Input string was not in a correct format.

It seems like the calculation for the total tax is still problematic. Let's focus on ensuring that the tax calculation correctly reads from the DataGridView and sums up the values.

Below is an adjusted version of your CalculateTotal method that emphasizes the correct calculation of the tax column:

public void CalculateTotal()
{
    decimal sum = 0;
    decimal subTotal = 0;
    decimal totalTax = 0;
    decimal discount = 0;
    decimal totalPrice = 0;
    decimal percentageDiscount = 0;

    // Check if txt_Discount.Text is a valid decimal
    if (!decimal.TryParse(txt_Discount.Text, out discount))
    {
        discount = 0; // Default discount to 0 if the input is invalid
    }

    // Check if lbl_TotalPrice.Text is a valid decimal
    if (!decimal.TryParse(lbl_TotalPrice.Text, out totalPrice))
    {
        totalPrice = 0; // Default total price to 0 if the input is invalid
    }

    // Check if lbl_Discount.Text is a valid decimal
    if (!decimal.TryParse(lbl_Discount.Text, out percentageDiscount))
    {
        percentageDiscount = 0; // Default percentage discount to 0 if the input is invalid
    }

    int itemCount = 0;

    for (int i = 0; i < dataGridView1.Rows.Count; i++)
    {
        if (dataGridView1.Rows[i].Cells[0].Value != null) // Ensure the row is not empty
        {
            decimal rowPrice = 0;
            decimal rowTaxPercentage = 0;
            int quantity = 0;

            // Parse row values safely
            if (!decimal.TryParse(dataGridView1.Rows[i].Cells[6].Value.ToString(), out rowPrice))
            {
                rowPrice = 0;
            }

            if (!decimal.TryParse(dataGridView1.Rows[i].Cells[4].Value.ToString(), out rowTaxPercentage))
            {
                rowTaxPercentage = 0;
            }

            if (!int.TryParse(dataGridView1.Rows[i].Cells[7].Value.ToString(), out quantity))
            {
                quantity = 0;
            }

            decimal rowTotal = rowPrice * quantity;
            decimal rowTax = (rowTaxPercentage / 100) * rowTotal;

            sum += rowTotal;
            totalTax += rowTax;
            subTotal += rowTotal;

            itemCount++;
        }
    }

    lbl_NoOfItems.Text = itemCount.ToString();
    lbl_SubTotal.Text = subTotal.ToString("#,##0.00");
    lbl_TotalTax.Text = totalTax.ToString("#,##0.00");

    decimal discountAmount = (discount / 100) * subTotal;
    lbl_Discount.Text = discountAmount.ToString("#,##0.00");

    decimal grandTotal = (subTotal + totalTax) - discountAmount;
    lbl_GrandTotal.Text = grandTotal.ToString("#,##0.00");

    lbl_OverallGrandTotal.Text = grandTotal.ToString("#,##0.00");

    // Calculate change if amount received is entered
    decimal amountReceived = 0;
    if (decimal.TryParse(txt_AmountReceived.Text, out amountReceived))
    {
        lbl_Change.Text = (amountReceived - grandTotal).ToString("#,##0.00");
    }
    else
    {
        lbl_Change.Text = "0.00";
    }
}

Key Points:

1. Row Validation: Ensure each row is not empty before performing calculations.
2. Tax Calculation: Correctly calculate the tax by taking the tax percentage from the DataGridView and applying it to the row total.
3. Accurate Summation: Sum up the totals correctly for the number of items, subtotal, total tax, discount, and grand total.

Make sure the column indices in the code (such as Cells[6], Cells[4], and Cells[7]) correctly correspond to the Price, Tax %, and Quantity columns in your DataGridView.

This approach should ensure the tax is calculated correctly and displayed in the label. If there are still issues, please verify the content and format of the data in your DataGridView.

Hello Sartha, I have been able to modify part of the code and is no longer over calculating the overallgrandTotal but the only problem is the total tax is not able to calculate the tax column in the DataGridView into the label 

Hello Sarthak

Thank you so much, but there is one issue here.

the Tax has been added to the subtotal at the stockin page already, there is over calculating of overallgrandTotal, the item quantity, and the price sum up should be equal to subtotal and I want only the tax coulmns calculated in the lable and as you can see in the snip shot and additionally when I enter the discount the value of the overallgrandTotal doesn't change