I can't seem to calculate back to how the COS is calculated on the Sales by product/service summary I have used the weighted average ,method but its not matching.

Hi there, sohail. I'm here to explain why your calculation and the Sales by Product/Service Summary report did not match.

Before anything else, please know that QuickBooks Online employs the First In, First Out (FIFO) method, which means that the first units of inventory purchased are considered the first units sold. This process helps to adjust your assets and Cost of Sales (COS) accordingly whenever sales of stock items are entered. 

The Product/Service Summary report is based on FIFO, not the weighted average, which is why your manual calculation does not match.

For more information, you can read this article with sample scenarios. It will also help you understand the concept of FIFO in QuickBooks Online: What is FIFO? 

In addition, should you wish to compare the calculation in QuickBooks Online to your manual calculation, you may utilize the Cost of Goods Sold calculator. 

Moreover, I'll be adding these informative articles to help you know when to restock and see insights on what you buy and sell:

• Track your stock in QuickBooks Online. 
• Fix negative stock issues. 
• Use reports to see your sales and inventory status. 

I'm always around to help whenever you need me. Enter any queries you have about calculating the cost of sales by clicking the reply button below.