Retirement planning is complex, so it's no surprise that people harbor many misconceptions about it. These popular untruths can be dangerous, especially if they lull you into a false sense of security. If you save too little, you could end up without enough money to pay bills in your golden years.

Here's a closer look at three retirement planning myths that could land you in serious trouble if you believe them.

1. Retirement is so far away that I don't need to start saving now.

Many people wrongfully assume there's no need to begin saving for retirement in their 20s or 30s, because they have 30 or 40 more years to sock away funds. But the truth is, the later you begin planning for retirement, the more difficult it is to save enough.

Elderly woman

Image source: Getty Images.

Let's say there are two people who both plan to retire at 65, and they have a retirement savings goal of $1 million. Both dutifully save money in their 401(k), enjoying an average annual return of about 7%. The only difference is when they started saving: One began at 25, while the other waited until they were 45.

The person who started saving at 25 would only have to put away roughly $381 per month in order to hit the $1 million by their retirement. The person who waited until 45 must save $1,920 per month.  That adds up to over $23,000 per year, which is actually more than you're allowed to contribute to a 401(k) in one year, as of 2019, unless you're over age 50 when you can make catch-up contributions. Unless the 45-year-old saver is also contributing to an IRA or a nonretirement investment account, that goal will never be reached.

The underlying reason for the difference between these two examples is the power of compounding. When you put money into your retirement account, it will grow over time because it is invested in the stock market, which produces the most superior returns of any asset class. Ideally, your nest egg should see an average growth rate of 7% per year.

Here's how compounding works: The first year, your initial contributions grow by 7%. The next year, your new, larger balance will grow by 7% again, and so on, getting exponentially bigger with time. When you wait to begin contributing to your retirement account, you're reducing the amount of time your money has to grow and consequently costing yourself a lot more money -- and potentially your retirement security. The best part of compounding is that it takes almost no effort at all on your part. Simply save as much money as you can, invest it in a tax-advantaged retirement account and the market will do the rest.

2. Medicare will cover all my healthcare expenses in retirement.

You become eligible for Medicare as soon as you turn 65. The federal health program covers many health expenses, including inpatient hospital care, lab tests, preventive services, and more. But it covers very little in its entirety. Most people won't have to pay a premium for Part A (hospital insurance), but Part B (medical insurance) and Part D (prescription drug coverage) have deductibles, premiums, and co-insurances, just like most health insurance policies.

And there are things that Medicare doesn't cover at all, like long-term care, dental care, hearing aids, and eye exams. You will have to pay for these expenses on your own unless you have a supplementary policy that covers them.

The average 65-year-old couple retiring today will need about $285,000 to cover their medical expenses in retirement, according to Fidelity. If you haven't factored this into your retirement plan because you were relying on Medicare, a serious illness or injury could derail your retirement. You may have no choice but to draw down your retirement accounts faster than you anticipated, leaving you short in the final years of your life.

You can avoid this by first understanding what Medicare does not cover, and then building the costs of healthcare into your retirement plan. For example, if you're concerned about potentially needing long-term care, you may want to invest in a long-term care insurance policy to cover these costs if they should arise. Saving for future healthcare costs in an HSA account is a great option if you have a high-deductible health plan.

3. I'll only need 70% of my pre-retirement income in retirement.

You've probably heard that most retirees only need about 70% to 80% of their pre-retirement income to live on in retirement. That may be true for you, but it's not a good claim to take at face value. Everyone's needs are different, and if you live in an expensive city or you plan to do a lot of traveling, it's highly likely that you'll need more than 70% of your pre-retirement income.

It takes some time, but the best way to be sure of how much money you need in retirement is to actually crunch the numbers. First, estimate your life expectancy. The average life expectancy in the U.S. is 78.6 years, but since it's an average, this may be far younger than you'll actually live. One in four 65-year-olds today can expect to live past 90, according to the Social Security Administration (SSA), and one in 10 will live past 95. Begin here and adjust this number up or down based on your current health, your family history and your lifestyle.

Next, estimate your annual retirement living expenses, keeping in mind some of your current expenses may disappear entirely in retirement. Don't forget to factor in inflation, which is impossible to predict with pinpoint accuracy, but using 3% per year as an estimate works. A simpler way to find out how much you'll need in retirement is to use a retirement calculator, or several to get a ballpark range. Once you have your total number, subtract the amount you expect to receive from Social Security, a pension, or other sources besides your retirement savings. This is how much you need to save on your own, also known as your retirement number or savings target.

No one ever said retirement planning was easy, but it's crucial that you do your best to plan appropriately and start as early as you can. Your future financial security is at stake, and if you wait to correct these errors, it can become too late to catch up.