What this tool does
Your average cost is everything you paid divided by everything you hold. Put your current average and number of shares on the first line and the planned purchase on the next, and the calculator shows the new average cost per share, the total shares, the total invested and how far the average moved. Add up to twenty lines to copy in a whole history of partial buys.
Add today's price to see the position's market value, the dollar gain or loss and the return, next to the return you had before the extra buy. It also tells you how much the price has to rise to get back to your average. Fill in a target average and a buy price, and it works backwards: how many more shares that takes and what they cost.
All sums use exact decimals kept to eight places, so 3 shares at $10.10 plus 3 at $20.20 average exactly $15.15 and a loss never drifts by a cent. It does arithmetic only and does not suggest when to buy or sell.
How to use it
- Pick dollars, or won for Korean shares.
- On the first line enter the average cost and share count from your brokerage app. If you prefer, enter each past purchase on its own line instead.
- On the next line enter the price and number of shares you plan to buy; the results update right away. Use Add a line for more buys.
- Enter today's price for gain, loss and break-even, or a target average to see how many shares it would take.
How the average is worked out
New average = (current average × shares held + buy price × shares bought) ÷ (shares held + shares bought). Holding 12 shares at $187.35 and buying 8 more at $171.20 gives $2,248.20 plus $1,369.60, or $3,617.80 for 20 shares: a new average of $180.89, down 3.45%.
How far the average falls depends more on how many shares you add than on the price. A small buy next to a large holding barely moves it, and no amount of buying can take the average below the price you are paying.
Averaging down does not erase a loss
A lower average makes the percentage loss look smaller, because more money is now in the position, but the dollar loss is the same and grows if the price keeps falling. That is why the calculator shows the return before and after the extra buy side by side.
A rebound has to be bigger than the drop: a stock that fell 20% needs a 25% rise to get back where it started. The break-even line shows the rise needed from today's price to your new average.
Shares needed for a target average
Shares needed = (current average − target) × shares held ÷ (target − buy price). The answer is rounded up to whole shares, with the exact fractional amount shown as well for brokers that sell fractions. If the target is at or below the buy price, it cannot be reached, and the calculator says so.
An optional commission rate is added to the planned buys only, since the average your broker shows for the shares you already own may include past commissions. Leave it empty if your broker charges nothing.
Your data
Prices and share counts are worked out in this browser and never sent anywhere. They are not saved either; only your choice of currency and the commission rate are remembered in this browser for next time.
Questions and answers
Why is my broker's average a few cents different?
Brokers differ on whether commissions go into the average and how many decimals they keep. This calculator adds no commission unless you enter a rate, applies it only to new buys, and rounds money to the cent.
I sold some shares. What do I enter?
Enter the shares you still hold and the average your broker shows for them on the first line, then any new buys below. Selling part of a position usually leaves the average cost of the rest unchanged.
Does it work for buying higher (averaging up)?
Yes. Enter a price above your average and it shows how much the average rises. The target section is for bringing the average down.
What if I leave the buy price empty in the target section?
If you entered today's price, the calculator assumes you buy at that price. Otherwise it asks for a buy price.
Can it tell me whether to buy?
No. It only does the arithmetic. Whether to add to a position is your call, based on your cash, time frame and how much loss you can take.
