Sizing an Operating Cash Reserve

The 3-6 month rule and its limits

The most-cited heuristic is to hold 3-6 months of operating expenses in a separate reserve account. JPMorgan Chase Institute research on 600,000 small businesses found the median had only 27 cash buffer days — well short of even the lower bound — and that businesses below 14 days were significantly more likely to fail.

Three months covers most operational shocks (one big customer leaving, a delayed launch, a 90-day collections drought). Six months covers a serious downturn or a founder transition. Less than three months means a single bad quarter can be terminal.

Adjusting for your situation

Revenue concentration: if your top 3 customers are 60%+ of revenue, target the high end of the range or beyond. Subscription/recurring revenue: you can run thinner because revenue is predictable. Project-based or seasonal: target the high end because revenue is lumpy.

An undrawn line of credit can substitute for some of the reserve, but only some — banks pull credit lines exactly when you need them. A defensible structure is: 2 months in cash, 2 months in available credit, with the credit reviewed annually for reliability.

How to actually build the reserve

Most businesses don't get to the target reserve in one move. The realistic path is to set a target (say, 90 days of operating expenses), set a monthly contribution that's painful but possible (typically 5-10% of monthly gross profit), and treat that contribution as a non-negotiable expense in the operating budget. Within 18-36 months, most businesses can reach a meaningful reserve from operations alone.

Hold the reserve in a separate account at a separate bank if possible. Friction matters: a reserve sitting in your operating checking account is a reserve that gets used for opportunistic purchases. A reserve in a money-market account at a different bank requires a deliberate transfer and a 2-3 day delay, which is exactly the friction that prevents impulse drawdowns.

Re-baseline the target annually. As payroll, rent, and contractual commitments grow, the dollar reserve needed to cover N months of expenses grows too. A target set in dollars three years ago has probably eroded to fewer months of cover than you think; refreshing the target as a percentage of trailing-twelve-month operating expenses keeps it honest.

When calculating the target reserve, use cash operating expenses, not GAAP operating expenses. Depreciation, amortization, and stock-based compensation are real expenses for P&L purposes but don't require cash to fund — including them in the reserve calculation overstates how much cash you actually need to cover N months of runway and may cause you to under-deploy cash unnecessarily.