The two main account structures give the same result if your tax rate is the same when you contribute and when you withdraw. The choice turns on whether you expect your rate to be higher or lower in retirement.
12 min read · Level 2
I find it clarifying to say plainly that pre-tax and post-tax accounts are not better or worse inventions, just different timings of the same tax. Once people see that, the marketing on both sides loses its force, and the conversation becomes about the one thing that matters: what tax rate you will face later. Since nobody knows that with confidence, spreading contributions between both is often a reasonable admission of uncertainty rather than indecision.
There are two broad structures:
Pre-tax contribute before tax, pay tax on withdrawal Post-tax contribute after tax, qualifying withdrawals tax-free
Take 1,000 of salary, an investment that triples over time, and illustrative tax rates.
Case 1: 30% tax now, 30% in retirement.
Pre-tax: 1,000 invested, grows to 3,000, taxed 30% = 2,100 Post-tax: 700 invested after tax, grows to 2,100, no tax = 2,100
Identical. When the rates are equal, the order of tax and growth does not matter.
Case 2: 30% now, 20% in retirement.
Pre-tax: 3,000 taxed at 20% = 2,400 Post-tax: 700 grows to 2,100 = 2,100
Pre-tax wins by 300.
Case 3: 20% now, 30% in retirement.
Pre-tax: 1,000 grows to 3,000, taxed at 30% = 2,100 Post-tax: 800 grows to 2,400, no tax = 2,400
Post-tax wins by 300.
The structure is effectively a choice of which tax rate applies to the money: today's or retirement's. Other factors can matter too, such as rules on required withdrawals, how withdrawals affect other benefits, and the risk that tax rules change.
Because people often choose based on the appeal of a tax break today, or of tax-free withdrawals later, without seeing that both are the same deal in different order unless tax rates differ.
It also explains why many people use both. Nobody knows their future tax rate with certainty, and holding some money in each structure spreads that uncertainty.
This is educational information, not investment, securities or financial advice. Rules differ by country and change — get advice from a qualified professional in your jurisdiction before acting.
Educational, not personalised financial advice. Nothing in the Investor Academy is a recommendation to buy, sell or hold anything — what is right for you depends on your own circumstances and the rules where you live.
Sources: U.S. IRS - retirement plans; UK Government - tax on your private pension; U.S. SEC - Investor.gov
An early-career worker. They earn a modest salary and are in a low tax band now, but expect higher earnings later. If they also expect a meaningful retirement income, their future tax rate may be similar or higher.
A post-tax account lets them pay tax at today's low rate. A pre-tax account would save tax at a low rate now and potentially pay it at a higher rate later.
A high earner near retirement. They are in a high tax band now and expect lower taxable income in retirement. Pre-tax contributions save tax at a high rate now, likely taxed at a lower rate on withdrawal.
Both decisions follow from the same arithmetic. Neither is universally right.
Choosing an account type only because the tax relief today feels larger. The relief is valuable only if the tax paid later is lower, or at least not higher.
Find the rate that applies to your next unit of income in the current year.
Consider expected retirement income from all sources and how it would be taxed under current rules, knowing rules may change.
Run the three cases above with your own rates to see which structure is favoured.
If you are unsure about future rates, holding both types can reduce the risk of guessing wrong.
When choosing between pre-tax and post-tax accounts or deciding how to split contributions.
Do not treat the illustrative tax rates as yours. Check current official rules and consider professional advice, especially where benefits or required withdrawals interact with tax.
Governments periodically change retirement account rules - contribution limits, allowances, required withdrawal ages and tax treatment of lump sums. For example, the U.S. SECURE Act of 2019 and SECURE 2.0 Act of 2022 changed required minimum distribution ages and other plan rules, and the UK has repeatedly adjusted pension allowances.
Decisions that depend on future tax rules carry the risk that those rules change. Diversifying across account types is one way to reduce that risk.
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