Cash Flow Forecasting: The 13-Week Rolling Forecast Method

This guide provides a simple, CFO-grade method to manage your company's liquidity. By building a 13-week rolling cash forecast, you can see your cash runway by the week, strategically steer collections and payments, and eliminate financial surprises. It's a system built for founders that works in perfect concert with your Beancount ledger.

Why 13 Weeks?​

A 13-week forecast is the gold standard for operational cash management for several key reasons:

• Short-Term Control: It covers approximately one business quarter, giving you a clear view of your immediate liquidity. This horizon is long enough to include 2–3 payroll cycles, tax remittances, and typical vendor payment terms, but short enough to remain highly accurate and actionable.
• Receipts & Disbursements View: The forecast uses the "direct method," focusing purely on cash in and cash out. This isn't about accrual accounting or profitability; it's about what will actually hit or leave your bank account, ensuring the forecast ties directly to your bank balance.
• Rolling, Not Static: This isn't a one-time budget. Each week, you drop the week that just passed, add a new week at the end (week 13), and update your assumptions. This keeps the forward-looking horizon constant, turning forecasting into a dynamic, weekly discipline.

What You’ll Build​

1. A Single Spreadsheet: The core of the system is one sheet with 13 columns (Week 1 through Week 13) and clearly defined sections: Opening Cash, Receipts, Disbursements, Net Cash, and Ending Cash.
2. Category Mapping: A simple system to map transactions from your ledger to the forecast categories (e.g., all payments from Stripe are mapped to "Customer Receipts"; Gusto payments are mapped to "Payroll").
3. A Weekly Rhythm: A repeatable process for updating the forecast, tracking variance (forecast vs. actual), and a set of pre-defined triggers for taking action when financial thresholds are hit.

Structure (The Rows You Need)​

Your forecast sheet should be structured with the following rows to capture all cash movements.

• Opening Cash Balance (This must tie to the prior week’s Ending Cash Balance)

• Receipts (Cash In)

  • Customer Receipts: Cash you expect to collect from existing invoices (Accounts Receivable).
  • New Bookings/Prepaids: Upfront payments you expect from new deals closing within the 13-week window.
  • Other Inflows: Any other cash coming in, such as tax refunds, interest income, or grant funding.

• Disbursements (Cash Out)

  • Payroll: The full cash cost, including net pay to employees and all employer-side payroll taxes.
  • Contractors & Freelancers: Payments to non-employees.
  • Cloud/Hosting (COGS): Core infrastructure costs like AWS, GCP, etc.
  • SaaS/Tools: All your software subscriptions.
  • Marketing: Ad spend, agency fees, and other brand-related costs.
  • Rent/Office: Physical office costs.
  • Legal & Accounting: Professional services fees.
  • Hardware/Capex: Purchases of laptops and other equipment.
  • Sales Tax Remittances: Payments of collected sales tax to government agencies.
  • Debt Service: Both principal and interest payments on any loans.
  • One-Offs: Lumpy, infrequent payments like annual insurance premiums or security deposits.

• Net Cash Flow (= Total Receipts − Total Disbursements)

• Ending Cash Balance (= Opening Cash + Net Cash Flow)

The Rolling Mechanics (Copy These Formulas)​

The logic of the rolling forecast is simple and powerful.

• Opening Cash (Week n) = Ending Cash (Week n−1)
• Net Cash (Week n) = Σ Receipts (Week n) − Σ Disbursements (Week n)
• Ending Cash (Week n) = Opening Cash (Week n) + Net Cash (Week n)

Your Weekly Monday Morning Rhythm:

1. Roll the Window: Move the entire forecast forward one week. The old Week 2 becomes the new Week 1. Drop the week that just passed and add a new Week 13 at the end.
2. Update with Actuals: Replace last week's forecast with the actual cash movements from your bank and ledger.
3. Re-estimate the Future: Update your forecast for the upcoming 2–4 weeks with the freshest information you have (newly sent invoices, upcoming vendor payments, confirmed payroll dates).

Mapping from Beancount to Your Forecast​

• Cash Actuals Source: Your weekly actuals are the sum of all postings to Assets:Bank:* and payments from Liabilities:CreditCard:*, grouped by date.
• Receipts Categories: Map payouts from Stripe, PayPal, etc., to your "Customer Receipts" line. Map non-operating inflows to "Other."
• Disbursements Categories: Create a simple mapping of vendors to your forecast buckets. For example, AWS and GCP map to "Cloud/Hosting"; Gusto or ADP map to "Payroll"; your law firm maps to "Legal/Accounting."
• Handling Sales Tax: Even though sales tax isn't revenue, it's a cash flow item. Treat collections of sales tax as a cash receipt and the remittance to the government as a disbursement. The revenue impact lives in your accrual books, but the cash movement matters here.

Tip: Keep a small "Vendor Mapping" tab in your spreadsheet. This ensures that you classify vendor payments consistently from month to month, which is crucial for accurate variance analysis.

The Update Rhythm (30–45 Minutes Weekly)​

1. Pull Actuals (15 min): Download transactions from your bank and credit card accounts. Confirm that your "Ending Cash" for the prior week perfectly matches your actual bank balance. This reconciliation is non-negotiable.
2. Review Accounts Receivable (10 min): List all outstanding invoices and slot them into the week you expect payment. Be conservative and apply realistic collection lags based on past performance.
3. Review Accounts Payable & Payroll (10 min): Slot the due dates for all known upcoming bills. Prefill your payroll dates and amounts for the entire quarter. Stage non-critical disbursements for Fridays to preserve cash optionality during the week.
4. Variance Meeting (10 min): Briefly compare last week’s forecast to the actual results. Note the causes of any significant differences and decide if you need to adjust your forecasting rules going forward.

Accuracy and Decision-Making​

Accuracy Rules of Thumb​

• Weeks 1–2: Aim for ±5–10% error. These dates and amounts should be highly certain.
• Weeks 3–6: Expect ±10–20% error. This period will be a mix of known bills and pattern-based estimates.
• Weeks 7–13: This part of the forecast is directional. It's driven by your sales pipeline and run-rate expenses.

Confidence Codes: To make the forecast easier to read, mark each forecast row with a confidence code: Committed (e.g., payroll, rent), Likely (e.g., invoices to good customers), or Upside (e.g., new deals from the pipeline).

Triggers & Actions (Decide These in Advance)​

A forecast is useless without a plan. Pre-define your actions for when you hit certain thresholds.

• Minimum Cash Floor: For example, your rule might be "We must maintain cash ≥ 1.5× the next full payroll amount at all times." If the forecast shows you'll breach this floor, you immediately execute a pre-agreed plan, such as a collections sprint and a pause on all discretionary spending.
• Runway Guardrail: For example, "If the Ending Cash in Week 13 implies less than X months of burn, we will initiate our financing plan." This could involve seeking a term sheet, offering customers a discount for revenue prepayment, or drawing on a credit line.
• Large Outflow Rule: For example, "Any single non-payroll disbursement greater than 5% of our current cash balance must be approved two weeks in advance and have a fallback plan."

Template and Scenarios​

Simple Category Set (For a Seed-Stage SaaS)​

• Receipts: Customer Receipts, Other Inflows (interest, refunds, grants)
• Disbursements: Payroll (net + ER taxes), Contractors, Cloud/Hosting (COGS), Software/SaaS (OpEx), Marketing (Paid/Brand), Rent/Office, Legal/Accounting, Taxes & Fees, Debt Service, One-Offs / Annuals
• Calculated: Net Cash, Ending Cash

Template (Copy to Your Spreadsheet)​

Label each column with the week-starting date (e.g., 2025-08-18, 2025-08-25, etc.). Freeze the header row and the first column.

Scenario Toggles (Keep it Lightweight)​

You can build simple scenario planning without creating a complex model. Add a "toggle" cell at the top of your sheet for key drivers. For example:

• Collection Days Toggle: [1.0] (Change to 1.2 to model a 20% slowdown in collections)
• New Bookings Toggle: [1.0] (Change to 0.8 to model a 20% miss vs. plan)

Multiply the relevant forecast rows by these toggles to see the impact.

Learning and Avoiding Mistakes​

Variance Tracking (Make Learning Compound)​

In the week that just closed, add two columns: "Last Week's Forecast" and "Actual." Calculate the variance. When you review, tag the reasons for major differences: collection delay, scope slip, unplanned vendor purchase, timing shift. If the same type of variance repeats, change your model's underlying rule. For example, if collections are consistently a week late, change your default collection lag assumption from 21 days to 28 days.

Common Pitfalls (Avoid These)​

• Mixing Accrual and Cash: This forecast is for cash only. Revenue recognized, depreciation, and other accrual concepts belong in your main ledger, not here.
• Forgetting Lumpy Annuals: Annual insurance premiums, large SaaS renewals, and quarterly tax payments can be huge surprises. Schedule them in your forecast as soon as you know about them.
• Ignoring Sales Tax Cash: Even if it’s a pass-through liability, the cash is in your bank account until you remit it. Model both the inflow and the outflow.
• Not Reconciling: If your forecast's Ending Cash doesn't match your actual bank balance, you have a mapping error. You must fix it before you can trust the forecast.
• No Clear Owner: Assign one person the responsibility of updating the forecast every single week. Name a deputy for vacations.

Quick Beancount Tie-Ins​

• Chart of Accounts: Keep your cash buckets clean (e.g., Assets:Bank:Checking, Liabilities:CreditCard:Amex). This makes pulling weekly actuals trivial.
• Fava Checks: Run an Income Statement in Fava with interval: week to sanity-check your forecasted burn rate against your recent actual burn.
• Documentation: When you have a large one-off item, attach the invoice PDF in your Beancount documents/ folder and link to it in your forecast's notes column.

Board/Investor Pack (One Slide)​

1. Graph: A simple line chart of your Ending Cash by week for all 13 weeks. Add a horizontal line showing your minimum cash floor.
2. Table: A small table showing the W1–W13 Ending Cash numbers, plus a bulleted list of the top 5 largest inflows and outflows expected in the quarter.
3. Notes: A few bullet points on key assumptions that have changed since the last update and any triggers you have hit or expect to hit.