How to Calculate Startup Runway for Founders: A Simple Guide

You have $340,000 in the bank. You're burning $28,000 a month. So you have twelve months of runway, right?

Wrong. You have about eleven, and if two of your customers pay late — which they will — you have closer to nine. That gap between the number you write on the whiteboard and the number that actually keeps you alive is where most founders get into trouble. Not because they can't divide, but because they divide the wrong things.

Calculating startup runway sounds like arithmetic. It's really a modelling problem, and most of the advice out there stops at "cash divided by burn" and calls it a day. That formula is correct and almost useless on its own. Here's how to actually build a runway number you can make decisions with.

How to calculate startup runway (and why the standard formula misleads you)

The textbook version: take your current cash balance, divide by your monthly net burn, and you get a number of months. That's the skeleton. Everything that matters is in the flesh you put on it.

Picking the right burn figure is the whole game

Net burn is what leaves your account each month minus what comes in. Simple. The trap is that which month you pick changes the answer dramatically, and nobody tells you the rule.

Here's the rule I use:

• Trailing three-month average for planning. It smooths out a big one-off invoice or a quiet month without hiding a trend.
• Last month's actual when you're making a hire, signing a lease, or deciding whether to cut. You need the current reality, not a blended one.
• Trailing six months only when the business is stable and flat. If you're growing or shrinking, six months is a lie you tell yourself.

Worked example. You have $340,000 cash. Last three months of net burn: $24,000, $31,000, $29,000. Trailing three average: $28,000. Runway: 340 ÷ 28 = 12.1 months.

Now use last month only: 340 ÷ 29 = 11.7 months. And if your burn is climbing $2,000 a month because you're hiring, the flat division overstates your runway by roughly two months. That's the difference between "we have time" and "we start raising now."

The denominator decides, so stress-test it

Founders obsess over the numerator — the cash figure — because it's the one number they check every morning. The denominator, the burn, is where the real leak is. A $3,000/month error in burn on a $340,000 balance is nearly a month and a half of runway. And burn errors are easy to make: a contractor invoice that hits in a lump, annual software renewals you forgot to amortise, payroll taxes you mentally excluded.

Build your burn from your actual bank statement, not from your budget. The budget is a plan; the statement is the truth.

Pre-revenue vs post-revenue: two different calculations

If you have no revenue, your runway is brutally simple: cash ÷ monthly spend. Every month you burn, you burn the same amount. The only lever is cost.

Once revenue exists, the calculation gets harder because revenue recognised is not cash collected. This is the mistake that humbles founders. You sign a $120,000 annual contract, book it as $10,000/month of MRR, and feel richer. But if that customer pays quarterly in arrears, you don't see a dollar for three months. Your runway calculation using MRR-inclusive burn looks great. Your bank account disagrees.

Adjusting runway for cash timing

Take your net burn, then subtract only the cash you actually expect to collect in each month, not the revenue you expect to recognise. If your average customer takes 45 days to pay, model collections one and a half months behind invoicing. On a business with $60,000 monthly invoiced revenue, that lag alone can swing your runway by a full month or more in the early quarters.

I once built a runway model that assumed all invoices were collected on time. The model said fourteen months. The reality was closer to ten because three of our largest customers consistently paid at day sixty. We found out the hard way when payroll came due and the money hadn't landed. That was a bad week, and it was entirely preventable.

Scenario modelling: base, pessimistic, optimistic

One runway number is a guess. Three runway numbers is a plan. Build a base case, a pessimistic case, and an optimistic case — with the pessimistic one being the one you act on.

The variables that move the most:

• Revenue growth rate (or lack of it)
• Hiring timing — a single senior hire can add $15,000/month fully loaded
• Payment terms from your biggest customers (the DSO question)
• One-off costs: legal, hardware, a conference you committed to before you knew better

In the pessimistic case, assume revenue grows at half your base rate and collections slip by 30 days. If the pessimistic scenario still gives you nine months, you're safe. If it gives you four, you have a decision to make now, not in three months. That decision is almost always: cut now, or start raising now.

How much runway does a startup need?

Eighteen to twenty-four months after a raise is the working target. Twelve feels comfortable and is usually too little, because raising a round takes three to six months of founder time — and you should be starting that process with at least six months of runway left, not when you're down to your last two.

Pre-seed and seed companies often run on thinner runway out of necessity, and that's fine as long as you know it and plan the next raise accordingly. What kills companies isn't a short runway; it's discovering the short runway too late to do anything about it.

What is the 80/20 rule for startups?

The 80/20 rule — the Pareto principle — says roughly 80% of your results come from 20% of your inputs. For a founder managing runway, that translates to a blunt prioritisation question: which 20% of your spending produces 80% of your growth?

In practice, it means auditing your cost base and asking of every line: does this actually move revenue or product forward? The expensive tool nobody opens. The contractor whose output you can't point to. The conference sponsorship that produced two leads. Cutting those doesn't just save money — it buys you months of runway, which is the most valuable currency a founder has.

The same logic applies to your time. If 20% of your customers generate 80% of your revenue, then 80% of your customer-support effort may be spent on the wrong 20%. Redirecting it is a runway decision disguised as an operations decision.

Is 1% equity in a startup good?

There's no universal answer, but here's the framework I use. 1% of a company that reaches a $100 million valuation is worth $1 million. 1% of a company that dies is worth zero. So the question isn't "is 1% good?" — it's "what's the realistic probability this company reaches a meaningful exit, and what am I giving up to hold that 1%?"

For a founding employee joining early with meaningful responsibility, 1% can be generous. For a late-stage hire joining at a $500 million valuation, 1% is unusual. For a founder watching their own stake get diluted through multiple rounds, 1% after several raises might represent a good outcome. The percentage alone tells you nothing without the stage, the strike price, the vesting schedule, and — critically — the company's realistic path.

If someone offers you 1% and the company is pre-revenue with no product, that 1% is a lottery ticket. Treat it as such.

Building a runway model you actually trust

Here's the structure I'd put in a spreadsheet tonight if I were starting over:

Input	What to use	Why it matters
Starting cash	Bank balance today, not budgeted	The only number that's real
Monthly burn	Trailing 3-month average of actual outflows	Smooths noise without hiding trends
Expected collections	Invoiced revenue shifted by average DSO	Captures the gap between revenue and cash
Hiring plan	Fully loaded cost, start date, ramp period	Hiring is the biggest discretionary lever
Scenario multiplier	Base / pessimistic (-30% revenue, +30 days DSO)	Forces you to plan for the worse case

Update it monthly. Not weekly — weekly is anxiety management, not planning. Monthly, on the same day, using the bank statement. Then look at the trend line, not the single number. A runway that shrinks by one month every month is a different situation from a runway that's holding steady.

The number that matters isn't how many months you have. It's how many months you have before you need to make a decision. If your runway says twelve months but you need six months to raise, you actually have six months of runway and a decision to make right now.

Which is the point. Runway isn't a comfort metric. It's a deadline, and the whole reason to calculate it precisely is to know exactly how much time you have before the deadline starts moving.