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What are boxes 435 and 440 on the CT600? (Corporation Tax chargeable)

Reviewed by Lee Jones, Founder · Updated 16 July 2026
The short answer

Box 440 is called "Corporation Tax chargeable" and it is your company's Corporation Tax bill for the period, before any final adjustments. Box 435, "Marginal relief", is a discount that some companies get on the way there: if your profits sit between £50,000 and £250,000, a reduction comes off the full-rate tax, and what is left lands in box 440. For most small companies, box 440 is the number they actually end up paying.

Official source. This guide is a plain-English summary of official GOV.UK guidance, not advice. The authoritative source is The Company Tax Return guide on gov.uk. Always rely on that over our summary.

What goes in them?

These two boxes are the last steps of a short chain that starts with your profit.

Step 1: your taxable profit. Box 315 holds your company's taxable profit for the period. Our guide on box 315 explains where that figure comes from.

Step 2: the rate is applied. The form has a set of rows, one for each "financial year" your accounting period touches, where the profit is multiplied by the tax rate. The rate is 19% if your profits are £50,000 or less, and 25% if they are over £250,000. The tax from those rows is added up in box 430, which the form calls simply "Corporation Tax".

Step 3: the discount, if you get one. If your profits are between £50,000 and £250,000, the rows use the full 25% rate first, and then a reduction called Marginal Relief goes in box 435 to bring the bill back down. It is worked out with a set formula, and HMRC has a free online calculator that does it for you.

Step 4: the bill. Box 440 is box 430 minus box 435. That is your Corporation Tax chargeable.

Two quick examples, both for a normal 12-month year with no other companies in the picture:

  • Profits of £30,000. Tax is 19%, so £5,700 goes in box 430. There is no Marginal Relief, so box 435 stays empty and box 440 is also £5,700.
  • Profits of £100,000. The rows use 25%, so £25,000 goes in box 430. Marginal Relief of £2,250 goes in box 435, and box 440 shows £22,750. So the company pays roughly 22.75%, not the full 25%.

What usually goes wrong?

The classic mistake is with profits between £50,000 and £250,000: people either forget Marginal Relief exists and pay the full 25%, or they try to type a lower rate straight into the rate rows. Both are wrong. The form wants 25% in the rows and the discount shown separately in box 435.

Two things quietly shrink those £50,000 and £250,000 limits, and catch people out. If your accounting period is shorter than 12 months, the limits shrink to match. And if your company is connected to other companies (HMRC calls these associated companies), the limits are shared out between them: with 3 other associated companies, the limits drop to £12,500 and £62,500. If either applies to you, do not use the headline limits.

One more wrinkle: if your accounting year crosses 1 April, the form splits your profit across two sets of rows, one for each financial year. Our guide on the financial year rows covers this.

Do I have to work this out myself?

No. Any filing software does this arithmetic for you, and HMRC's free online calculator will check a Marginal Relief figure if you want a second opinion. SimpleReturns builds the whole calculation, including box 435 and box 440, from your company's bank statement and a few plain-English questions, shows you every figure before anything is sent, and £99 covers both filings.

If your company has associated companies or is part of a group, an accountant is genuinely the right call instead.

Common questions

My profits are under £50,000. Do I need box 435?

No. At £50,000 or less your whole profit is taxed at 19%, there is no Marginal Relief, and box 440 simply matches box 430.

Is box 440 the amount my company actually pays?

Usually yes, for a small company. A few rare reliefs that most small companies never use can come off between box 440 and box 475, and some extra charges can be added further down the form. Box 475 shows the running total after those reliefs, but before any extra charges are added. Our guide on box 475 picks up the story from here.

Why does my software show 25% when my profits are only £100,000?

Because that is how the form works. The rows always use the full rate for profits over £50,000, and the discount is shown separately in box 435. Box 440 shows the real bill, roughly 22.75% in this example.

Want the boxes filled in for you?

SimpleReturns builds the tax calculation, including Marginal Relief, from your company's bank statement and a few plain-English questions, and shows you boxes 430, 435 and 440 with every figure explained before anything is sent. It is free to start, no card needed, and £99 flat covers both filings.

Start your return

And if your company has associated companies or sits in a group, we will say plainly that an accountant is the right call rather than pretend otherwise.

General guidance, not advice. This guide explains how the rules generally work for small UK limited companies. It isn't tax advice for your specific situation, if you're unsure, check with us or an accountant.