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How Dollar-Cost-Averaging Can Work to Your Advantage for Your 401(k)

Average Afternoon on Highway 401

When you invest in your 401(k) plan with salary deferrals from each and every paycheck, you are taking part in a process known as Dollar-Cost-Averaging (DCA).  This process can be advantageous when investing periodically over a long span of time, by smoothing out the volatility of the market and giving you an average cost of your investment shares over time.

How does this work, and how can it be advantageous?

Dollar-Cost-Averaging

When deferring income with each paycheck, typically you will be investing in your 401(k) plan each pay period, whether monthly, bi-weekly, or weekly.  Each pay period the same amount is deferred and invested, no matter what the price of the underlying investments are at the time.  Since you’re always putting the same amount into the investment, when the price of the shares is higher, you purchase fewer shares; when the price is lower, you are purchasing more shares.

Note: DCA can be used with any type of investment account, including a 401(k), 403(b), IRA, or even a non-tax-deferred investment account.  We’ll refer to 401(k) accounts throughout the article since this is one of the more common accounts where DCA is employed.

For example, let’s say that you defer $100 every two weeks into your 401(k) plan, and your investment is an index fund.  For the first pay period the price of the fund is $10.  When you make your deferral and purchase this time, your $100 purchases 10 shares.

Then, in the next pay period the price of the shares of your index fund has increased to $10.50.  Now your $100 purchases 9.5238 shares, and you have a total of 19.5238 shares, at a price of $10.50 per share, for a total account value of $205.

On the following pay period the price of your index fund has fallen to $9.50 per share.  Your $100 deferred will purchase 10.5263 shares of the fund – you now have a total of 30.0501 shares at a price of $9.50, with a total account value of $285.48.

The table below plays out purchases with random amounts over a year and then tallies the result:

Pay Period Amount Deferred Price Per Share
Shares Purchased
Total Shares Total Value
1 $100 $10.55 9.4787 9.4787 $100.00
2 $100 $10.44 9.5785 19.0572 $198.96
3 $100 $9.92 10.0806 29.1378 $289.05
4 $100 $10.33 9.6805 38.8183 $400.99
5 $100 $11.95 8.3682 47.1865 $563.88
6 $100 $11.36 8.8028 55.9893 $636.04
7 $100 $9.14 10.9409 66.9302 $611.74
8 $100 $9.54 10.4822 77.4124 $738.51
9 $100 $11.67 8.569 85.9814 $1003.40
10 $100 $9.76 10.2459 96.2273 $939.18
11 $100 $10.46 9.5602 105.7875 $1106.54
12 $100 $9.62 10.395 116.1825 $1117.68
13 $100 $10.23 9.7752 125.9577 $1288.55
14 $100 $10.70 9.3458 135.3035 $1447.75
15 $100 $10.40 9.6154 144.9189 $1507.16
16 $100 $11.52 8.6806 153.5995 $1769.47
17 $100 $11.37 8.7951 162.3946 $1846.43
18 $100 $10.91 9.1659 171.5605 $1871.73
19 $100 $11.55 8.658 180.2185 $2081.52
20 $100 $10.37 9.6432 189.8617 $1968.87
21 $100 $10.19 9.8135 199.6752 $2034.69
22 $100 $9.98 10.02 209.6952 $2092.76
23 $100 $11.89 8.4104 218.1056 $2593.28
24 $100 $11.82 8.4602 226.5658 $2678.01
25 $100 $10.33 9.6805 236.2463 $2440.42
26 $100 $11.41 8.7642 245.0105 $2795.57

The table above was created by generating random prices between $9 and $11.99 over the 26 periods. In real life, your investment wouldn’t likely have such wildly-fluctuating values during the course of 26 pay periods – I used this degree of fluctuation to demonstrate the benefit of DCA when the investment is relatively volatile.

The Advantage

If, instead of investing $100 every two weeks you saved up the entire $2600 and invested it at the end of the 26th pay period, you would be purchasing all of the shares at $11.41, for a total of 227.8703 shares.  By DCA, your $2600 has increased in value such that you hold 245.0105 shares, with a value of $2795.57 – a net benefit of $195.57.

On the other hand, if you had $2600 to invest at the beginning of the table when the price was $10.55 per share, you would have purchased a total of 246.4455 shares, which would be worth a total of $2811.94 at the end of the 26 periods.

You can see from the table that by Dollar-Cost-Averaging, you achieve an average price per share over the period that is beneficial to you – since you’re purchasing exactly the same dollar amount of shares every time.  When the price is high, you buy fewer shares, and when the price is low you buy more shares.  By doing this over a long period of time, such as 30 years, you will avoid the risk associated with saving up a large sum of money and (perhaps) purchasing shares in an investment at a relatively high price by comparison over the savings period.

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