Consulting invoices carry two risks that other trades don't worry about as much: they often bundle a retainer, a project fee, and expenses into one document, and a careless line description can put confidential client information in an email that gets forwarded. A clear structure solves both.
Separate the retainer from everything else
If the engagement has a retainer, give it a single line that states the period and the cap: "Advisory retainer — May, up to 8 hours". Below it, list any work that fell outside the retainer at your stated overage rate, and any distinct project deliverables on their own lines. Mixing all three into one number makes it impossible for the client to see whether they are using the retainer well, and impossible for you to renegotiate it later with evidence.
Decide and write down what happens to unused hours. The common answer is that they lapse at the end of the period, because the client is paying for your availability, not for a bank of time. If hours do roll over, cap how far.
Put expenses in their own section
Reimbursable costs — flights, hotels, rail, parking, software bought for the engagement — belong under an "Expenses" heading, one line per category, each at the amount on the receipt. Attach the receipts to the invoice or reference where they are filed. If you charge a per-diem instead of itemising meals, show the daily rate and the day count. Keeping expenses visibly separate from your fee is what stops a client's finance team from querying the whole invoice.
Use per-diems for travel-heavy work
A per-diem is a flat daily allowance that saves everyone the effort of processing a dozen small receipts. Agree the rate in the engagement letter, then invoice it as "Per-diem — 3 days at $75". Travel time itself, if you bill for it, is a different line at a rate you have agreed — often lower than your advisory rate.
Write descriptions that respect the NDA
Your line items will be read by people outside your main contact, and the invoice may be stored in a system you don't control. Describe the category of work and the timeframe, not the substance. "Board preparation support, first half of April" is safe. Naming the acquisition target, the restructuring, or the individuals involved is not. When in doubt, keep it vaguer and let your status reports carry the detail through a secure channel.
Set terms suited to corporate clients
Larger organisations often run on net 30 or net 45, and their systems may require a purchase order number on the invoice before it can be paid. Ask for the PO reference before you send, state your terms as a date, and name any late-payment interest. For a new corporate client, an upfront month of the retainer is a reasonable request.
Whether your fees carry VAT or another sales tax turns on your registration and where the client sits, so treat the tax line as something to confirm with an accountant rather than a detail this page can settle. It is general guidance, not tax or legal advice.
Keep a clean record of what you billed
Consulting engagements can run for months across several workstreams, so keep a simple ledger alongside the invoices: the period, the retainer and overage amounts, expenses claimed, and the date each invoice was paid. When it is time to review or renew the engagement, that record lets you show how the retainer was actually used rather than negotiating from memory. Store each invoice as the document you sent, and if a figure needs correcting, issue a credit note against the original number instead of resending an edited copy.
Before you send
- Your name or firm, address, and tax number
- The client entity, billing contact, and PO number if required
- A unique invoice number and both dates
- Retainer line with its cap; overage and project work on separate lines
- An "Expenses" section at cost, with receipts, or a stated per-diem
- Descriptions that name the work type and period only
- Payment terms, methods, and late-payment interest
Fill in the fields above, keep your descriptions general, and download the PDF. A free NeatDue account stores your clients and numbers each invoice so recurring retainers take a minute to send.