Corporation tax is one of the few taxes a company can compute exactly from published numbers. There are two rates, two thresholds and one relief that bridges them, and the only things that move the answer are your taxable profit, the length of your accounting period and how many companies you control. This page gives you the number and, more usefully, shows the working, so you can see which of those three is actually driving your bill.
The two rates and the band between them
GOV.UK states the position in one line: the Corporation Tax rate for company profits is 25%. That is the main rate, and it applies in full once a company's taxable profit passes £250,000. Below £50,000 a company pays the small profits rate, which is 19%. Between the two there is no third rate: a company in that band pays the main rate and then claims Marginal Relief, which HMRC describes as providing a gradual increase in Corporation Tax rate between the small profits rate and the main rate. The effect is a rising effective rate across the band rather than a cliff at either end, which matters if your profit is near a threshold and you are deciding whether to bring a cost forward.
What actually moves the answer
Three things, and only one of them is the profit figure. The first is the accounting period: if it is shorter than twelve months, HMRC proportionately reduces both thresholds, so a six-month period has a £25,000 lower limit and a £125,000 upper limit. The second is associated companies, which reduce the same thresholds by division; HMRC's own example is a company with three other associated companies, where the limits are divided by four and become £12,500 and £62,500. The third is which financial year the profit falls in, because the rate that applies is the rate for the company's accounting period, and a period straddling 1 April is split across the two rates by days.
Why the answer is not your tax bill on its own
The figure this page computes is tax on taxable profit, and taxable profit is not accounting profit. Capital allowances come off before the rate is applied, so a company that bought plant during the period may have a far lower taxable profit than its accounts suggest. R&D relief and patent box both change the figure again, and a director's overdrawn loan account adds a separate charge under section 455 that is assessed with the company's corporation tax but is not part of the rate calculation at all. Each of those has its own page here. Compute the rate first, then work through what reduces the profit it applies to.
Questions people ask about corporation tax calculator
What is the corporation tax rate for a UK company?
GOV.UK states that the Corporation Tax rate for company profits is 25%, applying in full above £250,000 of profit. A company making £50,000 or less pays the small profits rate of 19%, and a company in between pays the main rate and claims Marginal Relief.
Does the calculator handle associated companies?
Yes, because it has to. The £50,000 and £250,000 thresholds are proportionately reduced by the total number of associated companies, so a company that controls others reaches the main rate at a much lower profit. HMRC's own worked example divides the limits by four for a company with three associates.
What if my accounting period is not twelve months?
The thresholds are proportionately reduced for short accounting periods, exactly as they are for associated companies. A nine-month period carries a £37,500 lower limit and a £187,500 upper limit.