물타기 (Multagi) 사용 가이드

What Multagi is

Multagi (물타기) is an iPhone calculator that shows how your average purchase price changes when you buy more of a stock you already hold. According to its App Store description it does two things: it calculates your average price automatically, and it calculates how many additional shares you need to buy to reach a target average price. The interface is kept simple so you can enter numbers and read the result quickly.

It does not fetch quotes, place trades or recommend securities, and nothing in its description claims it does. It only does arithmetic on the numbers you type in. That is also a strength, because it is always clear where a result came from.

Who it suits

• People who want to see in numbers what their average price would be before making an additional purchase
• People who want a quick answer to "how many shares do I need to bring my average down to this price?" without a spreadsheet
• People who want to see how much extra money a plan would require and judge whether it is within their means

The calculation in one picture

The core of averaging down is a weighted average. In the picture below, the width of a bar is quantity and its height is price, so a bar's area is its purchase amount. A rectangle 200 shares wide with the same total area as the 100 shares held at 50,000 plus the 100 shares just bought at 40,000 has a height equal to the new average price, 45,000.

As the picture shows, the average is pulled toward whichever side has more shares. If the added quantity is far smaller than what you hold, the average barely moves; if equal, it lands exactly in the middle; if larger, it moves closer to the new price. The app's two functions, computing the average price and computing the quantity needed for a target average, solve this relationship forward and backward.

How to use it

Field labels in the app may differ between versions, so follow what you see on screen. The general flow is as follows.

Calculating the average price

1. Enter your current quantity and average purchase price. You can find both on the holdings screen of your brokerage app.
2. Enter the price and quantity of the additional purchase.
3. Read the new average price and total quantity.
4. Change the quantity or price to compare several scenarios.

Finding the quantity needed for a target average

1. Enter your current quantity, your average price and the price at which you would buy more.
2. Enter the target average price you want to reach.
3. Read the number of additional shares needed, then multiply by the purchase price to estimate the money required.

The target must be lower than your current average and higher than the price you would buy at. If the purchase price is at or above the target, no number of shares can reach it. The math behind this is explained with formulas and examples in How average purchase price works, and what to check before averaging down.

Privacy: offline calculation

According to the SETLOG privacy policy, all of Multagi's calculations are performed locally on your device, offline, and no personal or financial data is collected or transmitted. Numbers that may feel sensitive, such as your holdings and buy prices, therefore stay on the device. See the privacy policy for details.

Following along with an example

Say you hold 100 shares at an average of 50,000 and plan to buy 100 more at 40,000. The total paid is 9,000,000 for 200 shares, so the average is 45,000. If you instead set a target average of 48,000, the additional quantity needed is 25 shares. Entering these numbers into the app and checking that you get the same results is a good way to understand how it calculates. The figures are hypothetical and unrelated to the price of any real security.

Notice how a small change in the target average changes the required quantity and money a great deal: with a target of 41,000 the quantity grows to 900 shares. Seeing that non-linear relationship in numbers rather than by intuition is the main reason to use a calculator at all.

Check it yourself with a calculator

The calculator below is not the app but an average price calculator built into this site, and you can enter the example above as is. Replace the sample values, putting 100 shares at 50,000 on the first row and 100 shares at 40,000 on the second, and you get 200 shares, a total cost of 9,000,000 and an average of 45,000. The interface of this calculator is in Korean (quantity is 수량, price is 매수 단가). Numbers are computed only in your browser and are not sent anywhere. The calculator sits in the Korean section: jump to it.

Practical tips

• Do not look at just one scenario; vary the quantity and compare. You will see that doubling the purchase does not halve the gap in your average.
• Once you have a required quantity, check what share of your total assets it represents. The larger a single position becomes, the more risk is concentrated in it.
• Fees and taxes are separate. The app does a simple weighted average, so the real average shown by your broker may differ slightly. Treat your broker records as the authority.
• Before calculating, write down why you want to buy more. "To lower my average" alone is not a sufficient reason.

What does not change when the average price drops

Using the same example, let us weigh what lowering the average price actually does. The numbers are hypothetical and unrelated to any stock.

As the table shows, a lower average price reduces the rise needed to break even, but the loss in money at the current price stays the same. The loss percentage shrinks only because the invested amount, its denominator, got bigger, and if the price falls further the loss in money grows faster. So averaging down is less a way to cut losses than a choice to put a larger amount on the same stock in order to lower the recovery bar. Before adding, try answering these questions.

• If I were buying this stock fresh today, would I buy this amount at this price?
• After adding, what share of my total assets will this stock be?
• How far can I tolerate it falling further, and what will I do then?
• Is my reason for lowering the average a reasoned judgment, or reluctance to lock in a loss?

These are general checking questions and do not recommend for or against any trade.

Limits and cautions

• The app only computes the numbers you enter. It tells you nothing about whether the price will rise or fall.
• A lower average price does not mean a smaller loss. You have put in more money, so your exposure is larger.
• This app and guide are not investment advice. Consult a qualified professional if you need advice.

What this guide has verified

• Why there are no app screenshots. This guide relies only on what the App Store description and the SETLOG privacy policy say. Screens and input labels may differ by version, so follow the app's own instructions. The picture was drawn by us to explain the calculation.
• Price lookups, stock recommendations and automated trading are not covered. The functions confirmed by the App Store description are computing an average price and computing the quantity needed for a target average. Price and purchase terms follow the store page.
• The calculation is a simple weighted average. Fees, taxes, exchange rates, rounding rules and adjustments such as dividends or splits are not included, so it can differ from your broker app's average. Use your broker's records for real decisions.
• The version listed on SETLOG's site is for iPhone. Android and other platforms are not announced.

How it compares

The formula and more checks are in how average purchase price works and what to check before averaging down.

Sources and further reading

1. Wikipedia: Weighted arithmetic mean
2. Financial Supervisory Service FINE portal (Korean): investor cautions

FAQ