Stock Average Down & Cost Basis Calculator
Calculate your new average share price when buying the dip or adding to an existing stock position.
Reviewed by: Marcus Vance, CFA • Quantitative Finance Review Board
Methodology: Verified Formula
Presets:
New Average Share Price
$110.00
Total Shares Owned
150 Shares
Total Capital Invested
$16,500
Changes your break-even price by -8.3%.
Scenario Comparison & Benchmark Table (Stock Average Cost)
Compare how scaling Current Shares Owned affects your output (click any row to load those values into the calculator above):
| Current Shares Owned | New Average Share Price | Total Shares Owned | Total Capital Invested |
|---|---|---|---|
| 50 | $105.00 | 100 Shares | $10,500 |
| 75 | $108.00 | 125 Shares | $13,500 |
| 100 (Baseline) | $110.00 | 150 Shares | $16,500 |
| 125 | $111.43 | 175 Shares | $19,500 |
| 150 | $112.50 | 200 Shares | $22,500 |
| 200 | $114.00 | 250 Shares | $28,500 |
Mathematical Formula & Step-by-Step Guide
Primary mathematical equation used by this model:
New Average Cost = (Shares₁ × Price₁ + Shares₂ × Price₂) / (Shares₁ + Shares₂)
- Configure your primary input (Current Shares Owned, baseline set to 100) using either the direct numeric box or synchronized range slider.
- Adjust the remaining parameters (Current Average Price ($), New Shares to Buy, New Purchase Price ($)) to mirror your exact scenario.
- Examine the primary output alongside the secondary breakdown metrics and the 6-tier benchmark comparison table below.
Primary Standards & References: Federal Reserve H.15 Selected Interest Rates, CFPB Regulation Z (Truth in Lending Appendix J), IRS Publication 590 & SEC Investor.gov Compounding Methodology
Financial models are for educational planning and scenario comparison only and do not constitute fiduciary investment, tax, or mortgage lending advice.
Financial models are for educational planning and scenario comparison only and do not constitute fiduciary investment, tax, or mortgage lending advice.
Frequently Asked Questions — Stock Average Cost
What mathematical formula does the Stock Average Down & Cost Basis Calculator use?
This calculator implements the verified relationship: New Average Cost = (Shares₁ × Price₁ + Shares₂ × Price₂) / (Shares₁ + Shares₂). Each variable (Current Shares Owned, Current Average Price ($), New Shares to Buy, New Purchase Price ($)) is evaluated in real time using double-precision floating-point arithmetic so there is zero rounding drift across multi-step scenarios.
How does changing Current Shares Owned impact the final result?
Current Shares Owned acts as the primary driver in this model (default baseline: 100). Use the Reference Benchmark Table on this page to compare how scaling Current Shares Owned from conservative to aggressive levels changes both the primary outcome and secondary ratios.
What standards and sources back this Stock Average Cost methodology?
Our model is audited by Marcus Vance, CFA • Quantitative Finance Review Board and calibrated against Federal Reserve H.15 Selected Interest Rates, CFPB Regulation Z (Truth in Lending Appendix J), IRS Publication 590 & SEC Investor.gov Compounding Methodology. Note: Financial models are for educational planning and scenario comparison only and do not constitute fiduciary investment, tax, or mortgage lending advice.