Ultimately, retirement planning is like a math equation — you input several variables and estimate whether what you'll have will pay for what you will need in retirement. The challenge is that many of the variables are future values that are unknowable today.

But that doesn't mean you can't make some educated guesses. So let's examine the “what you'll need” part of the equation — that is, how much the retired life will cost you each year — to see if we make the murky crystal ball any clearer.

How much do retirees need?

The standard rule of thumb is that retirees need 70 to 80 percent of their pre-retirement income. Fortunately, we can examine how spending changes as we age by looking at the Consumer Expenditure Survey that is produced every year by the U.S. Bureau of Labor Statistics. That can help us figure out if this 70-to-80-percent estimate has any basis in reality.

The following table highlights average income and expenditures of households led by people in different age groups. (I selected particular categories from the much larger Consumer Expenditure Survey.)

EXPENSE ITEMS	AGE 25-34	AGE 35-44	AGE 45-54	AGE 55-64	AGE 65-74	AGE 75+
Income before taxes	$59,613	$76,128	$79,589	$68,906	$49,711	$31,782
Avg. number of persons per household	2.9	3.3	2.8	2.2	1.9	1.6
Average annual expenditures	$46,617	$55,946	$57,788	$50,900	$41,434	$31,529
Food at home	$3,338	$4,255	$4,369	$3,681	$3,213	$2,643
Food away from home	$2,753	$3,227	$2,861	$2,387	$1,935	$1,230
Housing	$16,845	$20,041	$18,900	$16,673	$14,420	$11,421
Apparel and services	$2,087	$2,040	$1,966	$1,571	$1,186	$708
Transportation	$8,231	$8,763	$9,255	$8,111	$6,086	$4,288
Health care	$1,800	$2,583	$3,261	$3,859	$4,922	$4,754
Entertainment	$2,251	$3,058	$3,088	$2,683	$2,341	$1,374
Reading	$61	$80	$104	$126	$147	$135
Education	$839	$963	$2,094	$917	$240	$140
Pensions and Social Security	$5,151	$6,664	$7,227	$5,932	$2,261	$763
Personal taxes	$1,055	$1,992	$3,323	$2,295	$1,116	$144

Factors that reduce living expenses among retirees

As you study the table, you notice that expenditures peak somewhere between age 45 and 54 — then they gradually decline. Here are some of the reasons that drive the trend:

• Fewer people under the roof. Eventually, kids leave the house and you wind up spending less money on food, utilities, education, and Febreze. Also — and this is the sad part — a spouse will pass away. When a two-person household goes down to a one-person household, expenses drop by approximately 30 percent.
• We eat less as we age. As our metabolisms slow down, so does our need for calories. Another unfortunate reason some older people eat less is that they find it more difficult to go shopping and to cook meals.
• The mortgage eventually gets paid off. Roughly 55 percent of households in the 45-to-54 age group have a mortgage, whereas just 13% of the 75-and-older group still have that monthly payment.
• We just slow down. As we age, we spend less on entertainment, clothes, travel, and other semi-discretionary expenses. As a writer and former English teacher who is married to a writer, I was heartened to see that expenditures on reading generally increase as we age, with just a slight dip after age 75.
• We don't save for retirement forever. Once you retire, you'll stop paying the 7.65 percent FICA tax that pays for Social Security and Medicare (15.3 percent if you're self-employed) and you'll stop contributing to your 401(k)s, IRAs, and other savings vehicles. This alone could shave 15 percent to 25 percent off your pre-retirement expenses.
• Uncle Sam likes older people. Senior citizens pay much less in taxes, for several reasons: They receive a higher standard deduction, most Social Security is not taxed, and other sources of income — such as qualified dividends, municipal bond interest, and long-term capital gains — are taxed at lower rates than ordinary income. Plus, as you can see from the first row in the table above, income declines as we age, which puts most older people in the bottom two tax brackets.

Not every expense decreases as we age — notably, health care costs increase. Also, there's a legitimate question about whether senior spending declines out of choice or necessity — i.e., retirees would spend more if they had more. However, for many of the categories, the spending declines are the logical result of getting older and not working anymore. Thus, on the whole, the evidence indicates that the old rule of thumb that you'll need 70 percent to 80 percent of your pre-retirement income in retirement has its foundation in reality.

How to translate statistics to your retirement plans

However, while the average retiree spends less than the average 50-year-old, this is not the case for every retiree. Many spend quite a bit more, especially in the first few years of retirement, as they fill their newfound free time with travel, hobbies, classes, and other forms of recreation. Others see their income needs drop to half of their pre-retirement income. So when it comes to your own financial planning, especially once you're within a decade of retirement, it's important to look at and refine your budget, estimating how much you'll actually need after you kiss the working world good-bye.