Welcome. If you run a small UK limited company on your own, or you've just incorporated and the words "corporation tax", "trial balance" and "CT600" make your shoulders rise, this tutorial is for you. The goal here is simple: by the end of five weeks you will know what your accounts mean, where the numbers come from, how they turn into a corporation tax bill, and how that bill gets filed with HMRC and Companies House. You will still want a good accountant for anything tricky, but you'll no longer be handing over a shoebox of receipts and hoping for the best. You'll be the kind of director who can read their own statutory accounts, spot an obvious mistake, and ask better questions. That confidence is worth quite a lot, both in money and in peace of mind.
This tutorial is written for:
- Solo limited company owners who do their own bookkeeping in Xero, QuickBooks or FreeAgent and want to understand what comes out the other end.
- Freelancers and contractors who have recently incorporated and are now staring at year-end for the first time.
- Founders of very small Ltds (one to three people, modest turnover) who want to understand the books before raising money, hiring, or handing more to an accountant.
- Anyone who has been quietly nodding through accountant meetings for a few years and would prefer to know what's actually being said.
This tutorial is not for:
- Qualified accountants or FCAs. You will find it slow and over-explained.
- Companies with consolidated group accounts, foreign subsidiaries, large R&D claims, share schemes, or anything that touches transfer pricing.
- Anyone with an HMRC enquiry already open. Get a chartered accountant or tax adviser today.
- Replacing professional advice when something genuinely complicated happens. We'll be clear about where those lines are.
If you are a tiny Ltd that fits comfortably inside the FRS 105 micro-entity thresholds (turnover under £1m, balance sheet under £500k, ten or fewer employees), this tutorial is aimed squarely at you.
After working through all five weeks, you will be able to:
- Read a trial balance out of Xero or QuickBooks and explain every line.
- Produce a basic profit and loss account and balance sheet and reconcile them back to the trial balance.
- Post simple year-end accruals, prepayments and depreciation journals.
- Prepare a set of FRS 105 micro-entity accounts suitable for filing with Companies House.
- Walk the bridge from accounting profit to taxable profit, adding back disallowables and substituting capital allowances for depreciation.
- Calculate corporation tax including the small profits rate, the main rate, and marginal relief.
- Map a trial balance to the right CT600 boxes.
- File a CT600 plus iXBRL accounts using commercial software (and know why HMRC's free service no longer exists).
- File micro-entity accounts with Companies House via WebFiling.
- Spot common mistakes in your own Xero file before they become an expensive problem.
To keep things concrete, every example, journal and number in this tutorial belongs to one fictional company: Acorn Studio Ltd.
- Trade: Freelance web design and branding for small UK businesses.
- Director and sole shareholder: Sarah Brown.
- Incorporated: 1 April 2024.
- Year-end (accounting reference date): 31 March.
- Bookkeeping: Xero, with a bank feed from Starling Business.
- VAT registered? Not yet in Weeks 1 to 4 (turnover sits below the £90,000 threshold). Becomes VAT registered partway through Week 5.
Weeks 1 to 4 cover Acorn Studio's year ended 31 March 2026 a clean, mid-sized small-company year with a turnover of £74,800. Week 5 then advances time by one year and adds the kinds of complications real companies actually run into: VAT registration, an employee, dividends large enough to matter, an overdrawn director's loan, and a question about whether some software development counts as R&D.
You're going to know Sarah and Acorn Studio very well by the end. That's deliberate. Real understanding sticks better when it's attached to a particular set of numbers.
Five weeks. Each week is about 90 minutes of focused reading and worked examples, plus a 30 minute exercise you do in your own Xero account or on paper. You can compress that into a weekend if you want, but spaced repetition works better.
| Week | Topic | Files | Focused study | Exercise |
|---|---|---|---|---|
| 1 | Books and the trial balance | Week-01-Books-and-Trial-Balance.md | ~90 min | ~30 min |
| 2 | Micro-entity statutory accounts | Week-02-Micro-Entity-Accounts.md | ~90 min | ~30 min |
| 3 | From accounting profit to taxable profit | Week-03-Tax-Adjustments.md | ~90 min | ~30 min |
| 4 | The CT600 and filing | Week-04-CT600-and-Filing.md | ~90 min | ~30 min |
| 5 | A messier year: VAT, payroll, DLA, dividends, R&D | Week-05-Messy-Example.md | ~120 min | ~45 min |
Plus the reference file you'll keep coming back to: Glossary.md.
A rough total of ten to twelve hours, spread over five weeks. That's the price of being able to read your own books.
You don't need much, but please have these to hand before you start Week 1:
- A free trial of Xero or QuickBooks (both offer 30 days free; FreeAgent is also fine if you already use it). You'll get most value if you can actually click around as you read.
- A notebook or text file to scribble in. Some of this stuff only clicks when you write it out.
- A basic calculator, or a spreadsheet open in another window.
- Around two hours a week for five weeks, and the willingness to sit with concepts that feel uncomfortable for a bit.
- Optional but useful: a copy of your own company's trial balance for last year, even if you can't share it. Applying each week's lesson to your own numbers is the single biggest accelerator.
You do not need any prior accounting knowledge. We start from the absolute beginning.
There are three sensible ways to work through it:
- Top to bottom, week by week. Read the file, do the worked examples in your notebook, take the mini-quiz at the end. Don't skip ahead. The five weeks are deliberately sequenced.
- Top to bottom plus the bonus exercise. Each week ends with a "What's in your Xero" sidebar telling you exactly where to click. Do that. Even five minutes of clicking around in a real ledger after each chapter compounds enormously.
- Apply it to your own Xero file. The ambitious path. Pause after each week and redo the lesson with your own company's numbers. You'll find things you didn't know were broken. That's the point.
Each week file follows the same shape:
- A learning-objectives box at the top so you know what you're aiming for.
- Worked examples using Acorn Studio Ltd's numbers.
- Bold key terms on first use, linked to the Glossary.
- A Common errors to watch for section near the end.
- A mini quiz of five questions, with answers in a collapsible block.
- A What's in your Xero sidebar.
- A short Next week preview so you know what's coming.
A few important caveats. Please read these:
- This tutorial is educational, not advice. It is general information about how UK corporation tax works for small limited companies. It is not personal tax advice, regulated accountancy advice, or a substitute for a qualified professional.
- It covers UK companies only. It assumes a company incorporated in England, Wales, Scotland or Northern Ireland and tax-resident in the UK. Cross-border situations are out of scope.
- It is current as of 28 May 2026. Rates, thresholds and deadlines used here reflect the position on that date, including FY2025 and FY2026 corporation tax rates, the £90,000 VAT registration threshold, the £1m AIA, FRS 105 micro-entity thresholds from 6 April 2025, and the closure of HMRC's free CATO joint-filing service on 31 March 2026.
- Tax law changes. A rule that's true today may not be true next year. Before relying on anything here for a real filing, sanity-check the current position on GOV.UK or with an accountant.
- Get an accountant for non-trivial situations. Some specific flags: an HMRC enquiry; a director's loan over £10,000; an R&D claim of any size; a share scheme; selling the company; bringing in an investor; group structures; or anything that has you genuinely worried. A few hundred pounds of professional advice will save you more than that.
This tutorial is written first as a set of Markdown files so the content can stand on its own. When it gets ported to an interactive web course, here's what the interactive layer should add:
- Progress tracking per user. Mark each week complete, save quiz scores, resume where the reader left off.
- Collapsible callouts for Tips, Warnings, and worked examples. Reader can expand or hide all at once.
- Embedded trial balance simulator for Weeks 1 and 2. The reader can post journals against Acorn Studio's chart of accounts and see the trial balance, P&L, and balance sheet update live.
- CT600 box-mapping drag-and-drop for Week 4. Drag trial balance lines onto the right CT600 boxes and get instant feedback.
- CT computation calculator for Week 3. Plug in profit-before-tax, depreciation, disallowables and qualifying capital spend; see the taxable profit, marginal relief and CT charge calculated step by step.
- Checkpoint quizzes at the end of each week, scored, with optional retry. Aggregate score shown on a final dashboard.
- Printable PDF cheat sheets for each week: a one-page summary of the key concepts, rates and journals.
- CSV upload of your own trial balance. A reader who's brave enough can upload their Xero TB export and the tool will run through the Week 3 adjustments alongside Acorn Studio's, side by side.
- Glossary tooltips. Hover over any bolded term to see its glossary definition without leaving the page.
Start here:
- Week 1: Books and the Trial Balance
- Week 2: Micro-Entity Statutory Accounts
- Week 3: From Accounting Profit to Taxable Profit
- Week 4: The CT600 and Filing
- Week 5: A Messier Year
- Glossary
Good luck. Take it slow. You're going to surprise yourself.