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The short version
- Every invoice gets a number you never use again.
- Count up by one. A plain sequence like 1041, 1042, 1043 is all most small businesses need.
- Never delete or reuse a number. Mark a mistake as void, keep it, and issue a new invoice.
- Keep estimates on their own sequence, and keep every invoice for as long as the tax rules ask.
Why invoice numbers matter
The number is the one thing on an invoice that is unique. Two invoices can share a client, a date and even an amount. They should never share a number. It is how you and your client talk about the invoice, how a payment gets matched to it, and how your accountant checks that nothing is missing.
The Canada Revenue Agency’s list of details a business client needs to claim back GST/HST does not include an invoice number. You still want one: it is how you prove which invoice a payment was for, and a clean, unbroken sequence makes your records easy to check.
Numbering formats that work
| Format | Example | Good for |
|---|---|---|
| Simple sequence | 1041, 1042, 1043 | Almost everyone. Easy to say on the phone. |
| Year first | 2026-014, 2026-015 | Seeing at a glance which year an invoice is from. |
| Client code | SHAH-003, SHAH-004 | A few clients you bill often, who like their own sequence. |
| Job or project | K12-01, K12-02 | Long jobs billed in stages. |
The simple sequence wins for most small businesses. It never needs a decision, it sorts correctly everywhere, and a gap in it stands out at once. Formats with codes are fine too, as long as every number is still unique across your whole business, not only within one client.
Where to start
You can start at any number. Many businesses start at 1001 instead of 1, so a first client does not receive "Invoice #1". After that, count up by one with each invoice you issue.
If you are moving from another app, carry on from your last number rather than starting again. That way an old invoice and a new one can never share a number.
Mistakes, voided invoices and gaps
Never delete an invoice you have sent, and never reuse its number. If it was wrong, mark it void (or cancelled), keep it in your records, and send a new invoice with the next number. If the client already paid part of it, a credit for the difference keeps the trail clear.
Worked example
A week of invoices with one mistake
- #1041 to Priya Shah
- $472.91, paid
- #1042 to Lee Dental
- $1,130.00, sent
- #1043 to Harbor Cafe (wrong rate)
- Void, kept on file
- #1044 to Harbor Cafe (replaces #1043)
- $904.00, sent
No number is missing and no number is used twice. Anyone checking your records can see what happened to #1043.
Draft invoices you never sent are different: if a number was never issued to anyone, most people simply keep counting. If your app assigns numbers to drafts, it is still best to void rather than delete, so the sequence has no unexplained gaps.
Put the number where payments arrive
A number only helps if it travels with the money. Ask clients to quote it when they pay, and use it yourself everywhere the invoice goes:
- In the email subject: "Invoice #1044 from Northline Plumbing, due Dec 1".
- In the payment details: "Please put 1044 in the message of your transfer" or "Write 1044 on your check".
- In the file name of the PDF, so it sorts and searches cleanly: "Invoice 1044 Harbor Cafe.pdf".
- On the receipt or thank you note when it is paid, so both of you close the same invoice.
When a payment arrives without a reference, match it by amount and client, then ask the client to include the number next time.
Estimates get their own numbers
Number estimates on a separate sequence from invoices. Many estimates never become invoices, and mixing the two leaves holes in your invoice sequence. When an estimate turns into an invoice, the invoice takes the next invoice number, and you can mention the estimate number in its notes ("As per Estimate #23").
Not sure where an estimate ends and an invoice begins? See invoice vs estimate vs quote.
More than one business
Each business should have its own sequence. If you run a renovation company and a separate property maintenance business, their invoices are separate records with separate numbers, even when you send both from the same phone.
In Sumocat, invoices and estimates are numbered for you, each counting up on its own, and the number is kept unique across all your devices. With Pro Plus PRO PLUS you can run up to 5 businesses, and each one keeps its own numbering, clients and settings.
Start with a numbered invoice
Make one in your browser now, then download the PDF. No sign up.
How long to keep invoices
In Canada, the CRA asks you to keep your records for six years from the end of the last tax year they relate to. In the US, the IRS names invoices among the supporting documents you should keep, and says the general period is 3 years, longer in some situations. Keep voided invoices too: they explain the gaps.
Sumocat backs up every invoice to the cloud for free when you sign in, and the Accountant Pack exports any whole tax year with every invoice for your accountant, also free.
Questions
Can two invoices have the same number?
They should not. A duplicate number makes it hard to match payments and can raise questions when your records are checked. If it happens, void one and reissue it with a new number.
Should I restart numbering every year?
You can if your format includes the year, like 2026-001 and then 2027-001, because the full number stays unique. With a plain sequence, keep counting so no number repeats.
Is it a problem if my numbers skip?
A gap is not wrong in itself, but you should be able to explain it. Voiding instead of deleting means every number in the sequence has a record behind it.
Do estimates and invoices share numbers?
They are best kept apart. Estimates that are never accepted would otherwise leave gaps in your invoice sequence.
Sources
The official pages this guide relies on, with the date we last checked each one. Rules and rates change, so check the page itself before you rely on it.
- Canada Revenue Agency, Input tax credits: records you need to support your claim. www.canada.ca/en/revenue-agency/services/tax/businesses/topics/gst-hst-businesses/calculate-prepare-report/input-tax-credit.html Checked .
- Canada Revenue Agency, RC188, Keeping records. www.canada.ca/en/revenue-agency/services/forms-publications/publications/rc188/keeping-records.html Checked .
- Internal Revenue Service, What kind of records should I keep. www.irs.gov/businesses/small-businesses-self-employed/what-kind-of-records-should-i-keep Checked .
- Internal Revenue Service, How long should I keep records?. www.irs.gov/businesses/small-businesses-self-employed/how-long-should-i-keep-records Checked .