Stop flying blind: How to calculate your cash runway in months

Your bank balance is $140,000. You have $60,000 in outstanding invoices from three clients. Your monthly payroll is $55,000 and you have $12,000 in vendor bills due in the next two weeks. You tell yourself you have about two and a half months of runway. But have you actually calculated it, or estimated it?

Most professional service firm owners estimate their cash runway by dividing their bank balance by a rough sense of monthly expenses. That number is almost always wrong, and it is wrong at exactly the moments when getting it right matters most. Numetix runs expert-led, AI-powered, human-in-the-loop accounting for professional service firms and builds the rolling cash runway model into every client's monthly financial package because the number a firm thinks it has and the number it actually has can diverge by months.

What does cash runway mean for a professional service firm, and why is the standard definition misleading?

The standard definition of cash runway is bank balance divided by monthly burn rate. For a SaaS company with predictable recurring revenue and fixed costs, this is accurate enough to be useful. For a professional service firm, it is misleading because the bank balance at any given moment does not reflect committed inflows (invoices that will be paid in the next 30 days) or committed outflows (payables due in the same period). The correct definition is the bank balance adjusted for near-term cash movements, divided by the actual monthly net cash burn rate based on recent history, not on theoretical budget.

Why the standard definition overstates runway. A service firm with $140,000 in the bank and $55,000 in monthly costs has 2.5 months of runway by the simple calculation. But if $40,000 in vendor payables and payroll taxes come due in the next 10 days, and only $20,000 of the outstanding invoices are expected to arrive before those bills come due, the actual runway looks different. The bank balance is not the starting point. Net near-term cash position is.

Why the standard definition sometimes understates runway. A firm with $140,000 in the bank and $80,000 in invoices due within the next 30 days from creditworthy clients who pay consistently has $220,000 in effective near-term cash. Dividing $140,000 by monthly burn and ignoring the $80,000 that is already earned and on its way produces a runway figure that is too short by a significant margin. Burn rate calculations that ignore near-term receivables consistently understate the true position for firms with healthy, reliable collections.

How do you calculate your cash runway accurately, and what four inputs does the calculation require?

Four inputs produce the correct calculation: current confirmed bank balance (from yesterday's statement, not memory), receivables due within the next 30 days from clients with a strong payment history, payables due within the next 30 days including payroll, taxes, rent, and any vendor bills, and the 90-day average monthly net cash outflow (total cash out minus total cash in, averaged over the last three full months). The formula: (bank balance + near-term receivables − near-term payables) ÷ 90-day average monthly net burn. The result is months of remaining runway adjusted for known near-term cash movements.

Step 1: Confirm your actual bank balance. Pull yesterday's statement balance for all business accounts. Include operating accounts and any reserve accounts that are accessible without penalty. Do not include credit facility availability or investment accounts that cannot be accessed quickly. Those are not operating cash for runway purposes.

Step 2: Identify receivables due within 30 days. Run the AR aging report. From the 0-30 day column, identify which clients have a consistent payment track record and exclude any clients known to pay late. Apply a collection rate: if 85% of 30-day invoices actually arrive within 30 days based on your history, use 85% of the 30-day balance, not 100%.

Step 3: Identify payables due within 30 days. List all committed outflows in the next 30 days: payroll (including employer taxes), rent or office costs, software subscriptions billed monthly, insurance premiums due, and any vendor invoices with due dates in the period. Include estimated payroll tax deposits if you are on a semi-weekly or monthly deposit schedule.

Step 4: Calculate 90-day average monthly net burn. Pull the last 90 days of bank statements. Total all cash outflows and all cash inflows for each of the three months. Subtract inflows from outflows to get the net cash burn per month. Average the three months. This average includes the variability of your actual cash flow pattern (the good months and the slow months) and is a more reliable basis for runway calculation than your budget or your best-case projection.

Note the difference from the intuitive calculation: $140,000 ÷ $55,000 monthly payroll alone = 2.5 months. The correct calculation shows 2.95 months because it accounts for additional receivables partially offset by all payables, not just payroll. In a different example with better receivable timing, the adjusted calculation could produce a longer runway than the bank balance alone suggests. The calculation can move in either direction depending on the AR and AP position.

Why do generic cash runway calculators produce the wrong number for professional service firms?

Three assumptions built into generic calculators that break for service firms: fixed monthly burn (assumes the same cash outflow every month), fixed monthly revenue or zero revenue (assumes predictable inflows or ignores them entirely), and a single balance point (uses today's bank balance without accounting for committed near-term movements). For a firm where revenue swings between $30,000 and $180,000 depending on project milestones, these assumptions produce runway estimates that are wrong by 40 to 60% in either direction: too optimistic in slow months and too pessimistic in strong months.

The most dangerous period for generic calculator errors is when the firm has just completed a large project and invoiced $100,000+. The bank balance looks excellent. The generic calculator says 8 months of runway. But no new projects are in the pipeline, payroll is running, and the next invoice date is 60 days away. The accrual-basis view versus the cash-basis view produces entirely different runway pictures depending on which lens you apply.

How do you build a runway model that updates as your pipeline and receivables change?

A dynamic runway model has three components: a current position section (bank balance, adjusted AR, and adjusted AP updated weekly), a burn rate section (the 90-day rolling average monthly net burn recalculated each month), and a forward scenario section (three scenarios: current trajectory, one large client pays 30 days late, and one large client does not renew). The model updates in 20 minutes per week. The output is a 12-week forward cash position by week. This is not a static calculation. It is a living document that converts current accounting data into a forward-looking position.

The current position section (updated weekly). Pull the bank balance. Pull the AR aging report and identify receivables due in 0-30, 31-60, and 61-90 days. Apply collection rates to each bucket based on actual historical payment patterns, not assumptions. Pull the AP aging report and identify all committed outflows in the same windows. Net these against the bank balance to produce the adjusted near-term cash position. This is the starting point for the burn rate calculation.

The burn rate section (updated monthly). Recalculate the 90-day average monthly net burn at the start of each month using the prior three months of actuals. If the firm added headcount in month two of the period, note that the average understates the current run rate. Adjust the burn rate upward by the incremental monthly cost of the new hire. If a one-time expense inflated one of the three months, note it and exclude it from the average or explain the distortion.

The forward scenario section. Three scenarios give a range rather than a false precision single number:

• Base case: Current trajectory continues. Receivables arrive on historical schedule, burn rate holds at 90-day average.

• Stress case: Your largest current receivable (or client contract) arrives 30 days later than expected. What happens to the cash position in week 6 and week 10?

• Downside case: Your largest client does not renew at the next renewal date. How many months does the resulting revenue gap extend the burn rate? At what week does the cash position fall below 60 days of operating expenses?

Running these three scenarios simultaneously converts the runway model from a measurement to a decision tool. The stress and downside cases are not predictions. They are the questions you need to have answers to before they happen.

What decisions does your cash runway number drive, and what action does each range trigger?

Four runway ranges, four response modes: 12-plus months is safe operating condition (monitor monthly), 6-12 months is caution mode (monitor weekly, review pipeline actively), 3-6 months is action mode (accelerate collections, review all discretionary spending, and validate that new revenue will arrive before cash runs low), and under 3 months is crisis mode (immediate intervention required before the window for action closes). These are not emotional responses to a number. They are pre-committed action protocols that remove the "we still have time" rationalization that causes most service firm cash crises.

What each action actually means.

At 6-12 months: Weekly runway calculation. Review the full client pipeline every two weeks, covering not just opportunities but close probability and expected invoice date. Run the stress case (largest receivable 30 days late) to see how it affects the bottom of the runway range.

At 3-6 months: Contact every client with an outstanding invoice personally. Review all subscriptions, vendor contracts, and discretionary expenses. Pause or cancel anything non-essential. Confirm that revenue expected to land in the next 60 days is real: signed contracts and work in progress, not verbal commitments. Explore whether a credit facility can bridge the gap if needed, before the gap arrives.

At under 3 months: The options narrow significantly. Defer every payable that does not have immediate legal or operational consequences. Call the largest invoice clients directly (not email, phone). Evaluate whether any non-essential assets can be liquidated. Talk to your accountant about credit options. The financial planning conversation that should have happened at 6 months now must happen in days. The runway number did not create this situation. Knowing it accurately and early enough is what determines whether you have options or you do not.

Frequently asked questions

Should you include revenue from signed but unstarted contracts in your runway calculation?

Include it with a discount for probability and timing. A signed contract invoicing in 60 days is a committed inflow: include it in the 60-90 day receivables window. A contract that will not invoice for 90-plus days belongs in the forward projection section, not the base calculation. The runway figure should reflect cash you can reasonably rely on, not the optimistic total of everything under contract.

How do you handle a month where runway calculation is distorted by a large one-time payment in or out?

Exclude the one-time item from the burn rate average and note it separately. If a $60,000 equipment purchase hit in one of the three months, remove it before averaging and note why. Apply the same logic to one-time revenue: a $200,000 project completion should not inflate the average in a way that makes ongoing runway look stronger than it is.

What is a healthy cash runway for a professional service firm, and does the target change at different growth stages?

Six months is the minimum for a stable service firm with predictable revenue. Nine to twelve months provides buffer for business development cycles, client concentration risk, and unexpected team changes. For firms in active growth, twelve months or more is appropriate because growth consumes cash before new revenue materializes. Early-stage firms below $500K should target 90 days of operating expenses as a minimum, building toward six months as revenue stabilizes.