenario, student loan repayments do not apply because the director's income is taken as dividends rather than salary, and HMRC does not collect student loan repayments on dividend income. Source: gov.uk/repaying-your-student-loan

The statutory minimum pension contribution under auto-enrolment is 5% employee and 3% employer on qualifying earnings. Our calculator allows users to set a pension contribution percentage between 0% and 15% of gross income, giving flexibility to model salary sacrifice arrangements where the contractor chooses to contribute more than the minimum to reduce their income tax and NI liability.

For inside IR35 and umbrella scenarios, pension contributions are treated as salary sacrifice (deducted before income tax and employee NI), which reduces both the tax and NI burden. For the outside IR35 (limited company) scenario, the pension contribution is deducted from gross income before corporation tax is calculated, reducing company profits and therefore corporation tax. This reflects the most tax-efficient approach for each structure.

The inside IR35 calculation models the treatment of a contractor as a "deemed employee" for tax purposes. The fee-payer (client or agency) must operate PAYE on the full contract value after certain deductions. The step-by-step calculation is:

1. Start with gross annual income: day rate × days per week × weeks per year
2. Subtract pension contribution: gross × (pension% / 100)
3. Calculate employer NI: 15% on (gross − pension − £5,000 secondary threshold). This is deducted from the contract value before the employee's taxable income is determined.
4. Calculate apprenticeship levy: 0.5% of the total pay bill (gross income). This is also deducted from the contract value.
5. Calculate employment income: gross − pension − employer NI − apprenticeship levy = deemed employment income
6. Calculate income tax: Apply progressive marginal bands (20/40/45% for England, or the five Scottish bands) to employment income minus the personal allowance of £12,570.
7. Calculate employee NI: 8% on employment income between £12,570 and £50,270, plus 2% on income above £50,270.
8. Calculate student loan: 9% of (employment income − plan threshold), if a plan is selected.
9. Net take-home: employment income − income tax − employee NI − student loan.

The order of deductions matters. Employer NI and apprenticeship levy are subtracted before income tax because they represent costs borne by the contract value before the employment income is determined. Income tax and employee NI are then calculated on the resulting employment income figure, not the original gross.

The outside IR35 calculation models a contractor operating through their own limited company. The company receives the gross income, pays corporation tax on profits, and distributes the remaining post-tax profit as dividends to the shareholder-director. The step-by-step calculation is:

1. Start with gross annual income: day rate × days per week × weeks per year
2. Subtract pension contribution: gross × (pension% / 100). This reduces company profits and therefore corporation tax.
3. Calculate corporation tax: Apply the tapered rate (19% to 25%) to (gross − pension).
4. Calculate post-corporation-tax profit: (gross − pension) − corporation tax = distributable profit.
5. Calculate dividend tax: Apply the dividend allowance (£500 at 0%) and then 8.75% / 33.75% / 39.35% to the remaining dividend income, based on which tax bands the dividends fall into.
6. Net take-home: distributable profit − dividend tax.

Our model assumes the director takes no salary and draws all post-tax profit as dividends. In practice many contractor-directors take a small salary (up to the NI primary threshold of £12,570) to qualify for State Pension credits while avoiding employee NI. We omit this for simplicity since the salary figure is typically small relative to total income and the marginal tax difference is negligible at most contractor income levels. The pension contribution is assumed to be a company contribution, which reduces corporation tax liability.

The umbrella calculation follows the same steps as the inside IR35 calculation, with one additional deduction: the umbrella company's margin. Umbrella companies act as the contractor's employer of record, handling payroll, tax, and compliance. They charge a fee for this service, typically a weekly or monthly amount.

1. Start with gross annual income
2. Deduct umbrella margin: £25 per week × working weeks per year (default: £1,150 at 46 weeks)
3. Follow all inside IR35 steps from that point — pension deduction, employer NI (15%), apprenticeship levy (0.5%), income tax, employee NI (8%/2%), and student loan
4. Net take-home: remaining amount after all deductions

The £25/week margin reflects a typical rate in the UK umbrella market as of 2026. Some providers charge more (£30–35) and some less (£15–20). The umbrella margin is deducted before all taxes, meaning the contractor also pays tax on the amount that goes to the umbrella — which is the correct treatment since HMRC considers the full contract value (including the margin) to be employment income from which the umbrella deducts its fee.

One of the most common questions contractors ask is: "If my client moves my role inside IR35, what day rate do I need to negotiate to maintain my current take-home pay?" The rate uplift algorithm answers this question precisely using an iterative binary search.

The Problem

Given a current day rate under an outside IR35 arrangement producing an annual net take-home of T, find the new day rate X under an inside IR35 (or umbrella) arrangement such that calculateNet(X, inside) = T. This is not a simple multiplier because tax bands are progressive — adding £50/day may push income into a higher bracket, changing the marginal rate. There is no closed-form inverse function for the inside IR35 calculation due to the conditional branching (pension thresholds, NI bands, student loan thresholds, Scottish vs English bands), so we use a numerical method.

The Algorithm

1. Initialise search bounds: Set the lower bound lo = current day rate, and the upper bound hi = current day rate × 2.
2. Expand search space if needed: If calculateNet(hi, inside) is still less than T, repeatedly double hi until the target is bracketed. This handles the edge case where the required uplift exceeds 100% (unusual but possible at very high pension contributions).
3. Binary search: For 60 iterations (or until convergence), compute mid = (lo + hi) / 2, compute netMid = calculateNet(mid, inside). If netMid is less than target, set lo = mid; otherwise set hi = mid. Stop early if |netMid − target| < £1.
4. Round: Round the result to the nearest £5, producing a realistic rate for contract negotiation. Day rates are almost always quoted in multiples of £5 or £10.

Why 60 Iterations?

The binary search halves the search space on each iteration. After 60 iterations, the interval is reduced by a factor of 2⁶⁰ ≈ 1.15 × 10¹⁸. Starting from an initial range the size of the day rate (e.g. £500), this guarantees convergence to within well under £1 — far more precision than needed. We use 60 rather than a stopping condition to ensure consistent behaviour across all inputs without edge-case failures. In practice the search converges within 30–40 iterations; the extra 20 iterations cost negligible compute time (a few microseconds in JavaScript).

Pseudocode

function calcUplift(targetNet, currentRate, days, weeks, pensPct, loc, loanPlan, scenario):
    lo = currentRate
    hi = currentRate * 2

    // Expand search space until target is bracketed
    while calculateNet(hi, days, weeks, pensPct, loc, loanPlan, scenario) < targetNet:
        hi += currentRate

    // Binary search (60 iterations = < £1 precision)
    for i in 0..60:
        mid = (lo + hi) / 2
        netMid = calculateNet(mid, days, weeks, pensPct, loc, loanPlan, scenario)

        if netMid < targetNet:
            lo = mid
        else:
            hi = mid

        if |netMid - targetNet|< 1:
            break

    return round(lo / 5) * 5  // nearest £5

The apprenticeship levy is charged at 0.5% of the total pay bill. Technically, the levy only applies when an employer's annual pay bill exceeds £3 million. However, in practice, many umbrella companies and fee-payers in the contractor supply chain pass through an equivalent 0.5% charge as a line item deducted from the contract value before paying the contractor. This is because the levy is calculated at the group level across all employees, and agencies/umbrellas that handle many contractors may exceed the £3 million threshold.

For completeness, our calculators deduct 0.5% of gross income in both the inside IR35 and umbrella scenarios. Most individual contractors will not trigger the levy themselves through their own limited company, so the apprenticeship levy is not applied in the outside IR35 calculation. If you are certain your umbrella company does not pass through the levy, you can mentally adjust the results — the difference is approximately £500–£750 per year depending on your gross income.

Our calculators provide estimates based on published HMRC rates and standard assumptions. They are designed for illustrative and educational purposes. The following limitations apply to all calculations on TaxRateHub:

• No other employment income modelled: All calculations assume the contractor's only source of income is from the contract being evaluated. Additional income from a second contract, part-time employment, rental income, or investment returns would change the tax bands applied to the contract income.
• Standard personal allowance only: We use the full £12,570 personal allowance. High-income earners (over £100,000) will have their personal allowance tapered by £1 for every £2 of income above £100,000, which increases effective tax rates. Our calculators do not model this tapering.
• No spouse salary splitting: Married couples can optimise tax by having the lower-earning spouse receive salary or dividends from the company. Our calculators assume a single director-shareholder.
• No pension carry forward: Our pension model applies a simple percentage deduction. We do not model the £60,000 annual allowance, the money purchase annual allowance (MPAA), or carry-forward of unused allowances from previous tax years.
• No R&D credits: Contractors developing new software or processes may qualify for Research & Development tax credits, which can significantly reduce corporation tax. These are not modelled.
• No VAT considerations: The calculators work with gross income before VAT. Contractors registered for VAT (whether flat rate or standard) will need to account for VAT separately. Our figures assume the contractor is not VAT registered.
• No capital gains: We do not model capital gains tax on the sale of company shares, assets, or investments. Entrepreneurs' Relief (Business Asset Disposal Relief) and other CGT reliefs are outside the scope of these calculators.
• No marriage allowance: The marriage allowance (transfer of £1,260 of personal allowance to a spouse) is not modelled.
• Assumes all Ltd company post-tax profit is drawn as dividends: In practice, many contractor-directors retain some profit in the company for future expenses, investments, or to manage dividend tax bands across multiple tax years. Our model assumes 100% distribution.
• Always consult a qualified accountant: These calculators are planning tools, not a substitute for professional tax advice. Tax legislation changes frequently, and individual circumstances vary significantly. Always verify your actual tax position with a qualified accountant or tax adviser.

All tax rates and thresholds used on TaxRateHub are sourced from official UK government publications. The following references apply to the 2026/27 tax year:

1. Income Tax rates and personal allowances — GOV.UK
2. Scottish Income Tax rates — GOV.UK
3. National Insurance rates and thresholds — GOV.UK
4. Corporation Tax rates — GOV.UK
5. Tax on dividends — GOV.UK
6. Repaying your student loan — GOV.UK
7. Workplace pensions and automatic enrolment — GOV.UK
8. Apprenticeship Levy guidance — GOV.UK
9. Off-payroll working (IR35) guidance — GOV.UK
10. HMRC rates and allowances — GOV.UK