Every year finance teams ask the same question: why does our accounting profit not match the income we pay tax on? The answer is book-tax differences. Understanding them stops you from over- or under-paying corporate income tax and explains that mysterious deferred tax line. This article breaks down the causes, the two types of difference, how to reconcile them, and the mistakes that trigger tax adjustments.

Why accounting profit and taxable income differ

Accounting profit follows accounting standards, which aim to show economic reality. Taxable income follows tax law, which aims to define what the state will tax and when. The two rulebooks have different goals, so they rarely produce the same number. The reconciliation starts from accounting profit before tax and adds or subtracts the items where the rules disagree.

Permanent differences

Permanent differences never reverse. An expense that is recorded in the books but is not deductible for tax stays non-deductible forever. Common examples in Vietnam include expenses without valid invoices, certain penalties, and costs exceeding capped limits. These change your tax bill but create no deferred tax.

Temporary differences

Temporary differences reverse over time because the books and tax recognize the same item in different periods. Depreciation is the classic case: if accounting depreciates an asset over five years but tax allows a different rate, the yearly expense differs, yet the total over the asset’s life is the same. These timing gaps create deferred tax.

How deferred tax works in plain terms

Deferred tax records the future tax consequence of today’s temporary differences. A deferred tax liability means you will pay more tax later. A deferred tax asset means you will pay less later, for example when you carry forward a tax loss that you expect to use against future profit. You only recognize a deferred tax asset when future taxable profit is probable; otherwise it is imprudent.

A worked example

A company buys equipment for 1,000 and depreciates it straight-line over 4 years in the books (250 per year). Tax allows depreciation over 5 years (200 per year). In year one, the accounting expense is 250 but the tax-deductible amount is 200. Taxable income is 50 higher than accounting profit. At a 20% rate, that is 10 of tax paid earlier than the books suggest. This creates a deferred tax asset of 10 that reverses in later years when accounting depreciation ends but tax depreciation continues.

How to reconcile step by step

• Start with accounting profit before tax
• Add back non-deductible expenses (permanent and temporary)
• Subtract non-taxable income and extra tax-deductible amounts
• Apply loss carry-forwards within the allowed period
• The result is taxable income; multiply by the CIT rate
• Separately, identify temporary differences and record deferred tax

Common mistakes and how to fix them

Mixing permanent and temporary differences. Teams often lump everything together, then miscalculate deferred tax. Fix: tag each adjustment as permanent or temporary in a working schedule.

Recognizing deferred tax assets that will never be used. Booking an asset on losses you cannot realistically absorb overstates equity. Fix: test for probable future profit before recognition.

Deducting expenses without valid invoices. These are permanently non-deductible in Vietnam and are a frequent audit adjustment. Fix: enforce invoice discipline during the year, not at filing.

Forgetting the loss carry-forward time limit. Losses can be carried forward for a limited number of years. Fix: track each loss year and its expiry.

Action checklist

• Maintain a book-tax reconciliation schedule updated quarterly
• Classify every difference as permanent or temporary
• Keep a fixed asset register showing both accounting and tax depreciation
• Document the basis for any deferred tax asset
• Reconcile the tax expense in the accounts to the CIT return

Conclusion and next step

Book-tax differences are logical once you separate timing gaps from permanent gaps. Build one reconciliation schedule that runs from accounting profit to taxable income and back to deferred tax. Next step: rebuild your fixed asset register to show accounting and tax depreciation side by side, since it drives most temporary differences.

Frequently asked questions

Is deferred tax a real cash payment?

No. Deferred tax is an accounting estimate of future tax effects. Only current tax is paid to the authority. Deferred tax smooths the tax expense to match the accounting periods that generated it.

Are all disallowed expenses permanent differences?

No. Some are timing differences that become deductible later; others, like invalid-invoice costs, are permanently non-deductible. The distinction determines whether deferred tax arises.

When should we not recognize a deferred tax asset?

When future taxable profit to use it against is not probable. Recognizing it anyway overstates assets and equity, which auditors will challenge.

Does a small company really need deferred tax accounting?

If temporary differences are immaterial, the effect is small. But depreciation and provisions almost always create some difference, so at least assess it rather than ignoring it.

References

• Law on Corporate Income Tax and its guiding circulars (Vietnam)
• Circular 200/2014/TT-BTC on the enterprise accounting regime
• Vietnamese Accounting Standard on Income Taxes (VAS 17)