IRA Calculator Guide: Roth vs Traditional & Compound Growth
IRA 计算器指南：Roth vs Traditional 与复利增长
An Individual Retirement Account (IRA) is one of the most powerful tools available for building long-term wealth, yet most Americans dramatically under-use it. The average IRA balance at Fidelity in 2024 was roughly $132,000, while the median sat closer to $43,000 — a gap that reveals how many account holders contribute sporadically rather than maxing out every year. This guide breaks down exactly how an IRA grows, the critical difference between Roth and Traditional, the tax math that should drive your choice, current contribution limits, income phase-outs, and the most common mistakes that quietly cost retirees real money.
How an IRA Grows
An IRA is a tax-advantaged container; what's inside does the growing. Whether you hold index funds, a target-date fund, or individual stocks, the growth follows the same compound-growth formula. With a starting balance P, annual contribution PMT, average annual return r, and n years, the future value is:
- FV — future value of the account
- P — starting balance (use 0 if starting fresh)
- PMT — annual contribution (made at year-end)
- r — expected annual return, as a decimal (7% → 0.07)
- n — number of years until withdrawal
The first term is compound growth on existing savings; the second is the future value of an ordinary annuity — your yearly contributions compounding over time.
Let's walk through a realistic example. Say you start from zero, contribute the 2024 limit of $7,000 every year, earn a 7% average annual return, and keep going for 30 years. Plugging in P = 0, PMT = 7,000, r = 0.07, n = 30:
FV = 7,000 × [ ((1.07)30 − 1) / 0.07 ] ≈ $664,000
That's the magic of compound growth in a tax-advantaged account: you contribute $210,000 out of pocket ($7,000 × 30 years) and end with more than $664,000 — over $450,000 of pure growth, sheltered from taxes the entire way. The earlier you start, the harder the compounding works, because each dollar earns returns that themselves earn returns.
Roth vs Traditional IRA
The single most important IRA decision you'll make is Roth versus Traditional. They use the exact same growth formula above — the difference is purely when you pay tax.
| Feature | Roth IRA | Traditional IRA |
|---|---|---|
| Contributions | After-tax (no deduction now) | Pre-tax (tax-deductible now) |
| Growth | Tax-free | Tax-deferred |
| Withdrawals in retirement | Tax-free | Taxed as ordinary income |
| Income limits to contribute | Yes (phase-outs) | No (to contribute; deduction has limits) |
| Required distributions (RMDs) | None in original owner's lifetime | Required starting at age 73 |
The Tax Math
The Roth-vs-Traditional choice ultimately comes down to one question: is your marginal tax rate higher today or in retirement? The numbers make this concrete. Suppose you're in the 25% bracket and have $7,000 of pre-tax money to put toward retirement.
With a Traditional IRA, the full $7,000 goes in pre-tax, so the entire amount is invested. With a Roth IRA, you pay 25% tax first, leaving $5,250 after tax to invest. After 30 years at 7%, the same compound-growth formula from above applies:
Traditional: $7,000/yr grows to ≈ $664,000 → taxed 25% at withdrawal → $498,000 net
Roth: $5,250/yr grows to ≈ $498,000 → tax-free → $498,000 net
When your tax rate is the same in both years, the two strategies produce identical results. The decision therefore hinges entirely on what you expect your future tax rate to be:
- Expect a lower retirement bracket (common — many retirees drop into the 15% bracket with no paycheck) → Traditional wins, because you defer tax and pay it at the lower rate.
- Expect a higher retirement bracket (pension income, big 401(k) balance, looming tax hikes) → Roth wins, because you lock in today's lower rate forever.
If you're young, early in your earnings curve, or expect promotions and higher income later, a Roth IRA is usually the better bet. You pay tax now while your bracket is low, and decades of growth compound completely tax-free. You can also withdraw your contributions (not earnings) at any time without penalty, which adds flexibility Traditional doesn't offer.
2024–2025 Contribution Limits
The IRS caps how much you can put into an IRA each year. The limits are per person, aggregated across all your IRAs — not per account.
| Your age | 2024 limit | 2025 limit |
|---|---|---|
| Under 50 | $7,000 | $7,000 |
| 50 or older | $8,000 (incl. $1,000 catch-up) | $8,000 (incl. $1,000 catch-up) |
You have until the tax filing deadline (typically April 15 of the following year) to make a contribution for that tax year. Hitting the limit every year is the single highest-leverage move most people can make — the example above shows how $7,000/year becomes more than $664,000 over 30 years.
Income Phase-Outs
High earners face restrictions. The Roth IRA has strict income limits — above a certain threshold you cannot contribute directly. The Traditional IRA's deduction is also phased out if you (or your spouse) have a workplace retirement plan.
For 2024, the Roth IRA contribution phase-out range is $146,000–$161,000 of modified AGI for single filers, and $230,000–$240,000 for married filing jointly. Below the lower bound you can contribute the full amount; above the upper bound you cannot contribute directly to a Roth IRA at all. The numbers shift slightly each year, so check the current IRS figures before filing.
If your income is above the Roth ceiling, the workaround is the backdoor Roth: contribute to a Traditional IRA (which has no income limit for non-deductible contributions) and then convert it to a Roth IRA. The conversion generates little to no tax if you have no other pre-tax IRA balance, effectively sidestepping the income cap. Be aware of the pro-rata rule: if you hold other pre-tax IRA money, the conversion is taxed proportionally across all your IRAs, which can erode the benefit.
Common IRA Mistakes
- Not maxing out. Even one missed year costs you decades of compounding on that contribution. Skipping a $7,000 contribution at age 35 isn't losing $7,000 — at 7% it's losing roughly $53,000 by age 65.
- Picking Roth or Traditional without thinking about tax bracket. Defaulting to whichever your friend picked ignores the core math above. Run the numbers on your current vs. expected retirement bracket.
- Tapping the account early. Withdrawals of earnings before age 59½ generally trigger a 10% penalty plus income tax. Roth contributions can come out penalty-free, but earnings have stricter rules — know them before you withdraw.
- Missing the backdoor Roth. If you're a high earner above the Roth income limit and assume you're simply locked out, you're leaving tax-free growth on the table. The backdoor conversion is a well-established, legal workaround.
- Ignoring Traditional RMDs. Traditional IRAs force Required Minimum Distributions starting at age 73, which can push you into a higher tax bracket and trigger Medicare surcharges. Roth IRAs have no RMDs during your lifetime, which makes them a powerful estate-planning tool.
Put It Into Practice
Reading about compound growth is one thing — seeing your own numbers is another. Use the CalcSpace IRA calculator to model your exact scenario: set your starting balance, annual contribution, expected return, and years to retirement, then toggle between Roth and Traditional to watch the after-tax outcome shift. You can also stress-test different contribution levels to see what maxing out every year is actually worth over 30 or 40 years. The numbers tend to be more motivating than any article.
个人退休账户（IRA）是积累长期财富最强大的工具之一，但大多数美国人远没有充分利用它。2024 年 Fidelity 的 IRA 平均余额约为 $132,000，而中位数仅为 $43,000——这一差距揭示了许多账户持有者只是零星供款，而非每年最大化供款。本指南将详细解析 IRA 如何增长、Roth 与 Traditional 的关键区别、决定你选择的税务计算、当前的 contribution limit、收入 phase-out 规则，以及悄悄让退休人士损失真金白银的最常见错误。
IRA 如何增长
IRA 是一个 tax-advantaged 容器；真正实现增长的是容器内的资产。无论你持有指数基金、目标日期基金还是个股，增长都遵循相同的复利公式。使用起始余额 P、年度供款 PMT、平均年收益率 r 和 n 年，未来价值为：
- FV — 账户的未来价值
- P — 起始余额（如从零开始则使用 0）
- PMT — 年度供款（年末支付）
- r — 预期年收益率，以小数表示（7% → 0.07）
- n — 到提取为止的年数
第一项是已有储蓄的复利增长；第二项是普通年金的未来值——你的年度供款随时间复利累积。
让我们来看一个现实的例子。假设你从零开始，每年供款 2024 年限额 $7,000 每一年，获得 7% 的平均年收益率，持续 30 年。代入 P = 0，PMT = 7,000，r = 0.07，n = 30：
FV = 7,000 × [ ((1.07)30 − 1) / 0.07 ] ≈ $664,000
这就是 tax-advantaged 账户中复利增长的魔力：你自掏腰包供款 $210,000（$7,000 × 30 年），最终获得超过 $664,000——其中超过 $450,000 是纯粹的增长，全程免税保护。你越早开始，复利效应就越强，因为每一美元赚得的收益本身还会继续产生收益。
Roth vs Traditional IRA
你需要做的最重要的 IRA 决策就是选择 Roth 还是 Traditional。它们使用完全相同的增长公式——区别纯粹在于你何时纳税。
| 特性 | Roth IRA | Traditional IRA |
|---|---|---|
| 供款 | 税后（现无扣除） | 税前（现可扣税） |
| 增长 | 免税 | 延税 |
| 退休时提取 | 免税 | 按普通收入纳税 |
| 供款收入限制 | 有（phase-out） | 无（供款；扣除有限制） |
| Required distributions (RMDs) | 原始所有者生前无需 | 73 岁起强制要求 |
税务计算
Roth 与 Traditional 的选择归根结底是一个问题：你的边际税率是现在高还是退休后高？数字会让这一点更具体。假设你处于 25% 税阶，有 $7,000 的税前资金可用于退休储蓄。
选择 Traditional IRA，全额 $7,000 税前投入，因此全部金额都被投资。选择 Roth IRA，你先缴纳 25% 的税，税后剩余 $5,250 可投资。以 7% 的收益率投资 30 年后，应用与上述相同的复利公式：
Traditional: $7,000/yr 增长至 ≈ $664,000 → 提取时缴 25% 税 → $498,000 净得
Roth: $5,250/yr 增长至 ≈ $498,000 → 免税 → $498,000 净得
当你两年的税率相同时，两种策略产生完全相同的结果。因此，决策完全取决于你对未来税率的预期：
- 预期退休后税阶较低（常见——许多退休人士在没有工资后降至 15% 税阶）→ Traditional 更优，因为你延税并在较低税率时缴纳。
- 预期退休后税阶较高（养老金收入、大量 401(k) 余额、即将到来的加税）→ Roth 更优，因为你永久锁定了今天较低的税率。
如果你还年轻、处于收入曲线的早期阶段，或预期未来会晋升和获得更高收入，Roth IRA 通常是更好的选择。你在税阶较低时缴税，数十年的增长完全免税复利。你还可以随时提取供款（非收益）而不受罚款，这增加了 Traditional 所不具备的灵活性。
2024–2025 年 Contribution Limits
IRS 限制了你每年可以向 IRA 供款的金额。限额是按人计算的，在你所有的 IRA 账户间汇总——而不是按账户计算。
| 你的年龄 | 2024 年限额 | 2025 年限额 |
|---|---|---|
| 50 岁以下 | $7,000 | $7,000 |
| 50 岁或以上 | $8,000（含 $1,000 catch-up） | $8,000（含 $1,000 catch-up） |
你可以在纳税申报截止日期（通常为次年 4 月 15 日）前为该纳税年度供款。每年达到限额是大多数人能做的杠杆率最高的单一行动——上面的例子展示了每年 $7,000 如何在 30 年内增长到超过 $664,000。
Income Phase-Outs
高收入者面临限制。Roth IRA 有严格的收入限额——超过特定阈值后你无法直接供款。如果你（或你的配偶）有雇主退休计划，Traditional IRA 的扣除也会逐步取消。
2024 年，Roth IRA 供款的 phase-out 范围为：单身申报者 modified AGI $146,000–$161,000，已婚联合申报（MFJ）$230,000–$240,000。低于下限可全额供款；超过上限则完全无法直接向 Roth IRA 供款。这些数字每年略有调整，申报前请查看 IRS 当前数据。
如果你的收入超过 Roth 上限，变通方法就是 backdoor Roth：向 Traditional IRA 供款（non-deductible 供款没有收入限制），然后将其 convert 为 Roth IRA。如果你没有其他税前 IRA 余额，转换几乎不会产生或不会产生税款，有效绕过了收入上限。请注意 pro-rata 规则：如果你持有其他税前 IRA 资金，转换将在你所有 IRA 中按比例征税，这可能会削弱其好处。
IRA 常见错误
- 未达到最大供款。即使错过一年，你也失去了该供款数十年的复利效应。在 35 岁时跳过 $7,000 的供款，并不是损失 $7,000——以 7% 的收益率计算，到 65 岁时大约损失 $53,000。
- 不考虑 tax bracket 就选择 Roth 或 Traditional。随意选择朋友选的类型而忽略了上述核心计算。应该对你当前的税阶与预期退休税阶进行计算。
- 过早提取账户。59½ 岁前提取收益通常会触发 10% 的罚款加所得税。Roth 供款可以免税提取，但收益有更严格的规则——提取前请了解相关规定。
- 错过 backdoor Roth。如果你是收入超过 Roth 限额的高收入者，并认为自己完全被排除在外，你就是在放弃免税增长的机会。backdoor conversion 是一个成熟且合法的变通方法。
- 忽略 Traditional RMD。Traditional IRA 要求从 73 岁起必须提取 Required Minimum Distributions，这可能将你推入更高的税阶并触发 Medicare 附加费。Roth IRA 在你生前没有 RMD，这使它们成为强大的遗产规划工具。
付诸实践
阅读复利增长是一回事——看到你自己的数字又是另一回事。使用 CalcSpace IRA 计算器来模拟你的确切场景：设置你的起始余额、年度供款、预期收益率和距离退休的年数，然后在 Roth 和 Traditional 之间切换，观察税后结果的变化。你还可以对不同的供款水平进行压力测试，看看每年最大化供款在 30 或 40 年后到底值多少钱。数字往往比任何文章都更有激励性。
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Enter your contribution, return, and timeline to see your projected nest egg — Roth vs Traditional, side by side.
输入你的供款、收益率和时间线，查看你预计的退休储蓄——Roth 与 Traditional 并排对比。
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