How much cash buffer should you actually hold?

The right question is not whether you feel comfortable with the balance in your account. Comfort drifts with your mood and your inbox. The real question is colder and more useful: if new revenue stopped tomorrow, how many months could you keep paying everyone on time without borrowing, cutting, or panicking? That number is your cash buffer, and most founders have never set it on purpose.

A cash reserve is money you deliberately set aside and agree not to touch in a normal month. It is not the cash sitting in your checking account, because that is already spoken for — payroll on Friday, rent on the first, the software bills all month. A reserve is the pile behind that, the one that only moves when something genuinely goes wrong.

What a reserve actually protects you from

Businesses rarely fail because of one enormous expense. They fail because several ordinary things go wrong at once and there is no slack to absorb them. A big customer pays 40 days late, a second one churns, and a payroll run lands the same week as your quarterly tax payment. None of these is fatal alone. Together, with an empty buffer, they force bad decisions fast — delay vendors, skip your own paycheck, or raise money on terrible terms.

The reserve exists to buy you time in exactly those weeks. It turns a crisis into an inconvenience, and it is worth being deliberate about how much you hold to do that.

The rule of thumb, and when to bend it

The common guidance is to hold three to six months of operating cash outflow. Operating outflow is the total cash that leaves the business each month to keep it running — payroll, contractors, rent, software, insurance, taxes — not one-time equipment purchases or loan principal. That range is wide on purpose, because the right spot inside it depends on how predictable your money is.

Push toward the higher end when your revenue is lumpy, seasonal, or concentrated in a few large clients. If two customers make up 60% of your income, losing one is a real event you should survive for months. Pull toward the lower end when revenue is recurring and predictable, or when you hold an undrawn line of credit you can tap in a pinch — a backup buffer, though a more expensive and less certain one than cash you already own.

• Concentrated revenue — a few clients drive most of your income: hold more
• Seasonal or project-based sales that arrive in bursts: hold more
• Long payment terms, so cash lands well after the work: hold more
• Recurring subscription revenue that renews predictably: hold less
• An undrawn, committed line of credit as backup: hold less
• Thin margins, where a slow month turns into a real loss fast: hold more

Do the math with a real number

Set the target with one line of arithmetic: monthly operating outflow multiplied by your chosen number of months. Take a company that spends $180,000 a month to operate. Its revenue is mostly recurring but leans on a handful of mid-sized accounts, so four months feels honest. The target reserve is $180,000 times four, or $720,000.

Now test it against a bad scenario instead of trusting the formula blindly. If the largest client left and collections slowed, could $720,000 carry the business for the four months it takes to cut costs or replace that revenue? If yes, the target holds. If things would get tight by month three, that is the signal to move to five months, or $900,000, rather than hope. The formula gives you a starting number; the stress test tells you whether to trust it.

Where to park it, and the cost of too much

A reserve has two enemies. The first is holding it somewhere that earns nothing. Cash idle in a checking account at 0% quietly loses value to inflation every month. On a $720,000 reserve, the gap between 0% and a safe 4% yield is about $28,800 a year — real money left on the table, since the safe options are just as accessible.

The second enemy is holding far more than you need. Cash beyond a sensible buffer is not safety, it is drag — money that could hire a salesperson or fund a product bet, sitting still instead. Holding $2,000,000 when $720,000 covers your worst realistic month leaves over a million dollars doing nothing. A reserve is insurance, and you do not over-insure a house for ten times its value.

The practical setup is boring on purpose. Keep about one month of outflow in your operating account so bills clear without a thought, and put the rest where it stays safe, liquid, and earning — a high-yield savings account, a money-market fund, or short-term US Treasury bills. All three let you reach the cash within a day or two, which is the point. Do not lock a reserve into anything with a withdrawal penalty or a price that can drop when you need it.

A reserve is not your runway — and do not raid it

Founders mix up reserve and runway, but they are different tools. Runway is a countdown: how many months until the bank hits zero at your current burn, the number that matters most when you spend more than you earn. A reserve is a floor you hold and defend even when profitable. A business making money can have effectively unlimited runway and still need a reserve, because profit this quarter does not protect you from a shock next quarter. Runway asks how long you last if nothing changes; the reserve is what you draw on when something does.

So do three things. Pick a target using the math above and write it down, because a buffer you have not defined is one you will spend by accident. Automate the top-up — a standing monthly transfer into the reserve account, treated as a bill you owe yourself, so it refills after any month you dip in. And guard it: the reserve is for revenue shocks and genuine emergencies, not a great hire or a discount that expires Friday. The moment a buffer becomes a slush fund, it stops being a buffer.