Corporation Tax calculator.

19% on profits up to £50,000 under the small profits rate, and 25% on profits over £250,000 at the main rate. Between those figures the calculation charges 25% and then deducts marginal relief of (£250,000 − profit) × 3/200, so the effective rate climbs gradually from 19% towards 25% rather than jumping in one step. Note that these thresholds are for a full twelve-month accounting period and a single company — a shorter period reduces them proportionately, and associated companies divide them.

Profit between £50,000 and £250,000 carries an effective marginal rate of 26.5% — higher than the 25% main rate — because each extra pound of profit also erodes the marginal relief. That is the number worth knowing in month nine of your year rather than after it. A £10,000 employer pension contribution or a qualifying capital purchase made while you are in that band reduces the tax bill by £2,650 rather than £1,900. The same spend after the year end saves nothing for that period at all, which is the single most common piece of avoidable Corporation Tax.

Yes, significantly, and this calculator assumes a single company. Where companies are under common control, the £50,000 and £250,000 thresholds are divided between them. Two companies each making £40,000 do not both get the small profits rate — the threshold becomes £25,000 each, so both fall into marginal relief. The test looks at control rather than shareholding alone, and it can catch companies you would not think of as connected, including dormant ones in some circumstances. If you have more than one company, get the associated company position confirmed rather than assumed.

Payment is due nine months and one day after the end of your accounting period; the CT600 return is not due until twelve months after it. So you pay before you file, which trips people up every year — you cannot wait for the accounts to be finalised before finding the money. Large companies with profits over £1.5 million pay by quarterly instalments instead. Interest runs on late payment from the due date, and HMRC pays a lower rate of interest on early payment. See key tax dates for the full calendar.

The reliable items, in rough order of value: employer pension contributions, which are deductible for the company and free of National Insurance; capital allowances on qualifying equipment, including full expensing for companies on new plant and machinery; salaries and employer's National Insurance; and genuine business costs properly recorded. Timing matters as much as the item — a purchase made a week before the year end lands in this period, a week after it does not. What does not work is a director's loan dressed up as an expense, or a claim with no invoice behind it.

Treat it as an estimate. It works from taxable profit, which is not the same as the profit in your accounts: depreciation is added back, capital allowances are deducted instead, entertaining is disallowed, and any losses brought forward or carried back change the result. It also assumes a single company with no associates and a full twelve-month period. For a real figure you need the tax computation that sits behind the return, not a headline calculation — which is why the number in your accounts and the number HMRC wants are rarely identical.