What it means
The idea is political rather than technical: when an event outside a company's control hands it unusually large profits, governments sometimes decide that the public should share in the gain. Wartime equipment makers and, more recently, energy producers during price spikes are the classic targets.
Two methods are used to define the normal profit that escapes the extra charge. The invested capital method allows a fixed percentage return on the capital employed in the business, while the historical earnings method allows the average profit of an earlier base period.
Everything above that allowance is the excess, taxed at an additional rate that has historically ranged from around 25% in peacetime windfall schemes to well over 80% in extreme wartime versions. Companies still pay ordinary corporation tax on the whole profit, so the two charges stack rather than substitute.
For a finance team the consequences are immediate and practical. Effective tax rate guidance given to investors has to be restated, deferred tax balances may need remeasuring, and any capital allowance built into the design creates a real incentive to invest rather than to pay dividends.
Critics argue that the tax discourages exactly the investment that would relieve the shortage causing high prices, and that normal profit cannot be defined fairly across different industries. Supporters argue that it captures windfalls owing nothing to skill or risk-taking.
Most versions are temporary and carry an explicit end date.
In practice
Real-world examples.
Example
A gas producer reports profits four times its five-year average after a supply disruption pushes prices sharply higher. A temporary levy on profits above a defined normal return adds roughly 15 percentage points to its effective tax rate for two years, and the company redirects part of its planned buyback into new production capacity to use an investment allowance in the rules.
Example
A container shipping line earns record freight rates during a period of port congestion. Its finance team spends most of a quarter rebuilding the tax model, because the base period chosen by the legislation includes two loss-making years that drag the allowed normal profit close to zero.
Example
A defence equipment manufacturer operating under a wartime excess profits regime renegotiates its government contracts on a cost-plus basis. Since profit above the allowance is taxed at a punitive rate, the company competes on volume and reliability rather than on margin.
Formula
Calculation
Excess profits tax = (Taxable profit - Allowed normal profit) x Excess profits rate, where Allowed normal profit = Capital base x Normal return percentage.
A refiner has a capital base of $20,000,000 and the legislation allows a normal return of 8%, so its shielded profit is $20,000,000 x 8% = $1,600,000. In the year in question it reports taxable profit of $5,600,000.
The excess is $5,600,000 - $1,600,000 = $4,000,000. At an excess profits rate of 25% the additional charge is $4,000,000 x 25% = $1,000,000. Ordinary corporation tax at 21% on the full $5,600,000 adds $1,176,000, so the total tax bill is $1,000,000 + $1,176,000 = $2,176,000 and the effective tax rate is $2,176,000 / $5,600,000 = 38.9%.Case study
Seen in the real world.
Meridian Fuels Holdings is an illustrative and entirely fictional refiner used here to show how the mechanics land on a real set of accounts. Its capital base was $20,000,000, the emergency legislation allowed a normal return of 8%, and its taxable profit for the year came in at $5,600,000 against a five-year average nearer $1,800,000.
The finance director's first calculation showed the shielded profit at $1,600,000, an excess of $4,000,000, and an additional charge of $1,000,000 at the 25% excess rate. Combined with $1,176,000 of ordinary corporation tax, the total came to $2,176,000, lifting the effective rate from 21% to 38.9%. Investor guidance issued three months earlier had assumed 21%, so a correction had to be issued before the interim results.
The board's response was to bring forward $3,000,000 of capital spending that qualified for the investment allowance, which reduced the taxable excess in the following year. The illustrative lesson is that excess profits taxes change behaviour as much as they raise revenue, and the design of the allowance decides which behaviour you get.
Watch out
Common mistakes.
- Assuming the excess profits tax replaces ordinary corporation tax, when in almost every version it is charged in addition to it.
- Modelling the charge on accounting profit when the legislation nearly always applies to a specially defined taxable profit with its own adjustments.
- Forgetting to remeasure deferred tax balances, which have to reflect the rate expected to apply when the temporary differences reverse.
Questions
People also ask.
Is an excess profits tax the same as a windfall tax?
They are close relatives, but a windfall tax is usually a one-off charge on a defined sector while an excess profits tax uses a formula comparing actual profit with an allowed normal return.
Who decides what counts as normal profit?
The legislation does, typically by allowing a fixed percentage return on invested capital or by using average profits from a stated base period.
Can a company plan around it?
Only within the rules, usually by taking up investment allowances, timing deductible spending and reviewing group structure, since aggressive avoidance of an emergency levy attracts both tax and political attention.
From the founder's library
Take it further with the book.
Build your financial confidence beyond this definition. Shihan's full-length guide, Accounting Fundamentals, takes the same plain-English approach and turns it into a complete, practical playbook for non-finance managers, business owners and students - with chapter-end quiz answers and presentation slides included.
25% off with code MMHQ25, applied at checkout. Priced in USD - checkout may show the equivalent in your local currency.
View the book and save 25%Related