Cash Flow Calculator UK

Cash flow is the lifeblood of any business. For UK business owners, managing cash flow effectively means more than just paying suppliers and staff – it means being prepared for tax payments, understanding your tax liabilities throughout the year, and avoiding the shock of unexpected bills. For the 2026/27 tax year, significant changes including Making Tax Digital for Income Tax, new capital allowance rules, and updated HMRC interest rates mean cash flow planning is more important than ever.

Why Cash Flow Matters for Tax

Cash flow problems are one of the leading causes of business failure. Even profitable businesses can run into trouble if they are not prepared for tax payments. HMRC does not accept cash flow difficulties as a reasonable excuse for late payment. Understanding your tax cash flow means knowing when payments are due and ensuring you have enough funds available.

For the 2026/27 tax year, several factors affect business cash flow:

• Making Tax Digital for Income Tax – From 6 April 2026, sole traders and landlords with income over £50,000 must submit quarterly digital updates. HMRC states that you will be able to "view up-to-date cash flow, fix any errors and see an estimate of your tax bill after sending each update". This means no more January surprises.
• Capital allowance changes – The main rate WDA has reduced from 18% to 14% from April 2026. However, the new 40% First Year Allowance from 1 January 2026 and the permanent £1 million AIA limit provide opportunities to reduce tax bills.
• HMRC interest rates – The late payment interest rate is 7.75% from 9 January 2026, making late payments more expensive than ever.

Key Tax Deadlines That Affect Cash Flow

Knowing when tax payments are due is the first step to managing cash flow. Here are the key dates for 2026/27:

Self Assessment Deadlines

Deadline

Action

31 July 2026

Second payment on account due (2025/26 tax year)

5 October 2026

Register for Self Assessment

31 October 2026

Paper Self Assessment returns due

30 December 2026

Opt into PAYE

31 January 2027

Online tax returns and first payment on account due

Corporation Tax Deadlines

Corporation Tax is due nine months and one day after your accounting period ends. For example, if your accounting period ends on 31 March 2026, payment is due by 1 January 2027. You have no choice about when you pay, so you must plan well in advance.

VAT Deadlines

VAT returns and payments are due one month and seven days after the end of your VAT period. For example, for the period ending 31 March 2026, payment is due by 7 May 2026. Missing this deadline results in surcharges and interest.

PAYE and National Insurance Deadlines

Employers must pay PAYE and National Insurance deductions by the 22nd of each month (or 19th if paying by cheque). These regular payments require consistent monthly cash flow management.

Understanding Payments on Account

The Payment on Account system is often a surprise for new self-employed taxpayers. It requires you to pay your Self Assessment tax and Class 4 National Insurance in advance, split into two instalments.

How it works:

• Your first payment on account is due on 31 January (the same date your tax return is due)
• Your second payment on account is due on 31 July
• Each payment is half of your previous year's tax bill

Example: If your 2025/26 tax bill was £8,000, you will pay:

• 31 January 2027: £4,000 (first payment on account for 2026/27)
• 31 July 2027: £4,000 (second payment on account for 2026/27)
• Plus a balancing payment for 2026/27 due on 31 January 2028

This means you need to save for tax throughout the year, not just when you file your return. Our Income Tax calculator can help you estimate your tax bill.

How Making Tax Digital Changes Cash Flow Planning

From 6 April 2026, Making Tax Digital for Income Tax (MTD ITSA) brings significant changes for sole traders and landlords with income above £50,000. Understanding these changes is essential for effective cash flow planning.

MTD requirements:

• Use MTD-compatible software
• Maintain digital records
• Submit four quarterly updates to HMRC
• Submit a final declaration by 31 January

How MTD helps with cash flow:

According to HMRC, you will be able to "view up-to-date cash flow, fix any errors and see an estimate of your tax bill after sending each update". This means:

• You will know your estimated tax bill throughout the year, not just in January
• You can fix errors as you go, avoiding unexpected adjustments
• You can see if your tax bill is higher or lower than expected and adjust your saving

Important: You do not pay your tax bill four times a year. The MTD quarterly updates are for reporting purposes only. Your actual tax payment remains due on 31 January.

For more information on tax registration and filing, our Self Assessment complete guide explains the process step by step.

Strategies to Improve Tax Cash Flow

1. Forecast Early and Often

"One of the most effective ways to manage corporation tax cash flow is through early forecasting. Rather than waiting until accounts are finalised". A 12-month rolling cashflow forecast is the foundation of stronger finances, updating every month to keep visibility ahead of decisions.

1. Save Monthly for Tax

For self-employed individuals, saving 25-30% of income for taxes is a common recommendation. This ensures funds are available when tax bills arrive. Consider opening a separate savings account specifically for tax.

1. Use Capital Allowances Strategically

The new 40% First Year Allowance (from 1 January 2026) increases tax relief for qualifying new investments. Full expensing remains available for limited companies, and the AIA limit of £1 million provides immediate 100% relief. Strategic use of capital allowances can reduce your tax bill and improve cash flow.

For example, if a business purchases machinery costing £80,000 and claims AIA:

• Tax saving: £80,000 × 19% = £15,200
• Effective cost after tax relief: £64,800
• This is £15,200 better for cash flow than without the relief

Our Annual Investment Allowance guide explains this in more detail.

1. Consider Cash Basis Accounting

The cash basis is a simpler way of reporting income and expenses for income tax purposes for small businesses. It allows eligible businesses to account for business income and expenses when money is received or paid out, not on the date goods or services are invoiced. This can help align your tax bill with your actual cash flow.

For example, if you invoice a client in March 2026 but they pay in May 2026:

• Cash basis: The income is taxed in 2026/27 (when received)
• Traditional basis: The income is taxed in 2025/26 (when invoiced)

This can be particularly helpful for cash flow management.

1. Consider VAT Cash Accounting

The VAT Cash Accounting Scheme enables businesses to account for VAT only when payment is made or received, aligning VAT liability with cash flow. It provides automatic protection against unpaid invoices and smooths cash flow.

1. Use HMRC Payment Support

If you are struggling to pay, HMRC's Time to Pay arrangements can help spread the cost. For Self Assessment bills up to £30,000, you can set up an arrangement online without contacting HMRC. For VAT, arrangements can be set up for debts up to £50,000. Interest accrues on the outstanding balance at 7.75% from 9 January 2026, so this should be used as a last resort.

Common Cash Flow Mistakes

Not forecasting tax liabilities early – waiting until accounts are finalised leaves no time to plan. By then, it is often too late to take corrective action.

Forgetting about payments on account – many taxpayers are surprised by the July payment on account, which is due seven months after the January payment. A common mistake is to assume the tax is all paid in January, only to face a second bill six months later.

Not setting aside money for tax – treating all income as available to spend. This is especially common among new self-employed people who are not used to paying tax directly.

Missing MTD quarterly updates – from 6 April 2026, this is mandatory for those above the £50,000 threshold. Penalties for late submission apply.

Ignoring capital allowance opportunities – strategic use of AIA, FYA and full expensing can significantly improve cash flow by reducing the effective cost of investment.

Assuming cash flow problems are a reasonable excuse – HMRC does not accept this as a valid reason for late payment. A shortage of funds is not, in itself, a reasonable excuse.