Interim Invoice vs Final Invoice: Differences and Examples
Interim invoice vs final invoice: when to bill for work in progress, how to deduct earlier payments and retention, with a worked example and a checklist.
Quick answer: An interim invoice bills for part of a project while work is still in progress, based on work completed, a milestone or a schedule of payments. A final invoice is issued when the job is finished: it shows the full contract value, deducts every interim invoice already billed, releases any retention and asks for the remaining balance.
Long jobs rarely wait until the end to get paid. Builders, consultants, agencies and software developers all bill along the way, and the way you structure those interim invoices decides whether the final invoice is a clean close-out or a month of reconciliation emails. This guide compares the two documents side by side and walks through a worked example with real numbers. You can produce both types in our free invoice generator by changing the line descriptions and using the amount-paid field.
What is an interim invoice?
An interim invoice is a bill for a portion of a larger contract, issued before the work is complete. The word "interim" simply means "in the meantime". Depending on your industry you will also hear it called a progress invoice, a progress billing, a stage payment invoice or an interim payment application.
The amount on an interim invoice is usually calculated one of three ways:
- Percentage of completion: you bill the share of the contract value that matches the work done, for example 40% complete on a $60,000 job.
- Milestones: you bill a fixed amount when an agreed deliverable is signed off, such as design approval or a working prototype. Our guide to milestone payments covers this model in depth.
- Time period: you bill monthly for work performed in that month, common on cost-plus and time-and-materials contracts.
An interim invoice is different from a deposit invoice. A deposit is paid before work starts; an interim invoice pays for work already done.
What is a final invoice?
A final invoice is the closing bill for a project or contract. It confirms the total amount earned, subtracts everything you have already invoiced, adds any approved variations or change orders, releases retention if the contract allows, and states the balance still due. Once it is paid, the account for that job should be at zero.
Because it summarizes the whole job, a final invoice is often the most scrutinized document in the project. The client's accounts payable team will compare it with the contract, the purchase order and every interim invoice, so it needs to reconcile to the cent.
Interim invoice vs final invoice: key differences
| Feature | Interim invoice | Final invoice |
|---|---|---|
| When issued | During the project, on a schedule or at milestones | When work is complete or the contract ends |
| Amount billed | Work done in the period, or cumulative work less earlier billing | Total contract value less all earlier invoices |
| Retention | Usually deducted from each interim invoice | Released in full or in part |
| Change orders | Included once approved | All approved variations reconciled |
| How many | Several | One per contract |
| Purpose | Keeps cash flowing during long work | Closes the account and the purchase order |
| Typical label | "Interim invoice 2 of 4" or "Progress billing, period to 31 May" | "Final invoice" with a summary of earlier invoices |
Who uses interim invoices?
Any business with a project that runs longer than one billing cycle can use interim invoicing. It is standard in:
- Construction and trades, where monthly progress claims are the norm. Our construction invoicing guide covers schedules of values, retainage and AIA-style pay applications.
- Consulting and professional services, where engagements last months and fees are billed monthly or by phase.
- Web and software development, often split into discovery, design, build and launch.
- Manufacturing of custom goods, with stage payments as materials are bought and production progresses.
- Events and creative production, where suppliers must be paid well before the event or launch date.
In the UK, the Housing Grants, Construction and Regeneration Act 1996 (the Construction Act) gives parties to most construction contracts a right to stage or periodic payments unless the work is expected to last less than 45 days. Where the contract does not set out a payment mechanism, the Scheme for Construction Contracts fills the gap. You can read section 109 on legislation.gov.uk. Other countries have their own security-of-payment laws, so check the rules where the work takes place.
How interim payments work, step by step
1. Agree the billing method in the contract
State whether you bill by percentage complete, milestone or month, how much retention (if any) the client holds back, how many days the client has to pay, and what evidence must accompany each interim invoice, such as timesheets, photos or a progress report.
2. Measure the work done
At each billing date, measure progress against the contract. On cumulative billing you value all work completed to date, not just this month's work.
3. Deduct retention and earlier invoices
Subtract the agreed retention percentage, then subtract everything you have already billed. What remains is the amount for this interim invoice.
4. Issue the invoice with full references
Give it a unique number (the invoice number generator can suggest a format), quote the client's purchase order and project reference, and label it "Interim invoice 1", "Interim invoice 2" and so on.
5. Close out with a final invoice
When the work is complete, issue the final invoice that reconciles everything and releases retention as the contract allows.
Worked example: three interim invoices and a final invoice
A contractor agrees a $60,000 fixed-price contract (figures exclude sales tax) with 5% retention deducted from every interim invoice and released on the final invoice. Billing is cumulative.
| Line | Interim 1 | Interim 2 | Final |
|---|---|---|---|
| Work completed to date | $18,000 | $42,000 | $60,000 |
| Less retention (5%) | ($900) | ($2,100) | $0 (released) |
| Earned to date, net of retention | $17,100 | $39,900 | $60,000 |
| Less previously invoiced | $0 | ($17,100) | ($39,900) |
| Amount due this invoice | $17,100 | $22,800 | $20,100 |
Check the arithmetic: $17,100 + $22,800 + $20,100 = $60,000, the full contract value. The final invoice of $20,100 is the last $18,000 of work ($60,000 − $42,000) plus the $2,100 of retention held back to date ($18,000 + $2,100 = $20,100), which is the same as $60,000 minus the $39,900 already billed. If the client approved a $2,500 change order during the job, it would be added to "work completed to date" on the invoice where the extra work happened, and the final total would become $62,500.
If the client paid only part of an interim invoice, the shortfall is still owed on that invoice; do not quietly roll it into the next one. Our guide to the partial payment invoice shows how to display amount paid and balance due.
What to include on an interim invoice
- The words "Interim invoice" and its sequence, for example "Interim invoice 2 of 4".
- Contract or project reference and the client's PO number.
- The billing period or milestone being claimed.
- Contract value, work completed to date and percentage complete.
- Retention held, previous invoices deducted and the amount due now.
- Tax, payment terms and due date.
What to include on a final invoice
- The words "Final invoice" so nobody mistakes it for another interim claim.
- Original contract value plus or minus each approved change order.
- A list of every interim invoice with its number, date and amount.
- Retention released and any amounts still held under a defects period.
- The final balance due and how to pay it.
Some clients also want a statement of account with the final invoice so they can see every invoice and payment on one page.
Tax on interim invoices
In most VAT and GST systems, issuing an invoice or receiving payment creates a tax point, so tax is charged on each interim invoice for the amount billed, not saved up for the final invoice. In the US, sales tax on services and construction varies by state. Calculate the tax on each invoice with the sales tax calculator and confirm the treatment of retention with your accountant, because some systems tax retention when it is billed and others when it is paid.
Common mistakes with interim and final invoices
- Mixing cumulative and period billing on the same project, which leads to double billing.
- Forgetting to deduct earlier invoices on the final invoice.
- Not releasing retention, or releasing it before the contract says you can.
- Reusing an invoice number with a suffix instead of issuing a new one.
- Billing unapproved extras. Get a signed change order first.
- Vague descriptions such as "progress payment" with no period or percentage.
Interim and final invoice checklist
- Billing method, retention and payment terms agreed in writing.
- Progress measured and evidence attached.
- Unique invoice number and clear "Interim" or "Final" label.
- Previous invoices and retention shown line by line.
- Tax calculated on the amount billed.
- Totals reconcile to the contract value plus change orders.
Next step: create your interim or final invoice
Open the invoice generator, choose a structured layout such as the Corporate template, and add one line for work completed to date, one for retention and one for previously invoiced amounts. Download the PDF and keep a copy with the project file so the final invoice is easy to reconcile.
Frequently asked questions
Is an interim invoice a legal tax invoice?
Usually yes. If it is a real demand for payment and contains the fields your tax system requires, an interim invoice is a full invoice and normally creates a tax point for the amount billed. If you only want to show the client what is coming, send a payment application or proforma instead and issue the tax invoice when payment is agreed.
How often should I send interim invoices?
As often as your contract allows. Monthly is the most common cycle on construction and long consulting projects, while creative and software work often bills at milestones. If your contract is silent, agree a schedule in writing before work starts so the client's accounts team expects each invoice.
Can the final invoice be lower than an interim invoice?
Yes. The final invoice only bills what is left after earlier invoices are deducted, so it can be small, zero or even negative if the project came in under budget or the client was overbilled. A negative balance is normally handled with a credit note rather than a negative invoice.
What should I write on a final invoice so the client knows it is the last one?
Label it clearly, for example 'Final invoice for Project 2026-014', list every earlier interim invoice number and amount, show the agreed contract value, and state that no further charges will follow unless a change order is approved. That summary makes it easy for the client to close the purchase order.
Do I need a new invoice number for each interim invoice?
Yes. Every interim invoice and the final invoice need their own unique number from your normal sequence. You can add a project code or a suffix such as 'Interim 2 of 4' in the description, but the invoice number itself should never repeat.
What is retention on an interim invoice?
Retention, also called retainage, is a percentage of each interim payment that the client holds back as security until the work is finished and any defects are fixed. It is still money you have earned, so show it as a separate line on every interim invoice. The contract should say the percentage, when it is released and whether part of it is kept for a defects period.
What is the difference between an interim invoice and a proforma invoice?
An interim invoice is a real demand for payment for work already done, so it normally goes into your sales records and creates a tax point. A proforma invoice is only a preview or quote of what you will bill. It is not a request for payment and usually has no tax effect. Send a proforma when the client needs approval before you issue the actual invoice.
How do I add a change order to an interim invoice?
Once the client has signed the change order, add it to the invoice for the period in which the extra work was done, ideally as its own line with the change order number. Increase the adjusted contract value by the same amount so the percentage complete still makes sense. Never bill a variation that has not been approved in writing, because it is the first line a client will dispute.
What can I do if a client pays an interim invoice late?
Follow your contract first: it may set interest, a late fee or a right to suspend work. Many countries also have their own rules. In the UK, for example, businesses can claim statutory interest on late B2B payments, and the Construction Act gives a right to suspend work after notice if payment is not made. Check the law where the work takes place before you suspend anything.
Can I issue the final invoice before the defects period ends?
Yes. Most contracts expect the final invoice at completion, with part of the retention held until the defects or warranty period ends. In that case the final invoice shows the retention still held, and you issue a short separate invoice to release it when the period is over. Write the release date on the final invoice so the client's accounts team can diary it.
Invoice Creator Editorial Team
Guides on invoicing, cash flow and small-business finance from Invoice Creator, operated by Next Software Development Company. Content is reviewed for accuracy against the fields our own invoice generator produces.
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