How to Invoice When You Work on Multiple Projects for One Client

One client, many projects, each with its own budget owner, purchase order (PO) and sometimes its own tax treatment. If you put everything into one undifferentiated total, you get delayed approvals, payments applied to the wrong work and sponsors who thought their budget was safe.

Large buyers match every invoice to a purchase order before they pay. US federal procurement is a well-documented example: under FAR 32.905, a proper invoice must include the contract or order number and line item number, along with a description, quantity and price of what was supplied. Private buyers set their own rules, but the principle is the same: make the matching easy.

Decide: one invoice or many

Separate invoices per project

Best when:

• Each project has a different PO
• Budget owners pay from different cost centers
• You need a clean milestone record for each stream
• The projects have different tax treatment or payment terms

One invoice with a section per project

Works when a single accounts payable team pays everything but internal sponsors still need visibility. Start each project's lines with its code, and put a per-project subtotal in the notes.

What to show on every multi-project bill

• Project name or code and PO number for each project, plus the PO line item if the client uses them
• Period covered (especially for hourly or retainer work)
• Expenses tagged to the project that incurred them (expense invoicing)
• Payment instructions once, not repeated per section unless the client requires it

Check the standard fields with how to write an invoice.

Worked example: three projects, one client

An agency runs three workstreams for one client in October: SEO (PO 4500-118), email (PO 4500-121) and a website build billed by milestone (PO 4500-130).

The client owes $2,500 + $1,830 + $6,000 = $10,330 across three invoices. If they pay a single transfer of $8,330 with no breakdown, ask which invoices it covers before you apply it. Under an oldest-first rule written into the agreement, it would clear INV-0210 and INV-0211 ($4,330) and leave $4,000 paid against INV-0212, with $2,000 still due on that invoice.

Allocation and partial payments

When clients pay several invoices with one transfer, request a remittance breakdown. Until you get one, allocate by oldest due date unless the contract says otherwise, and write that rule into the master agreement. See partial payments.

Approvals inside the client organization

Large buyers run approval workflows. Put the approver's name or department on the invoice when you know it; it speeds routing.

Follow-ups without spamming

Follow up per invoice, not with one message about everything the client owes, so a delay on Project A does not feel like pressure on Project B's sponsor, who paid on time. The payment reminder templates give you wording to adapt for each one.

Reporting for yourself

Track revenue and margin per project code even if the client pays in one lump. That shows which streams suffer scope creep or chronic late payment.

Reporting you should run internally

Each month, total revenue and gross margin by project code, even if the client pays with one transfer. Flag projects that take longer to pay than the client's other projects; often one sponsor is the bottleneck. Compare budgeted and actual hours if you track time, because drift predicts invoice disputes. Keep a change log when project codes or POs change mid-year.

Closing checklist

Each quarter, reconcile project codes between your project tool and your accounting records. Retire obsolete codes only after their invoices are paid. Check a sample of invoices for PO accuracy against the client's records. Discuss chronically late sponsors with the account lead, using dates and amounts. Update your notes on the client's approval workflow when new cost centers appear. Keep a one-page billing map showing which code and PO each workstream uses.

Metrics and cadence

Each month, track days to payment by project code within large accounts; one slow code can hide inside a client that looks healthy overall. Track how often invoices need revising; frequent edits mean unclear scopes. Measure the time from sending to the sponsor's acknowledgement. Review any discounts granted per code.

Invoicing multiple projects step by step

Step 1: Use project codes consistently

Weak: "Web design, $5,000". Better: "ACM-WEB, marketing site redesign, PO 4500-130, $5,000". Put the code on every invoice line and in your own time records.

Step 2: Keep one invoice sequence

Use one continuous invoice number sequence for all clients and projects, and put the project code in the lines and notes rather than in the invoice number. InvoiceQuickly, for example, numbers every invoice in a single sequence.

Step 3: Bill each project on its own trigger

Bill Project A when its milestone is accepted, Project B at month end for hours, Project C on completion. Holding one project's bill until the others are ready delays cash and confuses the client.

Step 4: Track projects outside the invoice if your tool can't

If your invoicing tool has no project module, keep a simple register (project code, PO, budget, invoices issued, paid) in a spreadsheet or project tool, and copy the code onto every invoice.

Step 5: Send a monthly summary for busy clients

For clients with many projects, a monthly email listing projects in progress, open invoices with amounts, recent payments and the total balance gives visibility without one oversized invoice.

Multi-project invoicing in InvoiceQuickly

InvoiceQuickly has no PO field. On Pro, a project holds a client, hourly rate, currency and status, and you can log time and expenses against it. What works today:

• Separate invoices per project for the same client, with the project code and PO in the line descriptions or notes (notes print on the PDF).
• One sequence: invoices are numbered with your prefix and a 4-digit counter (for example INV-0211); new estimates and credit notes have separate counters.
• One tax rate per invoice: if projects need different tax treatment, bill them on separate invoices.
• Partial payments can be recorded against each invoice on every plan, so you can apply a combined transfer invoice by invoice.
• Client portal (Pro): a private link where the client sees all their invoices and receipts in one place.
• Bill selected work (Pro): pick unbilled time and expenses for one client and turn them into one draft invoice you review, with one line per entry or lines grouped by project and description.
• Reports (Pro): invoices, payments, expenses, time, aging and unbilled work per currency, filtered by client or project; Pro's CSV export also lists invoices.

Frequently asked questions

Can I send one invoice for multiple unrelated projects?

You can, but it usually causes problems: different projects often have different POs, payment terms, approvers and cost codes. Separate invoices are simpler for both sides.

How do I handle a client who insists on consolidated invoicing?

Follow their accounts payable process, and give each project its own clearly labelled lines and a subtotal within the invoice. Keep your own tracking by project either way.

What about a retainer that covers work on several projects?

Report the split in the invoice notes, for example "October: 18 of 20 hours used (5 h Project A, 8 h Project B, 5 h Project C)". The retainer is still one fee; the breakdown is for visibility.

Does combining projects on one invoice change the tax?

No. Tax follows what you sold and where, not how you group it on an invoice. If projects are taxed differently, show each on its own lines, or use separate invoices if your software applies one rate per invoice, as InvoiceQuickly does. See sales tax.

How should I number invoices for several projects?

Use one continuous sequence and put the project code on the lines, rather than building project codes into the invoice number. A single sequence makes gaps and duplicates easy to spot.

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Keep each project's invoices separate and easy to match. Get started with InvoiceQuickly.

Last reviewed 28 October 2026.