Why Your Runway Estimate Can Drop in Half From One Month to the Next

You check your cash projection in August and see 8 months of runway. You check it again in September and it says 4 months. Nothing catastrophic happened. But the number cut in half. If you've experienced this, you're not doing the math wrong. You're probably running into a few predictable cash dynamics that projections capture in ways that feel alarming but are often manageable once you understand them.

What Owners Get Wrong and Why It Costs Money

The most common mistake is treating a runway estimate like a fixed fact rather than a snapshot. Runway is calculated from your current cash balance divided by your net monthly burn. Both of those inputs move every month. A runway number is only as accurate as the month it was calculated in.

When owners see runway drop sharply, the instinct is often to panic or to conclude the projection is broken. Sometimes it is broken. But often the swing is explained by real cash events that are scheduled but uneven, not by a fundamental change in the business.

The cost of getting this wrong is either overreacting to a number that doesn't mean what you think it means, or underreacting because you've convinced yourself the projection is always wrong so you stop reading it. Both responses leave you flying blind at a time when cash visibility matters most.

The Most Common Reasons Runway Drops Sharply

Lumpy revenue is the most frequent culprit. If a significant chunk of your revenue comes in quarterly or annually rather than monthly, your cash balance spikes when those payments land and drops between them. A projection built on your average monthly revenue will look stable. A projection built on your actual payment schedule will show the low points between large payments as genuine low-cash periods.

Scheduled large expenses are the second most common cause. Annual insurance premiums, quarterly tax instalments, equipment purchases, or a large vendor payment that comes due once a year all hit your cash balance in a single month. If those payments weren't clearly visible in your projection model, they look like a sudden change when they show up.

A third cause is a revenue timing shift. If a client delayed a payment by 30 days, or a deal you expected to close in one month closed in the next one, the cash you planned to receive in month 5 of your projection now arrives in month 6. That single shift can move your runway estimate more than you expect, especially if that payment was a large portion of a particular month's expected inflows.

A professional services business with about $1.5M in revenue ran cash projections monthly. In one quarter, the owner saw her runway drop from 7 months to 3.5 months in a single cycle. On review, three things happened simultaneously: a quarterly tax instalment hit, one large client pushed a $50K payment by 6 weeks, and a team bonus that was modeled as a year-end payment went out a month early. None of those events were surprises individually. Together in the same month they cut the runway reading in half. The business was fine. The projection was doing its job, surfacing the convergence of those events clearly.

How to Read a Runway Estimate Accurately

A single runway number, taken alone, is not very useful. What's useful is the shape of the cash curve over the next 90 days. Look at the month-by-month cash balance, not just the total months of runway. A business with 6 months of average runway that dips to $20K in month 3 has a real risk point at month 3, regardless of what the average says.

Separate your fixed monthly burn from your variable and one-time costs. Your fixed burn is rent, payroll, recurring subscriptions, and committed loan payments. These don't change. Your variable and one-time costs are everything else. When you see a runway drop, the first question is whether it's driven by a fixed burn increase or by a one-time or lumpy expense that's already scheduled and visible.

What to Do About It

1. Build your projection on actual scheduled payments, not monthly averages. Pull your bank statements and identify every expense over $2,000 that happens less than monthly. Put those on their actual dates in the model. The same for revenue from clients who pay quarterly or annually.
2. Add a 13-week cash flow view alongside your monthly projection. Thirteen weeks forces you to work at the week level, which catches timing issues that monthly projections smooth over. It's more work to maintain, but it's more accurate for short-term decisions.
3. Flag the months where large payments converge. Once your model has all scheduled large payments entered, scan for months where two or more large outflows land at once. Those are your risk months. Plan either to have cash reserves in place or to delay discretionary spending around them.
4. Track your actual versus projected each month. When the projection is wrong, find out why. Was it a timing difference or an actual new spend? Tracking variance tells you whether your model is improving or still missing recurring items.
5. Set a cash floor and treat it as a hard line. Decide the minimum cash balance you need to operate without stress. When your projection shows you approaching that floor, act then, not when you hit it. Options narrow fast once you're below your floor.
6. Review and rebuild your projection every month, not just when it looks bad. A projection you review monthly is a tool. One you review only when things feel tight is a fire alarm.

The Bottom Line

A runway estimate that drops sharply is not always a crisis. It's often the model doing exactly what it's supposed to do: showing you when cash timing events converge. The goal is to build the model well enough that the drops aren't surprises, and to review it consistently enough that you have time to act when a real low point is coming. If you want help building a cash projection that you can actually rely on, book a free call at peterxiacpa.com/book.

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