Online Tool Store
· 5 min read
How to Calculate Your New Average Share Price
Manesh Jayawardhana
CIO & Co-founder
You bought 100 shares at $52. The stock is now $38, you still believe in the company, and you’re thinking about buying another 100. The question you actually need answered before you click anything is simple: what would my average cost per share become, and where does the price need to get back to before I’m even?
People do this arithmetic in their head, get it slightly wrong, and end up with a mental break-even that’s five dollars off. That matters, because the break-even price is what most people use to decide when to sell.
What averaging down actually is
Averaging down means buying more shares of something you already own at a lower price, which pulls your average cost per share down. The maths is a weighted average, not a simple midpoint between the two prices.
Total cost divided by total shares. If you bought 100 at $52 and 100 at $38, that’s $5,200 plus $3,800 over 200 shares — $45 a share. Because both purchases were the same size, that happens to land on the midpoint. Buy 50 the second time instead and the average is $47.33, not $45. The size of each purchase is doing as much work as the price.
That’s the part people get wrong. A small top-up at a low price barely moves your average; a large one moves it a lot. Knowing which one you’re doing changes whether the trade is worth making at all.
Why people get stuck here
- Confusing average with midpoint. Averaging $52 and $38 gives $45 only when the share counts match.
- Forgetting fees. Commissions and stamp duty are part of your cost basis. Ignoring them leaves your real break-even slightly above the number you calculated.
- Multiple prior purchases. Once you’ve bought three or four times, the running average has to include all of them, and reconstructing it from a broker statement takes care.
- Currency conversion. If the stock trades in a different currency, the rate at each purchase date is part of your actual cost.
- Treating a lower average as a gain. Your average cost dropped. Your position’s value didn’t change at all.
What a good calculation looks like
It weights by share count
The formula is total money spent divided by total shares held. Every purchase contributes in proportion to its size, which is why the second trade’s quantity matters as much as its price.
It shows the break-even, not just the average
The number you’ll actually use day to day is “what price gets me back to zero” — and once fees are included, that’s a little above the raw average cost.
It makes the risk visible
Averaging down increases your exposure to a single position. A calculation that shows the new total invested alongside the new average is honest about what just happened: you now have more money in a stock that has fallen.
| Scenario | Second Purchase | New Average | What It Tells You |
|---|---|---|---|
| Equal size | 100 shares at $38 | $45.00 | Average lands at the midpoint |
| Half size | 50 shares at $38 | $47.33 | Small top-ups barely help |
| Double size | 200 shares at $38 | $42.67 | Bigger effect, bigger concentration risk |
Figures assume an initial 100 shares at $52 and exclude fees.
Common mistakes to avoid
- Averaging down on a thesis you no longer hold. If the reason you bought has broken, a lower average cost is just a cheaper way to be wrong.
- Using money you’ll need soon. Adding to a falling position is the opposite of raising cash.
- Ignoring position sizing. Two rounds of averaging down can quietly turn a 5% position into a 15% one.
- Recalculating from memory. Pull the actual fill prices and quantities from your statements; remembered prices drift toward what you wish you’d paid.
- Assuming a lower average means a shorter recovery. It lowers the price you need, but nothing about the company’s prospects changed because you bought more.
How to do it with Stock Average Down Calculator
Online Tool Store’s Stock Average Down Calculator works out the new average cost in your browser — no account, and nothing about your position is sent anywhere.
- Enter your existing holding: the number of shares and the price you paid.
- Enter the additional purchase you’re considering — quantity and price.
- Read the new average cost per share and the new total invested together, not separately.
- Try a couple of different second-purchase sizes to see how much the quantity actually moves the average.
- Add your broker’s fees to the cost side if you want a break-even you can act on.
For the position sizing question that usually follows, the Position Size Calculator and the Stock Profit Calculator are both in the tools directory.
Frequently asked questions
Does averaging down reduce my losses?
No. It reduces the price at which you break even, but your unrealised loss on the shares you already own is unchanged, and you now have more money at risk in the same company.
Should I include brokerage fees in the average?
For a break-even price you can act on, yes — fees are part of what the position cost you. For quickly comparing two possible top-up sizes, leaving them out is fine since they affect both options similarly.
What if I’ve bought the same stock five times already?
The formula is the same, just longer: add up every purchase’s total cost, add up every purchase’s shares, and divide. Work from your broker’s transaction history rather than memory so no partial fill gets missed.
Final thought
Run the number before the trade, not after. If seeing the new average and the new total invested side by side makes you hesitate, that hesitation is the useful output — not the average itself.