How to Build a 13-Week Cash Flow Forecast for Your Law Firm in 2026: The Step-by-Step Model That Turns WIP, AR, and Trust Into a Real Forecast

Law firms are profitable on paper and short on cash for one reason: the gap between doing the work and collecting for it. With median realization lockup around 43 days and collection lockup around 32 days, most firms are financing roughly 75 days of their own operations. A 13-week rolling cash flow forecast is the single most useful financial tool a managing partner can build — here is exactly how to build one.

Published: 2026-08-19T12:36:46.424Z · Category: Legal Accounting · 9 min read

💡 IN SHORT

A 13-week cash flow forecast projects every dollar entering and leaving your operating account for the next quarter, week by week. For law firms it is uniquely valuable because revenue is not recognized when work is done — it is realized when cash clears, and industry benchmarks put median realization lockup near 43 days and collection lockup near 32 days, roughly 75 days combined. This guide walks through the seven steps: define the boundary, build the opening balance, forecast collections from AR aging, layer in WIP conversion, schedule fixed and variable outflows, model the trust account separately, and run the weekly rolling update.

👥 Who should read this: Managing Partners Firm Administrators Legal Bookkeepers CFOs & Controllers

💸 Why Law Firms Specifically Need This

Most professional service businesses collect close to when they deliver. Law firms do not. A matter generates work in March, gets billed in April after pre-bill review, gets paid in May or June, and — if the client disputes two entries — gets partially written down in July. Four different months, one piece of work.

That lag is measurable. Industry benchmarking puts average realization near 88% (with 90–95% considered healthy) and average collection near 93%. Median realization lockup runs about 43 days and median collection lockup about 32 days. Combined, the typical firm waits roughly 75 days between doing work and banking the cash for it.

📊 Did You Know?

A firm billing $6 million annually with 75 days of combined lockup is carrying roughly $1.2 million of its own working capital at any given moment. That is not a hypothetical — it is the amount of the firm's money sitting in unbilled time and unpaid invoices. Reducing lockup by ten days on that book frees approximately $165,000 in cash without adding a single new client.

🧱 Step 1 — Define the Boundary of the Forecast

The forecast covers your operating account only. Trust money is not firm cash and must never appear in the operating forecast as available funds. This is the single most common error in law firm cash planning and it is also a compliance hazard, because a forecast that shows trust balances as firm liquidity invites exactly the decision that ends careers.

🚫 Red Flag

If your cash flow spreadsheet has a line item that combines the operating balance and the IOLTA balance into a "total cash" figure, delete it today. Client funds held in trust are the client's property until earned and disbursed. Any forecasting practice that treats them as a buffer normalizes the thinking that leads to commingling.

📍 Step 2 — Establish a Clean Opening Balance

Week zero is your reconciled operating bank balance — not the general ledger balance, and not the online banking balance. Use the balance from your most recent completed bank reconciliation, then adjust for outstanding checks that have not cleared and deposits in transit. Starting from an unreconciled number means every subsequent week inherits the error.

📥 Step 3 — Forecast Collections From Your AR Aging

This is where most of the forecasting value lives. Pull your AR aging and assign a collection probability and expected timing to each bucket based on your firm's actual history — not on optimism.

AR Aging Bucket	Typical Collection Probability	Forecast Timing
Current (0–30 days)	90–95%	Weeks 1–5
31–60 days	80–88%	Weeks 2–8
61–90 days	60–75%	Weeks 4–11
91–120 days	35–55%	Weeks 6–13
120+ days	15–30%	Exclude or heavily discount

Use your own numbers if you have them. Run a report of invoices issued 12 months ago and check what percentage of each aging bucket actually cleared and when. Firms are consistently surprised by how much worse the 90+ buckets perform than assumed — which is precisely why the forecast is worth building.

💡 Pro Tip

Forecast large invoices individually rather than by bucket. Any single invoice representing more than 5% of a month's expected collections deserves its own line with a named responsible attorney and a specific expected date. Bucket averages hide the one $180,000 corporate invoice whose timing determines whether you make payroll in week nine.

⏳ Step 4 — Layer In WIP Conversion

Unbilled work in progress is future cash, but only after two conversions: WIP must become an invoice, and the invoice must become a payment. Model both.

For each of the next 13 weeks, estimate (a) how much WIP will be billed, based on your billing cycle — if you bill monthly on the 5th, all conversion happens in specific weeks, not evenly; (b) your realization rate on that WIP, since the industry average of roughly 88% means a meaningful slice never becomes an invoice at all; and (c) the collection lag from invoice to payment, applying the same aging logic from step three.

⚠️ Watch Out

Contingency and flat-fee work break the WIP-to-cash chain entirely. Contingency matters generate no billable WIP and produce lumpy, unpredictable settlement inflows. Flat fees are typically collected up front into trust and earned over time. Both belong in the forecast, but as separate lines with their own logic — never blended into an hourly WIP conversion assumption.

📤 Step 5 — Schedule Outflows Week by Week, Not Monthly

Monthly averaging is what makes a forecast useless. Payroll lands on specific days. Rent hits on the first. Quarterly tax payments, malpractice premiums, and annual software renewals are lumpy and often forgotten until they clear.

👥

Compensation

Salaries, partner draws, guaranteed payments, payroll taxes, and benefits — placed on their actual pay dates, including the months with three pay periods.

🏢

Fixed Overhead

Rent, utilities, insurance, software subscriptions, and debt service. Predictable in amount, so the only work is dating them correctly.

📎

Advanced Client Costs

Filing fees, expert witnesses, medical records, court reporters. For contingency firms this is often the largest discretionary outflow — and the most controllable.

🧾

Vendor Payables

Pull directly from AP with actual due dates rather than estimating. Your payables subledger already contains this forecast.

🏛️

Taxes & Distributions

Estimated tax payments and partner distributions — the two items most often left out and most likely to cause a surprise shortfall.

💻

Capex & Renewals

Technology purchases, office build-outs, annual license renewals. Lumpy by nature and worth a dedicated line.

🔐 Step 6 — Model the Trust Account on a Separate Schedule

Trust does not belong in the operating forecast, but it does need a forecast of its own — because earned-fee transfers out of trust are a real and often significant source of operating cash.

Build a parallel schedule showing expected trust deposits (new retainers, evergreen replenishments, settlement proceeds), expected disbursements (filing fees, expert costs, client distributions), and expected trust-to-operating transfers for fees earned and properly billed. Only that last line feeds the operating forecast, and only after the fee has actually been earned and invoiced.

💡 Pro Tip

Firms with automated trust-to-operating transfer workflows forecast this line far more accurately, because the transfer happens on a defined trigger — invoice issued and fee earned — rather than whenever someone remembers to process it. Manual transfer processes create forecast noise and, worse, leave earned fees sitting in IOLTA where they do not belong.

🔁 Step 7 — Run It as a Rolling Weekly Update

A 13-week forecast built once is a document. Built weekly, it is a management system. Every Monday, drop the completed week, add a new week 13, and — critically — record the variance between what you forecast for the prior week and what actually happened.

That variance log is where the forecast becomes accurate. After eight weeks you will know that your 61–90 day bucket collects at 68% rather than the 75% you assumed, and that advanced costs run 15% above plan every month. Two months of honest variance tracking produces a forecast that is genuinely decision-grade.

📊 Did You Know?

The forecast's real payoff is not predicting a shortfall — it is predicting it far enough out to have options. A cash gap spotted in week eleven can be closed by accelerating collections, deferring a capital purchase, or timing a distribution. The same gap discovered in week one leaves only expensive choices: a line of credit draw, a delayed vendor payment, or a partner capital call.

⚙️ What Makes This Easy Versus Painful

Every input above already exists somewhere in your firm: AR aging, WIP by matter, AP due dates, trust balances by matter, and payroll schedules. The question is whether they live in one system or five.

When practice management and accounting are unified — as they are in CaseQube with LawAccounting built in — the forecast pulls from a single set of records: WIP from time entries, AR from the billing engine, payables from AP, and trust balances from matter-level IOLTA ledgers. When they are separate, the same forecast requires exporting from a practice management tool, exporting from QuickBooks, manually reconciling matter names across both, and rebuilding it all next Monday.

✅ Key Takeaways

Forecast the operating account only — never blend IOLTA balances into firm liquidity, in a spreadsheet or anywhere else.
Start from a reconciled bank balance, not the GL balance or the online banking figure.
Build collection assumptions from your own historical aging performance rather than generic percentages, and forecast large invoices individually.
Model WIP through two conversions — WIP to invoice at your realization rate, then invoice to cash at your collection lag.
Place outflows on their actual dates; monthly averaging hides payroll timing, quarterly taxes, and annual renewals.
Give trust its own parallel schedule, and feed only earned-and-invoiced transfers into operating.
Update weekly and log forecast-versus-actual variance — accuracy comes from the variance log, not the initial model.

Stop Rebuilding Your Forecast Every Monday

LawAccounting gives law firms real-time WIP, AR aging, AP, and matter-level trust balances in one legal-specific system — so a 13-week forecast is a report you run, not a spreadsheet you reconstruct.

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