How Accounted Calculates Your Tax in Real Time

The January Surprise Problem

Every January, millions of UK sole traders experience the same unpleasant shock. They sit down to file their self-assessment return (or their accountant does it for them) and discover that their tax bill is significantly more than they expected. Sometimes hundreds more. Sometimes thousands.

This happens because most sole traders have no real visibility into their tax position during the year. They know roughly what they've earned, they have a vague sense of their expenses, but the actual calculation -- income tax, Class 2 National Insurance, Class 4 National Insurance, payments on account -- remains a mystery until someone sits down with all the numbers.

The consequences are real. According to HMRC statistics, thousands of taxpayers enter Time to Pay arrangements each year because they can't afford their tax bill when it arrives. That's not because they didn't earn enough to pay -- it's because they didn't know the amount in time to save for it.

Accounted solves this with real-time tax calculations that update with every transaction Penny processes.

How Real-Time Tax Calculation Works

Every time a new transaction enters your Accounted books -- whether it's income received, an expense categorised, or a receipt matched -- Penny recalculates your estimated tax position. Not once a quarter. Not when you ask. Every single time your data changes.

Here's what's happening behind the scenes:

Step 1: Income Tracking

Every payment received into your connected bank account is analysed. Penny identifies business income based on the source, amount, and pattern. Regular payments from clients are recognised as turnover. Interest income, refunds, and personal transfers are correctly excluded.

Your gross income figure updates in real time.

Step 2: Expense Tracking

Every business expense is categorised against HMRC-allowable categories using the three-tier reasoning engine. The total of your allowable expenses updates as transactions are processed and categorised.

Step 3: Taxable Profit Calculation

Taxable profit equals gross income minus allowable expenses. This figure -- not your turnover -- is what you're taxed on. Penny calculates it continuously.

Step 4: Tax Calculation

With your taxable profit figure, Penny applies the current tax year's rates and thresholds:

Income Tax:

• Personal allowance: £12,570 (no tax on this amount)
• Basic rate: 20% on taxable income from £12,571 to £50,270
• Higher rate: 40% on taxable income from £50,271 to £125,140
• Additional rate: 45% on taxable income above £125,140

Class 2 National Insurance:

• Flat rate: £3.45 per week if profits are above the Small Profits Threshold (£6,725)

Class 4 National Insurance:

• 6% on profits between £12,570 and £50,270
• 2% on profits above £50,270

Payments on Account:

• If your previous year's tax bill was above £1,000 (and less than 80% was collected at source), you'll need to make payments on account -- advance payments towards next year's tax bill, each equal to 50% of the current year's liability

Penny applies all of these calculations using integer arithmetic (no rounding errors) and the correct tax year rates.

Step 5: The Result

All of this runs in the background. When you ask Penny for your tax position, or when she proactively shares it, you get a clear, current figure:

Penny: Here's your estimated tax position for 2025/26 (as of 28 February 2026):

• Gross income: £48,200
• Allowable expenses: £15,670
• Taxable profit: £32,530

Tax breakdown:

• Income tax: £3,992

• Class 2 NIC: £179

• Class 4 NIC: £1,198

• Total liability: £5,369

• Payments on account already made: £2,400

• Estimated balance due 31 Jan 2027: £2,969

• Estimated first payment on account for 2026/27: £2,685

Total due 31 Jan 2027: £5,654

That's your complete picture. Not an estimate made months ago. Not a rough calculation. Your actual position based on every transaction processed to date.

Why Real-Time Matters

You Can Save for Your Tax Bill

The most practical benefit: when you know your tax liability throughout the year, you can set aside money regularly rather than scrambling in January. If Penny tells you in March that you're on track for a £5,000 tax bill, you know to save roughly £400 per month. No surprises.

You Can Make Informed Decisions

Thinking about a large business purchase? Ask Penny how it would affect your tax bill. Considering taking on a new contract? Penny can tell you which tax band your additional income falls into. This kind of real-time tax awareness helps you make better financial decisions.

You Spot Problems Early

If your tax liability is growing faster than expected, you'll know in real time -- not nine months later. Maybe your expenses are lower than usual. Maybe you've had an unexpectedly good quarter. Either way, early awareness gives you time to adjust.

Payments on Account Are Less Shocking

Payments on account catch many sole traders off guard because they're calculated on the previous year's tax bill. Penny tracks these throughout the year and includes them in your total liability calculation, so the January figure doesn't come as a shock.

Tax Position for Different Business Types

Accounted's real-time tax calculation adapts to your business type:

Sole Traders

The standard calculation described above: income tax, Class 2 NIC, Class 4 NIC, and payments on account.

Landlords

For property income, Penny applies the specific rules:

• Mortgage interest restriction (Section 24) -- only a basic rate tax credit, not a full deduction
• Allowable expenses specific to property (repairs, insurance, management fees)
• Multiple property aggregation
• Distinction between revenue expenditure (repairs) and capital expenditure (improvements)

Limited Company Directors

Corporation Tax estimation plus personal tax on salary and dividends. Read our full limited company guide for details.

Employed and Self-Employed

If you have employment income alongside your self-employment, Penny factors in your PAYE tax already paid through your employer. Your self-employment tax is calculated on the combined income, minus what's already been collected through PAYE. This prevents the common mistake of calculating self-employment tax in isolation and being shocked when the combined figure is higher.

VAT Tracking

For VAT-registered sole traders, Penny tracks VAT separately and continuously:

• Output VAT: The VAT you've charged on your invoices (or that's implied in your income if you're on the flat rate scheme)
• Input VAT: The VAT you've paid on business purchases, extracted from receipts and bank transaction data
• Net VAT position: What you owe HMRC (output minus input) or what HMRC owes you

You: What's my VAT position?

Penny: For the current VAT quarter (1 Jan - 31 Mar 2026):

• Output VAT (collected): £3,240
• Input VAT (paid): £1,890
• Net VAT due: £1,350

Your VAT return is due by 7 May 2026.

This feeds directly into your MTD VAT submissions.

Proactive Tax Alerts

Penny doesn't just calculate your tax when you ask. She proactively alerts you to important tax events:

Threshold Alerts

If your income is approaching the VAT registration threshold (currently £90,000), the higher rate tax band, or the personal allowance taper, Penny tells you in advance:

Penny: Heads up -- your taxable profit has reached £48,500. If it goes above £50,270, you'll start paying 40% higher rate tax on the excess. You might want to bring forward any planned business expenses to stay within the basic rate band this year.

Opportunity Detection

Penny identifies tax-saving opportunities based on your data:

Penny: I noticed you haven't claimed any capital allowances this year. If you've bought any equipment, tools, or vehicles for your business, these could reduce your tax bill. Have you made any purchases like that?

Payment Reminders

As the 31 January deadline approaches, Penny reminds you of the exact amount due, broken down by balancing payment and payment on account, giving you time to ensure the funds are available.

Accuracy and Limitations

Penny's tax calculations are based on the current published tax rates and thresholds. They're accurate to the penny (no pun intended) based on the data available. However, a few limitations are worth noting:

• Estimates, not returns: The figures are estimates based on year-to-date data, projected to the end of the tax year. Your actual liability may differ if your income or expenses change in the remaining months.
• No capital gains: Capital gains tax (on selling assets, property, or investments) is not included in the standard estimate. This requires specialist calculation.
• Tax code complexities: If you have unusual tax codes, marriage allowance transfers, or other HMRC adjustments, the estimate may not capture these fully.

For the vast majority of sole traders, the real-time estimate is accurate within a few per cent of the final self-assessment figure. And it's infinitely better than having no idea until January.

Getting Started

Connect your bank to Accounted, and Penny starts calculating your tax position from your first transaction. The longer she has your data, the more accurate the projections become.

Sign up for free -- 14-day trial, no credit card required. Or explore all of Accounted's features to see the full picture.

Useful Resources

• HMRC's MTD for Income Tax requirements
• HMRC's list of MTD-compatible software