What it means
The deduction exists because interest is treated as a cost of doing business, in the same way as rent or wages. Its effect is to make debt cheaper than its headline rate: a loan at 8% costs a company paying 25% tax only 6% after the deduction.
This asymmetry between debt and equity is one of the main reasons companies borrow. Dividends paid to shareholders are not deductible, so funding with debt reduces the tax bill in a way that funding with equity does not.
Governments noticed that companies could push profits into low-tax countries by loading debt into high-tax ones, so most now limit the deduction. A widely used rule caps net interest deductions at around 30% of a measure of earnings similar to EBITDA, with disallowed amounts often carried forward to future years.
Other restrictions frequently apply. Interest on borrowings used to buy assets that generate exempt income may be disallowed, interest paid to related parties is scrutinised for arm's length pricing, and small businesses often sit below a de minimis threshold and escape the cap entirely.
The practical consequence for a finance team is that the tax benefit of borrowing is real but not automatic. A highly geared company with weak earnings may find a large slice of its interest non-deductible in exactly the year it can least afford the extra tax.
In practice
Real-world examples.
Example
A family manufacturing company borrows $2,000,000 at 6% to buy a competitor. The $120,000 annual interest reduces taxable profit and saves $30,000 in tax at a 25% rate, making the effective cost of the borrowing 4.5%.
Example
A property investor is told that interest on the loan funding a residential portfolio receives only restricted relief under local rules. The restriction changes the deal from marginally profitable to loss making, and the investor renegotiates the purchase price.
Example
A private equity backed retailer with $9,000,000 of interest and $20,000,000 of adjusted earnings is capped at $6,000,000 of deductions. The $3,000,000 disallowed is carried forward, and the group restructures part of the debt as equity to bring future interest under the cap.
Formula
Calculation
The tax saving is: Tax saving = Deductible interest x Marginal tax rate, where Deductible interest = min(Interest paid, Cap on deduction).
An engineering group pays $400,000 of net interest in the year. Its adjusted taxable earnings, broadly EBITDA for this purpose, are $1,000,000. It faces a 30% earnings-based cap and pays tax at 25%.
Cap on deduction = 30% x $1,000,000 = $300,000.
Interest paid of $400,000 exceeds the cap, so deductible interest = $300,000 and disallowed interest = $400,000 - $300,000 = $100,000, carried forward.
Tax saving this year = $300,000 x 0.25 = $75,000.
After-tax interest cost = $400,000 - $75,000 = $325,000, an effective after-tax rate of 81.25% of the headline charge rather than the 75% the group had budgeted. Had the full $400,000 been deductible, the saving would have been $400,000 x 0.25 = $100,000 and the after-tax cost $300,000, so the cap cost the group $25,000 of cash tax in the year.Case study
Seen in the real world.
Corbin Logistics is an invented haulage group used for this illustrative example. It bought a rival depot network using $12,000,000 of acquisition debt at 7%, producing $840,000 of annual interest, and the financial model assumed a full deduction at the 25% tax rate, worth $210,000 a year.
A freight downturn in the following year cut adjusted earnings to $2,200,000. The 30% cap limited deductible interest to $660,000, leaving $180,000 disallowed. The tax saving fell to $660,000 x 0.25 = $165,000, which was $45,000 less than the model assumed, in a year when cash was already tight.
Corbin's response was to test its covenants and tax position against a downside earnings case rather than only the base case. It also converted $3,000,000 of the debt into preference shares, which reduced interest to $630,000 and brought the whole amount comfortably inside the cap. This illustrative case shows that the value of an interest deduction depends on earnings, not just on the loan.
Watch out
Common mistakes.
- Assuming all interest paid is deductible, when earnings-based caps, related-party rules and asset-specific restrictions can disallow a large share of it.
- Building a financial model that applies the full tax saving in every year, including downside scenarios where low earnings shrink the deductible amount.
- Treating loan arrangement fees and similar charges as automatically deductible in the year paid, when they often have to be spread over the life of the loan.
Questions
People also ask.
Does an interest deduction reduce the tax bill dollar for dollar?
No, it reduces taxable profit, so the cash benefit equals the interest multiplied by the marginal tax rate.
What happens to interest that is disallowed by a cap?
In most systems it is carried forward and can be deducted in a later year when there is enough headroom under the cap.
Why do tax rules favour debt over equity?
Because interest is deductible while dividends are not, which is the tax shield that makes debt financing cheaper after tax.
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