There are two primary reasons that may cause a discrepancy with a manual calculation: 

1: Check for Sales that were Tax-Exempt
First, check to make sure there were no sales marked as tax-exempt during your date range. For instructions, see our related solution: How to find orders that were marked tax exempt

2: Altru's Calculation is rounded per Order
When Altru calculates the tax owed, the system is performing that calculation on a per order basis. The system calculates the amount taxable for each order, multiples that by your tax rate,  then rounds appropriately to the nearest cent. Therefore, the rounding may be different when you're calculating tax owed based on a larger pool of sales. To verify the rounding, you can create a sales order query and perform the same rounding per order in Excel. Here are instructions for a commonly used query to do this: 
  1. Go to Analysis > Information Library 
  2. Click Add an ad-hoc query
  3. Select the Source view of Sales Orders and click OK
  4. From the middle column, drag Transaction date to Include records where and set this to be equal to your date range on the Tax Report. 
  5. From the left column, highlight Sales Order Item. From the middle column, drag Type into Include records where and set this to be equal to Tax.
  6. Also from the middle column, drag Total to Results fields to display. This will be the amount of tax you collected. You can also click the pencil with the abz to rename any of these fields if desired.
  7. Next to find the amount of the taxable items sold during your date range, expand Sales Order Item on the left, then expand Sales Order Item Tax. Highlight Taxed Sales Order Item. From the middle column, drag Net Amount to Results fields to display. Click the Sigma button and check the option to SUM this field. Your final query should appear like this: User-added image
  8. Save this query if desired, then export it to Excel.
  9. When the data is in Excel, in column D, add a calculation to multiple the amount taxable on the order by your organization's tax rate. In this example, our tax rate is 8.8%: User-added image
  10. Next in column E, add a calculation to round this number to two decimal places. This will mimic what Altru does when it calculates the tax for each order:User-added image
  11. Once the calculations are complete, apply these formulas to the rest of the rows in your sheet. 
  12. Next, sum column B (Tax collected by Altru during your date range) and column E (Tax we calculated based on the amount of taxable items per order) separately. These totals should match to verify that Altru's rounding is correct.User-added image