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Invoice Adjustment Calculator
Work out what a credit or debit note should be when an invoice needs correcting.
As the invoice went out
As it should have been
Quick Tool Overview
The Invoice Adjustment Calculator takes the invoice as issued and the invoice as it should have been, and works out the correction — the net difference, the tax difference, and whether it is a credit note or a debit note.
- What it does
- Works out the correction an invoice needs
- You give it
- The invoice as issued and as it should be
- You get back
- The adjustment, its tax and its direction
- Best for
- Fixing an invoice that has already gone out
About the Invoice Adjustment Calculator
When an invoice is wrong, the fix is a second document rather than an edit — and the only hard part is working out what that document should say. Not the new total, which is easy, but the difference, and the tax on the difference, and which direction it goes in.
This takes the invoice as issued and the invoice as it should have been, and works out the adjustment: the net difference, the tax difference, whether it is a credit note or a debit note, and the wording for the reason. It handles the awkward version too, where the error is in the tax rate rather than the amount — a supply invoiced at 20% that should have been zero-rated needs the tax credited and nothing else touched.
The order matters here as much as anywhere. An adjustment has to be made in a way that leaves both parties’ records reconcilable: the original invoice stands, the adjustment references it, and the two together produce the correct figures. Editing the original leaves a customer holding a document that no longer matches yours.
Key features of the Invoice Adjustment Calculator
Direction decided
Credit note down, debit note up — named rather than guessed.
Rate errors handled
A 20% supply that should have been zero-rated, corrected properly.
Only the difference
Never the whole invoice again, which would double the debt.
Reason wording
Ready to copy onto the note, in terms tax authorities recognise.
Both ledgers reconcile
The original stands and the adjustment references it.
How to use the Invoice Adjustment Calculator
-
1
Enter the invoice as issued
The net, the tax rate and the total that actually went out.
-
2
Enter what it should have been
The corrected net and rate.
-
3
Read the direction
A credit note reduces what is owed, a debit note increases it.
-
4
Check the tax adjustment
Adjusted at the original rate, or at the corrected one for a rate error.
-
5
Use the wording
The reason statement is on the page, ready to copy onto the note.
Frequently asked questions
Issue a note. A sequential invoice that has been sent is part of your audit trail, and editing it leaves your customer holding a document that no longer matches yours. The original stands and the adjustment references it.
A credit note when the correction reduces what is owed — an overcharge, a return, a cancelled order. A debit note when it increases it — an undercharge, extra work, an omitted charge.
At the rate that applied to the original supply, on the difference in value. Where the error was the rate itself, the adjustment is the difference between the tax charged and the tax that should have been charged, with the net unchanged.
Most tax systems expect a correction within a defined period and in the return period in which the adjustment is made. Sooner is always better — an uncorrected error compounds across every subsequent statement.
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