Commercial rounding difference
Why the final amount can differ by a cent between net and gross invoices.
→ Open invoices in SimpliServ · Open default tax rates
In practice amounts are rounded to two decimal places (cents). It's exactly this rounding that can make the final amount differ by a cent, depending on whether an invoice is calculated net or gross.
In many cases this is not an error but a consequence of when rounding happens and which value is the basis of the calculation.
In short
A commercial rounding difference typically arises from a combination of:
- rounding at line-item level (each line is rounded to cents individually)
- conversion between net and gross (VAT as a percentage)
- summing up already-rounded individual values
Even with a few lines this can produce a discrepancy of 0.01.
Why does this happen?
There are two common rounding approaches:
1) Per-line rounding (horizontal)
Here each line is calculated and rounded individually. The rounded values are then summed.
Advantage: easy to follow, especially with multiple VAT rates (tax groups can be derived cleanly from the lines).
2) Total rounding (vertical)
Here totals are formed first (e.g. the net total) and only then is VAT calculated from that and rounded.
This can lead to a slightly different final amount than per-line rounding, because the rounding happens at a different point.
Example: a 1-cent difference (20% VAT)
In the example we use the Austrian standard rate of 20%.
Suppose a line has:
- Quantity: 3
- Net unit price: €0.99
- VAT: 20%
Net as the basis
- Net total:
3 * 0.99 = 2.97 - VAT:
2.97 * 0.20 = 0.594-> rounded0.59 - Gross total:
2.97 + 0.59 = 3.56
Gross as the basis
If you round the gross unit price to cents first:
- Gross unit price:
0.99 * 1.20 = 1.188-> rounded1.19 - Gross total:
3 * 1.19 = 3.57 - Net total:
3.57 / 1.20 = 2.975-> rounded2.98 - VAT:
3.57 - 2.98 = 0.59
Result: €3.56 (net basis) vs. €3.57 (gross basis).
The difference arises because the gross unit price was rounded to cents early and that rounding multiplies with the quantity.
What does this mean for the invoice?
- A discrepancy of
0.01is normal for certain price/quantity combinations. - What matters is that net, VAT and gross are internally consistent and that the tax is shown correctly per VAT rate.
Tip: fix the calculation basis early
If you want final amounts not to change due to a later switch:
- Decide whether you invoice net or gross by default.
- Avoid switching the calculation basis after a document has been communicated or posted.
FAQ
Is this an error?
Usually not. If the discrepancy is 0.01 and can be explained by rounding, it's a commercial rounding difference.
Can I prevent it?
You can reduce it by keeping the calculation basis (net or gross) consistent and avoiding a later switch. With cent rounding it can't be fully ruled out in every case.
Related topics
- Create an invoice — here you see net, VAT and gross per line and in the total
- Set up your company & organisation — where you set the default tax rates and the commercial base settings
- Products — maintain net or gross prices per product that feed into the calculation
- Invoice correction & credit note — when an amount does need to be corrected
How is this guide?