Most retirement advice falls into two categories: vague platitudes ("save what you can") or precise but unhelpful answers ("you need $1.5 million"). Neither tells you what you actually need to know — what specific dollar amount you should save each month, given your specific income, age, and goals. This guide walks through the actual math.

The Core Question: How Much Annual Income Do You Need?

Every retirement calculation starts with one question: how much money will you need each year in retirement? Two frameworks dominate.

The Replacement Ratio Approach

Financial planners traditionally target 70–80% of your final pre-retirement income as your annual retirement spending. The reduction comes from eliminating retirement contributions, payroll taxes, commuting, work clothing, and (typically) a paid-off mortgage. Higher earners often need lower ratios (60–70%); lower earners often need higher ratios (80–90%).

Example: Sarah earns $90,000/year at age 60. Using a 75% replacement ratio, she'll need $67,500/year in retirement. If she retires at 65, she needs to plan for 25–30 years of $67,500/year, increasing with inflation.

The Expense-Based Approach (FIRE Style)

Track your current expenses for 6 months. Subtract things that go away in retirement (commuting, retirement savings). Add things that increase (healthcare). The result is your projected retirement annual expenses — usually significantly different from a replacement-ratio estimate. Use our FIRE Number Calculator to convert annual expenses into a portfolio target.

The 4% Rule and the 25x Multiplier

Required Portfolio = Annual Expenses × 25

Withdrawing 4% of a portfolio annually with the rest invested at long-term market returns historically lasted 30+ years across all studied periods. Inverting that gives the 25x multiplier (1 ÷ 0.04 = 25). Sarah needs $67,500/year × 25 = $1,687,500.

The Inflation Problem

The 4% rule produces a target in today's dollars. At 3% annual inflation, $67,500 today equals $90,400 in 10 years and $121,000 in 20 years. Your retirement spending will be higher in nominal dollars, and your portfolio target grows with inflation. If retirement is 20 years away, Sarah will need roughly $3,050,000 in 2046 dollars to maintain today's $67,500 lifestyle. Use our Inflation Calculator to model this.

The Three Sources of Retirement Income

Source 1: Social Security

For average earners, Social Security replaces 35–40% of pre-retirement income. The Social Security Administration's "MyAccount" tool gives a personalized estimate. For Sarah's $67,500 need, Social Security might provide $25,000–$30,000/year.

Source 2: Pensions

Only 15% of US private workers have traditional pensions. Government, military, education, and unionized industries still have meaningful pensions. Get an exact projected monthly benefit from your plan administrator if applicable.

Source 3: Personal Savings

Your portfolio fills the gap between guaranteed income and your spending need. For Sarah needing $67,500 with $27,500 from Social Security: her portfolio funds $40,000/year. At 25x: $1,000,000 — significantly less than the original $1.687M figure.

Calculating Your Required Monthly Savings

Working backward to find required monthly savings:

Monthly Savings = Target ÷ FV Annuity Factor
FV Annuity Factor = ((1+r)ⁿ − 1) ÷ r × (1+r)

Where r is monthly return rate and n is months until retirement. Use our SIP Calculator for this directly. For Sarah (20 years to retirement, 7% annual return assumption, $1M target): she needs to save approximately $1,920/month — about 26% of her current income.

Why Starting Early Wins

The single most powerful retirement variable is time. Three savers all targeting $1M at 65 with 7% returns:

  • Saver A starts at 25: $381/month × 40 years = $183K total contributions
  • Saver B starts at 35: $820/month × 30 years = $295K total contributions
  • Saver C starts at 45: $1,920/month × 20 years = $461K total contributions
  • Saver D starts at 55: $5,800/month × 10 years = $696K total contributions

Saver D contributes 3.8x more than Saver A to reach the same target. Every decade of delay roughly doubles the required monthly contribution. Use our Compound Interest Calculator to see the math.

Healthcare: The Variable That Breaks Calculations

Pre-Medicare (50–64), ACA marketplace coverage typically runs $15,000–$30,000/year per couple. After Medicare at 65, premiums plus supplemental coverage average $7,000–$15,000/year per couple. Fidelity's research projects approximately $315,000 total healthcare expenses for a couple retiring at 65. For early retirees in their 40s and 50s, healthcare is often the single biggest financial obstacle to FIRE — budget $20,000–$30,000/year separate from other expenses.

Long-Term Care: The Often-Ignored Risk

About 70% of people 65+ will need some form of long-term care. Costs: assisted living averages $54,000/year, in-home care $61,000/year, nursing homes $108,000/year. Three approaches: long-term care insurance, self-fund, or rely on Medicaid after spending down assets.

Common Mistakes

Underestimating retirement length. A 65-year-old today has a 50% chance of living past 85. Plan for 25–30 years of retirement.

Ignoring sequence of returns risk. A portfolio averaging 7% can fail if it experiences major losses in the first 5 years while you're withdrawing. Cash buffers of 1–3 years of expenses help.

Forgetting taxes. Traditional 401(k)/IRA withdrawals are taxed as ordinary income. Roth withdrawals aren't. The mix matters enormously for after-tax spending power.

Putting It All Together

  1. Project annual retirement expenses
  2. Subtract expected guaranteed income (Social Security + pension)
  3. The remainder is your annual portfolio withdrawal need
  4. Multiply by 25 (or 30 for early retirement) for portfolio target in today's dollars
  5. Inflate to retirement-year dollars using assumed inflation rate
  6. Calculate required monthly savings using our Retirement Calculator
  7. Add buffers for pre-Medicare healthcare and long-term care
  8. Re-run annually as your situation evolves

Tools to Use

Retirement planning isn't a one-time calculation — it's an annual recalibration. Run the numbers, adjust your monthly savings, run them again next year. Compound math will do the heavy lifting if you give it enough time.