What payment terms are standard for consulting invoices?

Net 30 is the consulting default for established clients with signed engagements. New engagements typically have a deposit (25-50%) on signing plus Net 15 or Net 30 on milestone invoices. Top consulting firms (McKinsey, Bain, BCG) bill on Net 30 with no deposit because their bargaining power lets them. Smaller firms negotiate Net 15 or a deposit to manage cashflow.

Should consultants bill hours or fixed fees?

Fixed fees by milestone is the modern standard — easier for the client to approve, easier for you to forecast cash, and removes the "Why did this take 80 hours?" conversation. Hourly is fine for genuinely scope-uncertain work (deep technical investigations, regulatory response) where you can't commit to a fixed price upfront. Either way, state the billing model in the engagement letter and stick to it on every invoice.

How do I bill expenses on a consulting engagement?

Two options. (1) Pass-through at cost: you front the expense, attach receipts to the invoice, and the client reimburses exactly what you spent. (2) Cost-plus markup (often 10-15%): same but you add a small markup to cover the admin overhead of fronting expenses. Most engagement letters specify which. Always bill expenses on a separate invoice or clearly delimited line group — procurement approves expense reimbursement separately.

Do enterprise clients require a PO before they'll pay a consulting invoice?

Almost always, yes. Enterprise procurement systems (SAP Ariba, Coupa, etc.) physically cannot release payment on an invoice without a matching PO. Get the PO number before you send your first invoice — usually it's on the engagement letter, in the SOW, or you can email the procurement contact directly. An invoice without a PO sits in "needs research" indefinitely.

How do I invoice for consulting work that spans a retainer and project scope?

Use two invoice sections: a fixed retainer line ("Monthly advisory retainer — May 2026 — $5,000") and a separate project line below it ("Project work — data migration audit, 14 hours @ $250/hr — $3,500"). The retainer is predictable and should auto-recur; the project work varies and should reference the SOW or change order that authorized it. Never blend them into one line — it creates ambiguity about what the retainer covers and whether project work is in or out of scope.

Invoice Generator

Create professional invoices in minutes

ToolsFeedback

Details

Your Business

Client

Line Items

Review

Invoice Details

Set up your invoice information

Invoice Number *

Issue Date *

Due Date (Optional)

Currency *

Payment Terms

INVOICE

Invoice #: INV-001

From:

Hartwell Strategy Partners

Bill To:

Date: May 17, 2026

Terms: Net 30

Description	Qty	Price	Total
Strategic advisory retainer — current month	1	$4,500.00	$4,500.00
Expense reimbursement — Travel, materials	1	$380.00	$380.00

Subtotal:$4,880.00

Total:$4,880.00

INVOICE

Invoice #: INV-001

From:

Hartwell Strategy Partners

Bill To:

Date: May 17, 2026

Terms: Net 30

Description	Qty	Price	Total
Strategic advisory retainer — current month	1	$4,500.00	$4,500.00
Expense reimbursement — Travel, materials	1	$380.00	$380.00

Subtotal:$4,880.00

Total:$4,880.00

Overview

A consulting invoice bills clients for advisory work, deliverables, or ongoing retainer services. It differs from a standard service invoice by handling milestone payments, retainer drawdowns, expense pass-throughs, and engagement-letter references — all the things consulting engagements use that simple service invoicing skips.

Step-by-step

How to invoice as a consultant

Reference the engagement letter

Open the invoice with the engagement reference: "Per Engagement Letter dated [date]." Most enterprise clients route consulting invoices through procurement, who match every invoice against an approved engagement — no reference, no payment.

Choose your billing model — and stick to it on each invoice

Don't mix retainer + project + hourly on one invoice. Send a retainer invoice on the 1st of the month, separate milestone invoices when deliverables clear, and expense invoices monthly with receipts. Mixing models confuses AP and slows payment.

For retainers, state the period covered

Line: "Strategic advisory retainer — May 2026 — $4,500." Without a period, AP can't determine if it's a duplicate of last month's invoice. Use one retainer line per month, not bundled quarterly invoices.

For milestones, name the deliverable that triggered the bill

Line: "Discovery report — accepted [date] — $8,500." Tie the line to the contract's defined milestone. If your engagement has "interim deliverables" the client hasn't formally accepted, don't bill them yet — you'll spend three weeks on a dispute.

Bill expenses separately, with backup

Travel, materials, software seats — these go on their own invoice (or at minimum their own line group) with attached receipts. Most clients allow expense reimbursement at cost; some allow a percentage uplift. Read the engagement letter.

Set Net 30 for established clients, Net 15 with deposit for new ones

Established enterprise client on retainer: Net 30 is the norm. New client starting a project: 25-50% deposit on signing + Net 15 on milestone invoices. Don't consult on credit for a brand-new client over $25K without a deposit.

What to include

What every consulting invoice should include

Engagement letter reference (date + contract number) at the top

Period covered (for retainer invoices) or milestone identifier (for milestone invoices)

Your professional services description — not "consulting hours" but the engagement's named work

Retainer fees as their own line; project fees as their own line; never mixed

Hours-based billing only if your engagement is explicitly time-and-materials

Expense reimbursements on a separate invoice or clearly marked expense line group

Your tax ID and the client's legal entity name (procurement matches the entity)

Bank wire details — most enterprise clients pay by ACH/wire, not card

Late fee policy (consultants typically use 1.5%/month or "interest at statutory rate")

PO number if procurement issued one — without it, the invoice goes to "needs research"

Watch out

Common consulting invoicing mistakes

Mixing retainer and project work on one invoice. AP sees a confusing total, kicks it to "research," and you wait three weeks.

Billing hourly when the engagement letter says fixed-fee milestone. The client's AP team rejects on contract mismatch.

Billing for an unaccepted deliverable. Wait for written acceptance — most consulting contracts make payment contingent on deliverable approval.

Forgetting to reference the PO number. Most procurement systems literally cannot pay an invoice without the matching PO.

Burying expenses in the main project total. AP needs to separately approve expense reimbursement; merging it forces a manual split on their side.

Common questions

Frequently asked questions.

Retainers go on a fixed monthly recurring invoice — same amount, same date every month, line item names the period: "Strategic advisory retainer — May 2026." Project work goes on a separate milestone invoice when a deliverable is accepted: "Recommendations document — accepted [date]." Don't mix them on one invoice; procurement teams approve retainer and project budgets separately, so a mixed invoice has to be split before it can be paid.

Why PrimeBase

Why consultants move retainer billing into PrimeBase

A template invoice gets paid once. In PrimeBase, the signed engagement letter and SOW live on the client record alongside every retainer and project-work invoice. When the next month starts or a phase wraps, you raise the invoice in one click from the previous one's line items — retainer line and project line stay separate, no re-keying. AR aging shows which clients are slow-pay, and the client sees every paid and outstanding invoice in their branded portal.

Try PrimeBase free See PrimeBase accounting PrimeBase for consulting firms

✓ 14-day free trial · ✓ No credit card · ✓ Cancel anytime

Free consulting invoice template. Project, milestone, or retainer.

Invoice Generator

Invoice Details

INVOICE

From:

Bill To:

INVOICE

From:

Bill To:

How to invoice as a consultant

What every consulting invoice should include

Common consulting invoicing mistakes

Frequently asked questions.

Why consultants move retainer billing into PrimeBase

Related tools and templates.