Average Share Cost Calculator: a practical guide
How would buying more shares change my average cost?
Explore how an additional purchase changes your combined cost per share.
By VolatileShares · Example version 1 · Updated . Hypothetical inputs, not financial recommendations or live company data.
A worked example, in plain English
The idea in 10 seconds: Mix two purchases: total dollars paid divided by total shares gives the new average.
A new average. A bigger cash commitment.
Buying more commits more money and can increase losses. The desired price may never be available.
- Combined average cost / break-even before sale costs
- $23.40
- Total shares
- 15
- Combined purchase cost
- $351.00
Walk me through the full calculation
Your existing 10 shares have an entered average cost of $25.00. Adding 5 shares at $20.00, plus $1.00 in new purchase fees, brings the total to 15 shares with an average cost of $23.40 each.
Your average falls from $25.00 to $23.40, but you commit another $101.00 and have $351.00 in combined purchase cost. Selling at the new average would only break even before sale costs and taxes. A lower average is not a reason to add shares; the larger holding can create a larger loss.
Inputs used for this example
These are the calculator's example defaults, not suggested amounts or rates. Market calculators initially display USD. Change currency in the toolbar without creating a personal profile. Under optional Location & display settings, choose an example country for its currency and available historical CPI. Review any example household size and rent assumption before explicitly applying local assumptions: matching unedited budget/inflation fields change, while edited or restored inputs are kept. Lifestyle budgets are fictional examples, not measured national averages. Local tax, pension and borrowing links are guidance, not personal calculations. Switching currency alone does not change percentages, ages or share counts.
- Shares you already own
- 10
- Average price you paid per share (USD)
- 25
- Extra shares to test buying
- 5
- Price for the extra shares (USD)
- 20
- Total fees for this extra purchase (USD)
- 1
Work backwards from your goal
What purchase price keeps my average at or below this amount?
Start with the average you want and solve for the extra purchase price.
Choose Work backwards, enter your target and the few essential inputs, then choose Calculate my goal. The answer is checked with the same forward calculation and applied in one step. Example assumptions are disclosed under More options. Expand Calculation details & tools only if you want more detail. Impossible or out-of-range goals show an error instead of invented numbers. Long numbers display commas while editing; calculations and saved snapshots keep the original precision.
Return to the interactive calculator above →How the calculation works
Combined cost = existing shares x existing average cost + added shares x purchase price + new purchase fees. New average = combined cost / total shares.
What this cannot tell you
One existing position and one added purchase in the same share class and USD. Existing average cost must already include earlier purchase fees. Allows fractional shares. Excludes sale fees, taxes, splits and wash-sale adjustments; adding shares can increase losses.
Changing an assumption changes the example, not the future. Verify source dates and actual product, tax and lending terms before relying on any result. These tools do not replace appropriately qualified advice.
Average cost is weighted, not a midpoint
Buying 100 shares at $20 and another 50 at $10 gives a combined average of about $16.67, not $15, before fees. The larger first purchase carries more weight. A lower average can still mean more cash committed and more exposure to a falling stock.
Illustrative arithmetic, not market evidence or a promise of returns.
Keep a useful starting point
In the interactive calculator, compare different assumptions or explicitly save a named snapshot to My browser notebook. Saving is optional and browser-only, not an account or cloud backup. Revisit it on the same browser profile; restoring never refreshes prices or assumptions.