Financial Planning Guidelines with Numbers: A Toolkit for 403(b) Plan Participants
November 22, 2025
By Barbara O'Neill, CFP®, AFC®
A guideline is a recommended principle, course of action, or piece of advice. Unlike rules and laws, guidelines are typically not mandated but, rather, are followed by people voluntarily. Human beings like guidelines because they provide direction and structure, reduce uncertainty, and simplify decision-making.
There are dozens of personal finance guidelines that include numbers. Some require basic math functions while others involve ratios calculated from a combination of numbers. For example, if someone saves $6,000 in a 403(b) and has a $60,000 income, the savings ratio is 1:10 (10%). Ironically, many personal finance guidelines include the word “rule,” even though nobody requires that they be followed.
This post describes 30 personal finance guidelines that contain numbers, grouped into four categories. Also included is a summary of research studies about portfolio asset allocation, retirement savings withdrawals, and emergency fund adequacy. The post concludes with three “need to know” facts and six take-away action steps.
Budgeting and Spending
Budget Split Guidelines — Frequently cited budgeting rules include: 80/20 (spending/saving), 70/20/10 (living expenses/saving or investing/debt or giving), 60/30/10 (essentials/wants/savings and debt), and 50/30/20 (needs/wants/savings and debt). To determine which rule you most closely follow, track income and expenses for a month or two.
Three to Six Rule — This guideline for emergency savings suggests saving three to six months of essential living expenses (e.g., housing and food). For example, $9,000 to $18,000 with monthly essential expenses of $3,000. For many households, this is a stretch. A corollary guideline is any emergency fund savings is better than none.
20/4/10 Rule — This guideline for vehicle financing suggests a 20% downpayment, a loan no more than four years, and monthly expenses under 10% of gross income. For example, with a $6,000 monthly gross income, if a car costs $30,000 and 20% down is $6,000, a $24,000 (8%, four year) loan monthly payment is ~$585, which is less than $600.
30% Rule — The 30% rule suggests spending no more than 30% of gross income on housing costs (i.e., rent or mortgage, property tax, insurance). For example, if monthly gross income is $5,000, 30% equals a maximum of $1,500 spent on housing. This rule is considered unrealistic in many locations in today’s housing market.
0.01% Rule — The “0.01% Rule” from The Wealth Ladder: Proven Strategies for Every Step of Your Financial Life, states that if a potential purchase (above household expenses) costs 0.01% or less of your net worth, don’t worry because the amount will be replaced by earnings on your wealth. For example, if your net worth is $100,000, you can spend $10 more per day ($100,000 x .0001). If it’s $1 million, you can spend $100 extra ($1,000,000 x .0001).
10x Income Rule — The "10x income rule" for life insurance suggests purchasing a policy worth 10 times your annual income. Viewed as simplistic by many, this guideline should simply be used as a starting point for calculating how much coverage you need. A more accurate approach is a detailed needs analysis that considers personal factors such as debts, mortgage payments, and future goals like college expenses.
Rule of $100 — This guideline suggests giving yourself a “cooling off period” of one day for every $100 that a big discretionary purchase costs. It provides perspective and a chance to compare prices. The Rule of $100 is easy to implement with online purchases. Simply leave items in your shopping cart until you are ready.
Rule of Three — Establish a personal rule to always compare at least three different product or service providers for “big ticket” items. Key factors to consider include price, a product or service provider’s experience, product or service features, convenience, referrals from other people, and a vendor’s consumer complaint history.
Investing and Investment Growth
Compounding Rules — The Rule of 72 estimates how long it takes to double a sum of money by dividing the interest rate into 72 (e.g., 72 ÷ 6% = 12 years). The Rule of 115 estimates how long it takes to triple a saved sum (e.g., 115 ÷ 6% = 19.2 years) and the Rule of 144 estimates how long it takes to quadruple savings (e.g., 144 ÷ 6% = 24 years).
Three Fund Rule — This guideline is a simple investment strategy where an investment portfolio is built with three mutual funds with low expense ratios (expenses as a percentage of assets): a U.S. total stock market index fund, an international stock index fund, and a U.S. total bond market index fund.
Six Percent Rule — With this guideline, 6% is used as a long-term growth assumption for investment growth and retirement planning calculations. It reflects the fact that, while long-term average returns on stocks are around 10%, most investors do not invest all their money aggressively.
Ten and Fifteen Percent Rules — These are frequently cited rules for retirement savings: save at least 10% of gross income (e.g., $60,000 x .10 = $6,000) and save at least 15% of gross income (e.g., $60,000 x .15 = $9,000).
Stock Asset Allocation Guidelines — Guidelines for the percentage of stock to hold in a portfolio are 100 - age (conservative investors), 110 - age (moderate risk investors), and 120- age (aggressive investors). Example for a 50-year old: 50% (100-50), 60% (110-50), and 70% (120-50).
Expense Ratios — Two general guidelines are “lower is better” and read the prospectus. For example, the Vanguard Total Stock Market Index Fund (VTSAX) has an expense ratio of 0.04% (40 cents annually per $1,000 invested) vs. some actively managed funds with 1.5% or higher expense ratios (annual cost of $15 per $1,000 invested).
Break-Even Return on Investments — This is the minimum average return that your investments must earn to outpace taxes and inflation. The formula is Break-Even Rate = Inflation Rate ÷ (1 – Marginal Tax Rate). Example with 3% inflation and someone in the 22% marginal tax bracket: 3 ÷ (1-.22) = 3 ÷ .78 = 3.85%.
60/40 Rule — The popular 60/40 investment rule allocates 60% of a portfolio to stocks for growth and 40% to bonds for stability and income. This mix allows investors to benefit from diversification. When stocks fall, bonds often hold steady or rise. The strategy is especially popular among moderate-risk investors nearing or in retirement.
TMND Wealth Test — This guideline from the book The Millionaire Next Door states that someone’s expected net worth should be at least this formula-based number: ENW= Current Age x Annual Pre-Tax income ÷ 10. For example, the ENW of a 50-year old earning $70,000 = 50 x $70,000 = $3,500,000 ÷ 10 = $350,000.
Debt and Credit
Thirty-Six Percent Rule — This rule, used by many mortgage lenders, states that total monthly debt payments (mortgage PITI, car loan, student loan, credit cards, other installment debt) should not exceed 36% of gross monthly income. Example: gross monthly income = $6,000 and maximum total debt allowed = $2,160 ($6,000 x .36).
Consumer Debt-to-Income Ratio — In another measure of maximum debt, monthly consumer debt payments (excluding a mortgage) should not exceed 15% of net (take-home) pay and a 20% DTI ratio is considered a danger zone. Example: car loan payment ($400) + credit cards ($200) and $4,500 net pay: 600 ÷ $4,500 x 100 = 13.3%.
Credit Score Guidelines —‚ FICO (Fair Isaac Corporation) credit scores range from 300 (lowest) to 850 (highest). Scores of 740+ are considered the best evidence of creditworthiness and borrowers with those scores receive the best rates. Credit scores of 740-799 are considered very good and 800-850 exceptional.
Credit Utilization Ratio — This is the percentage of available revolving credit currently being used. It is calculated by dividing the total balance on revolving accounts by the total credit limit and multiplying by 100. This ratio is a significant factor in credit scores and experts recommend keeping it below 30%. Example: $5,000 ÷ $20,000 = 25%.
Retirement Planning
Four Percent Rule (and Alternatives) — This is where retirees withdraw 4% of their savings in year one of retirement and inflation-adjust the amount annually for a high probability of not outliving their assets. Example: with $500,000 of savings, $20,000 would be withdrawn in year #1 ($500,000 x 0.04). If inflation is assumed to be 3%, the year #2 withdrawal would be $20,000 x 0.03 = 600 + $20,000 = $20,600. Some researchers have suggested 3.7% and 4.7% withdrawals.
Rule of 25 — This guideline suggests a retirement savings target of 25 times desired retirement spending by the time you retire and assumes a 4% withdrawal rate to align with the 4% Rule. Example: If desired annual spending (after a pension and/or Social Security) = $60,000, required retirement savings = 25 x $60,000 = $1,500,000.
$1,000 of Monthly Income Rule — This guideline also aligns with the 4% Rule. It states that, for every $1,000 of monthly income needed to supplement guaranteed income sources, someone needs to save about $300,000. The math is as follows: $300,00 x .04 = $12,000 annually ÷ 12 months = $1,000 per month.
Retirement Savings Income Multiples — The investment companies Fidelity and T. Rowe Price developed frameworks for retirement savings based on multiples of income at various ages. Their recommendations differ based on different assumptions. Example: Fidelity suggests 10x income at age 67 while T. Rowe Price suggests 11x income at age 65.
2.5 by 65 Rule — This is an ambitious savings guideline to supplement guaranteed income sources and maintain a comfortable upper-middle class lifestyle in retirement. It suggests saving $2,500,000 by retirement age since the long-standing “holy grail” of $1,000,000 has been eroded by inflation over time.
Rule of 80 — This is a pension eligibility guideline used in many public sector pension systems. It states that if the sum of your age and years of service equals 80 or more, you qualify for full retirement benefits. Examples: age = 55/years of service = 25 and age = 60/years of service = 20.
73 and 75 Rules — Mandatory formula-based withdrawals from tax-deferred plans called required minimum distributions (RMDs) must begin at age 73 for taxpayers born from 1951 to 1959 and at age 75 for those born in 1960 or later. Income taxes are due on pretax saving contributions and retirement plan earnings.
Rule of 55 — This is an actual rule and part of the IRS tax code. It allows penalty-free withdrawals if you retire, quit, or are laid off in the year you turn age 55 or later. The Rule of 55 only applies to qualified employer plans such as 403(b)s but not to IRAs. Of course, the withdrawn amount is taxed as ordinary income.
72(t) Rule — This tax code rule can be used before age 55. It requires participants who want to tap their 403(b) early to take “substantially equal periodic payments” (SEPPs) for the later of at least five years or until they reach age 59½. Examples: a 51-year old must take SEPPs until age 59 ½ while a 57-year old must take SEPPs until age 62.
Research Results
A 2025 study found that investors following the 60/40 asset allocation rule sacrifice long-run returns compared to portfolios more heavily invested in stocks. Another study found that only 33% of U.S. households had financial assets sufficient to cover three months of spending. A third study confirmed a safe withdrawal rate from retirement accounts (to avoid outliving savings) of about 4.5%.
Three (More) Things
- Financial guidelines may not fit everyone’s unique circumstances and can be unrealistic for some people.
- Following guidelines, such as diversifying investments, can improve financial stability and reduce stress.
- While guidelines serve as useful benchmarks, they should be viewed as flexible tools rather than strict rules.
Six Smart Strategies
No. 1: Assess Your Finances — Review income, expenses, debts, and savings to understand your baseline before applying any guideline.
No. 2: Choose Relevant Guidelines — Select rules that fit your goals: e.g., 50/30/20 budget, 3-6 month emergency fund, or saving 15% of income.
No. 3: Adjust Personal Guidelines — Modify percentages or target amounts based on income variability, family needs, or high cost living areas.
No. 4: Track Progress Regularly — Use apps or spreadsheets to monitor spending and savings compared to your chosen financial guidelines.
No. 5: Stay Flexible — Adapt your guidelines as life evolves (e.g., marriage or retirement) to keep your plans realistic and effective.
No. 6: Test and Tweak — Experiment with guideline variations for a few months to see what feels sustainable and realistic.
In Summary
This is not a complete list of financial guidelines with numbers, but it is a good start. There are also guidelines for tipping (15%, 18%, 20%), tithing (generally 10%), and retaining tax records for a least three years and seven years “to be safe.” Start small by incorporating one or two relevant guidelines into your life.
This post provides general personal finance information and does not address all the variables that apply to an individual’s unique situation. It should not be construed as legal or financial advice. If professional assistance is required, the services of a competent professional should be sought.
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Dr. O'Neill is the owner/CEO of Money Talk: Financial Planning Seminars and Publications where she writes, speaks, and reviews content about personal finance. She is a Distinguished Professor Emeritus at Rutgers University and a long-time 403(b) plan participant.