function calculateShares() {
var buyPrice = parseFloat(document.getElementById('buyPrice').value);
var quantity = parseFloat(document.getElementById('quantity').value);
var buyCommission = parseFloat(document.getElementById('buyCommission').value) || 0;
var sellPrice = parseFloat(document.getElementById('sellPrice').value);
var sellCommission = parseFloat(document.getElementById('sellCommission').value) || 0;
var taxRate = parseFloat(document.getElementById('taxRate').value) || 0;
if (isNaN(buyPrice) || isNaN(quantity) || buyPrice <= 0 || quantity 0) {
var grossSale = sellPrice * quantity;
var netSale = grossSale – sellCommission;
var profit = netSale – totalCost;
var roi = (profit / totalCost) * 100;
var taxAmount = 0;
if (profit > 0) {
taxAmount = profit * (taxRate / 100);
}
var netAfterTax = profit – taxAmount;
// Breakeven (Price needed to sell to cover all commissions)
var breakEven = (totalCost + sellCommission) / quantity;
document.getElementById('netProfitDisplay').innerText = profit.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('netProfitDisplay').style.color = profit >= 0 ? "#2e7d32" : "#d32f2f";
document.getElementById('roiDisplay').innerText = roi.toFixed(2) + "%";
document.getElementById('taxDisplay').innerText = netAfterTax.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('breakEvenDisplay').innerText = breakEven.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('sellResultsArea').style.display = "block";
} else {
// Just show break even assuming a 0 sell commission if not specified
var breakEvenSimple = totalCost / quantity;
document.getElementById('breakEvenDisplay').innerText = breakEvenSimple.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('sellResultsArea').style.display = "none";
}
document.getElementById('shareResults').style.display = "block";
}
Understanding Share Calculations
Investing in the stock market requires more than just picking the right company. To be a successful investor, you must understand the mathematical reality of your trades. A share calculator helps you determine your true cost basis, breakeven points, and potential net profits after factoring in the "hidden" costs of trading.
1. Total Investment Cost (Cost Basis)
Many beginners make the mistake of only looking at the share price. However, your total cost basis includes the purchase price of the shares plus any commissions or brokerage fees. The formula is:
Total Cost = (Share Price × Quantity) + Buy Commission
2. The Breakeven Point
The breakeven price is the amount your shares must reach for you to exit the trade without losing money. This calculation must account for both the commission you paid to buy and the commission you will pay to sell.
If you don't calculate this, you might sell at a "profit" based on share price alone, only to find that after fees, you actually lost money.
Practical Example
Let's say you want to invest in 50 shares of a tech company:
Share Price: 200.00
Quantity: 50
Brokerage Fee: 10.00
Total Purchase Cost: (200 × 50) + 10 = 10,010.00
Even though the price was 200.00, your Effective Buy Price is 200.20 per share. You are "down" 10.00 the moment the trade executes.
3. Calculating ROI (Return on Investment)
ROI is the percentage gain or loss relative to your initial investment. It is calculated by taking your Net Profit (Total Sale Value – Selling Fees – Total Cost) and dividing it by the Total Cost.
ROI = (Net Profit / Total Cost) × 100
The Impact of Taxes
Capital gains tax can significantly impact your take-home profits. Our calculator includes a tax field to help you estimate your "Net After Tax" earnings. In many jurisdictions, short-term trades (held for less than a year) are taxed at a higher rate than long-term investments.
Expert Tip
Always keep a record of your total cost basis including fees. This is essential for accurate tax reporting and helps you make unemotional decisions about when to sell. If you are a high-frequency trader, even small $5 commissions can erode your capital over dozens of trades.