When generative AI cuts a two-hour research task to thirty minutes, you have gained time. The question is whether you charge for the time you spent or the time you would have spent — and under the Model Rules, the answer is not a gray area.

The billing ethics of AI-assisted work flow from Rule 1.5's requirement that fees be reasonable. That rule has always governed how lawyers bill; AI adds a new context in which applying it requires some thought. ABA Formal Opinion 512 directly addresses the question, as have several state bar opinions. This guide covers what the ethics require, how they apply across billing structures, and what to update in your practice.

The core principle: you bill what you do, not what you would have done

Rule 1.5(a) requires that a lawyer's fee be reasonable. The factors for evaluating reasonableness include the time and labor required and the results obtained, among others. The time and labor required is the time the task actually took — not the time it would have taken in the absence of AI.

ABA Formal Opinion 512 is direct on this: if AI reduces the time required to complete a task, the lawyer cannot bill for the time the AI eliminated. The efficiency benefit belongs to the client. Billing for hours that were not spent — at any rate — is not a reasonable fee.

This is a clarification of an existing principle, not a new rule. Before AI, a lawyer who developed a template and reused it for a client matter had long been ethically constrained from billing full research-and-draft time for a document they adapted in fifteen minutes. AI extends that principle to a technology that can produce a first draft of almost any document in seconds.

Warning

The most common ethical risk is not intentional overbilling. It is a billing workflow that was not updated when AI was adopted — one that still estimates time based on pre-AI experience or rounds tasks to minimums that no longer reflect actual time spent. Review your billing practices when you add AI to your workflow.

Hourly billing: the clearest case

For hourly matters, the rule is straightforward: bill the time you actually spent.

If AI produces a contract draft in three minutes and you spend forty-five minutes reviewing, editing, and applying professional judgment to it, you bill forty-five minutes — not the three hours you would have billed for the same task without AI. If AI identifies ten relevant cases in two minutes and you spend ninety minutes verifying, analyzing, and working them into a brief, you bill the ninety minutes, not the time pre-AI research would have taken.

The practical challenge is that lawyers often do not track time with the precision that AI-efficient tasks require. A task that used to take two hours and was rounded to 2.0 now takes forty-five minutes and should be billed at 0.75. If your billing system and habits are built around longer-duration tasks, the adjustment is operational as much as ethical.

Some specific questions that come up in hourly billing with AI:

Can you bill for the time spent prompting and reviewing the AI? Yes — the time you spend working with an AI tool is billable time if it is time spent on the client's matter. Writing a detailed prompt, reviewing the output, refining it, and verifying citations are all attorney work time.

Can you bill for learning to use a new AI tool on a client matter? Generally no, or with significant restraint. The competence duty under Rule 1.1 places the cost of keeping up with technology on the attorney, not the client. Time spent learning a new tool is generally overhead, not a client expense.

Can you bill for AI tool subscriptions? Subscription costs are typically overhead — they benefit your entire practice and often multiple clients simultaneously — and are not billed to individual clients as matter expenses, just as you would not itemize your law library subscription as a per-matter cost. If your fee agreement specifically provides for AI costs as billable expenses and the client has agreed to that, different rules may apply; otherwise, treat subscriptions as overhead.

Flat-fee billing: a different question

For flat-fee matters, the ethical analysis is different because the client is paying a fixed amount regardless of time. If AI allows you to complete a matter in half the time the flat fee was priced for, you keep the efficiency — that is how flat-fee pricing works.

But flat-fee pricing and the duty of competence interact in a way that AI makes worth examining. The question is whether a flat fee remains reasonable as AI consistently reduces the time matters take.

Rule 1.5's reasonableness standard applies to flat fees as much as hourly fees. If the market for a type of matter is changing because AI is reducing time and cost across the profession, a flat fee that does not reflect that change may eventually be a fee that is not reasonably related to the services rendered. This is not an immediate compliance question — markets adjust over time, and a lawyer who is earlier to adopt AI efficiency is entitled to a period of higher margin before market rates catch up. But it is a question worth thinking through as you build AI into your practice.

Matters that overrun: The inverse is also true. If you price a flat fee assuming AI efficiency and the matter turns out to be complex in ways AI cannot address, you carry the risk — not the client. Flat-fee pricing with AI works in both directions.

Contingency matters

For contingency matters, where the fee is a percentage of the recovery rather than based on time, AI efficiency does not directly affect the fee amount. The ethical question in contingency matters is different: whether the work product is competently produced, not how long it took. AI that helps you prepare better pleadings, better research, or stronger settlement analysis serves the client on a contingency matter without raising a billing ethics question.

Passing AI costs to clients

The question of whether you can charge clients for AI tool costs as disbursements — the way you might charge for court filing fees, deposition transcription, or expert costs — requires attention to your fee agreement and, where applicable, your jurisdiction's rules on expenses.

The general framework from ABA Formal Opinion 512:

Overhead costs cannot be charged as disbursements. A subscription that benefits your whole practice — an AI research assistant you use across all matters — is overhead. You cannot charge the full subscription cost to a single client any more than you could itemize your office rent.

Direct costs attributable to a specific matter may be chargeable. If your jurisdiction's ethics rules permit charging actual costs, and a specific AI cost is directly attributable to a single client matter (for example, a significant API compute cost incurred specifically for that matter), it may be chargeable as an expense — if your fee agreement covers it. This is a narrower category than most AI tool costs.

Your fee agreement governs what is chargeable. If your engagement letter does not specifically address AI costs as potential expenses, you are unlikely to be able to charge them later without a fee agreement amendment the client has agreed to.

Tip

If you want to address AI costs explicitly in your engagement letters, a simple provision works: "We may use AI tools in connection with your matter. Subscription costs for AI tools are overhead and are not charged as a matter expense. For any significant per-matter AI compute cost that exceeds [threshold], we will advise you in advance." This handles the question before it arises.

Disclosure considerations

When AI use materially affects the nature or cost of work, disclosure considerations arise under Rule 1.4. For billing specifically:

If a fee agreement was priced based on pre-AI time estimates and your actual time is significantly lower because of AI, consider whether transparency with the client about the change serves the relationship — even if you are not ethically required to reduce the fee
If you pass any AI costs to clients, the cost and the basis for it should be stated on the invoice
Some state bar guidance requires more specific disclosure when AI materially affects billing; check your state bar's current position

Updating your billing practices for AI

If you have added AI tools to your practice without revisiting your billing workflow, the following is worth reviewing:

Time tracking. Are you tracking time in a way that captures the actual time AI-assisted tasks take? Tasks that used to be estimated at 2.0 hours may now routinely take 0.75. Your invoices should reflect what you actually did.

Billing minimums. If your billing system applies a minimum entry (0.1 or 0.25 hours) regardless of actual time spent, review how that applies to AI-compressed tasks. A task that takes three minutes cannot reasonably be billed at 0.25 if the only work involved was prompting and reviewing AI output.

Engagement letters. Does your engagement letter address AI use, AI costs, and how AI efficiency affects billing? If not, adding a brief provision eliminates ambiguity for both you and the client.

Flat fee schedules. If you offer flat fees, is your pricing still reasonably related to the work involved given AI efficiency? This is a market question as much as an ethics question, but it is worth reviewing periodically.

For the full ABA Formal Opinion 512 analysis — including its treatment of fees alongside the other professional duties — see ABA Formal Opinion 512 explained. For putting your AI billing practices into a written policy, see writing your law firm's AI use policy.

Billing Ethics of AI-Assisted Legal Work: What You Can and Can't Charge