There are two versions and the difference matters. Gross burn is everything you spend in a month, regardless of income. Net burn is what you spend minus what you collect. Net burn is the number that governs survival, because it is what actually drains the account. Gross burn is still worth knowing, because it tells you the size of the problem if revenue stops entirely, which is what happens when a large client leaves or a season ends.
Calculate it from your bank account rather than your accounting software. Take the closing balance three months ago, subtract the balance today, and divide by three. That is your average monthly net burn, and it accounts for everything, including the transactions you forgot about and the ones you would have excluded if you were doing this by hand. Accounting software tells you about profit, which is a different question. Burn rate is about cash, and cash is what makes payroll.
Runway is the balance divided by net burn, expressed in months. Below six months, every decision becomes urgent and your negotiating position weakens in ways that cost real money. Between six and twelve months, you can make deliberate choices about growth. Above twelve months you have the rarest thing available to a new business, which is the ability to say no to work that does not fit. Most first year businesses discover they have less runway than they assumed, because they were reasoning from revenue rather than from the account balance.
The number that matters is not the burn in any single month but the trend across several months, because a single month distorts easily. An annual insurance payment, a quarterly tax deposit, or one large client paying late will make a healthy month look alarming and an alarming month look fine. Track it monthly, look at the three month average, and recalculate whenever something structural changes: a hire, a new subscription, a price change, or a client leaving. The businesses that fail on cash are almost never surprised by the arithmetic. They simply were not doing it.