SimpleReturns

What are boxes 380 to 395 on the CT600? (rates and marginal relief)

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

Boxes 380 to 395 are one row of the small tax table on the CT600. Box 380 names a financial year, box 385 says how much of your profit is taxed in that year, box 390 gives the tax rate, and box 395 gives the tax that comes out. You only reach this row when your company's year overlaps two of HMRC's financial years, because boxes 330 to 375 handle the first year and boxes 380 onwards handle the second. One thing this row is not: the marginal relief box. That discount, if you get it, goes in box 435 further down.

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?

This part of the form is a sum written out in a grid: profit, times rate, equals tax, shown separately for each financial year.

HMRC's financial year runs from 1 April to 31 March. If your company's accounting year runs, say, 1 July to 30 June, it sits across two of HMRC's years, so the form gives you a block for each: boxes 330 to 375 for the first, boxes 380 to 425 for the second. Box 380 holds the second year (as a year, like 2026), box 385 the slice of profit that falls in it, box 390 the rate, and box 395 the tax on that slice.

The rate in box 390 is one of two numbers:

  • 19% if your company's profit is £50,000 or less (the small profits rate)
  • 25% if it is more than £250,000 (the main rate)

In between those two, your company pays the 25% rate in the grid but then gets a discount called marginal relief, which is entered lower down in box 435 and taken off the grid's total to give your final bill in box 440. In one sentence: the discount is biggest just above £50,000 and shrinks to nothing at £250,000, so your overall rate climbs smoothly from 19% towards 25% as profit grows. For example, a company with £100,000 of profit works out tax at 25% (£25,000), gets £2,250 of marginal relief, and ends up paying £22,750.

If your profit is in the 19% band or the marginal relief band, there is one more small job: put an X in box 329, and enter your number of associated companies in box 326 (zero if you have none), or in boxes 327 and 328 if your year spans two financial years.

What usually goes wrong?

The trap in this part of the form is associated companies. The £50,000 and £250,000 limits are not per person, they are shared out. If your company has one associated company (broadly, another company under the same control), the limits are split between the two of them and drop to £25,000 and £125,000. With three associated companies they fall to £12,500 and £62,500. So a company with £40,000 of profit pays 19% on all of it if it stands alone, but loses the small profits rate if its owner also controls one other company, because £40,000 is then above the halved £25,000 limit. A short accounting period shrinks the limits in the same way.

Two smaller stumbles: forgetting the X in box 329 when you are claiming the small profits rate or marginal relief, and trying to squeeze the marginal relief pounds into this grid. The grid only ever shows profit at 19% or 25%; the relief itself lives in box 435.

One honest limit: certain dividends your company receives can also count towards the £50,000 and £250,000 tests, and the associated company rules have real depth. If you have several companies, or a group, an accountant is the right call for this bit.

Do I have to work this out myself?

No. Any filing software does this arithmetic for you, splitting the profit across the years, picking the rate, and computing the relief. SimpleReturns builds the whole grid from the profit it works out from your bank statement and a few plain-English questions, works out any marginal relief, and shows you every number before anything is sent. It is free to start, no card needed, and filing costs £99 flat for both filings (the tax return to HMRC and the accounts to Companies House).

Common questions

My profit is £30,000. Do these boxes matter to me?

Only lightly. Your whole profit sits in the 19% band, so the grid shows it at 19% and there is no marginal relief to claim. Just remember box 329 and box 326.

Where does the marginal relief amount actually go?

Box 435. The form then does box 430 (the tax from the grid) minus box 435 to give box 440, the Corporation Tax chargeable. Our guide to boxes 435 and 440 walks through it.

What counts as an associated company?

Broadly, another company controlled by the same person or people as yours, at any point in the period. The detailed rules go deeper than that, so if you own or part-own more than one company, check with an accountant before relying on the full £50,000 and £250,000 limits.

Want boxes 380 to 395 filled in for you?

The software fills the whole tax calculation grid for you: it splits profit across HMRC's financial years, picks the right rate, works out any marginal relief, and shows every number before anything is sent. Free to start, no card needed, £99 flat when you file.

Start your return

If you have several companies, or a group, an accountant is the right call for this bit, and we say so plainly.

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.