How to Calculate Your Financial Runway

“How long could I last?” is one of the most useful questions anyone working for themselves can answer. The answer is your financial runway: how long your available cash would cover the gap between what you earn and what you spend. This guide shows how to calculate it, with worked examples, and explains where simple calculations stop being reliable.

What runway means

Runway is a measure of time. It tells you how many months you could continue at your current level of income and spending before your cash runs out — or, more usefully, before it falls below a level you consider safe.

Runway only exists when you're spending more than you earn. If your income covers your expenses, your savings aren't being drawn down, and the runway calculation doesn't apply in the usual way.

The basic formula

Monthly cash flow = monthly income − monthly expenses

If monthly cash flow is negative:
Runway (months)   = available cash ÷ monthly shortfall

Here, the monthly shortfall is the negative cash flow written as a positive number. If you earn €2,500 and spend €4,000, your cash flow is −€1,500 and your shortfall is €1,500.

Use net figures: income after taxes and payment fees, and all the expenses you actually pay, including irregular ones averaged per month.

Monthly cash flow vs. cash balance

These two numbers are easy to confuse, but they answer different questions:

• Cash flow is a rate: how much your money changes each month (+€800 or −€1,500).
• Cash balance is a level: how much money you have at a point in time (€18,000 today).

Runway combines them. A large balance with a small negative cash flow can last years; a small balance with a large negative cash flow might last weeks. Improving either one — raising the balance or reducing the shortfall — extends your runway.

Worked examples

Example 1: no income at all

You have €18,000 in savings, monthly expenses of €4,000, and no income.

Cash flow = €0 − €4,000 = −€4,000
Runway    = €18,000 ÷ €4,000 = 4.5 months

Example 2: some income remains

Same savings and expenses, but you still earn €2,500 a month from other clients.

Cash flow = €2,500 − €4,000 = −€1,500
Runway    = €18,000 ÷ €1,500 = 12 months

Keeping some income nearly triples the runway, even though income still doesn't cover expenses.

Example 3: an uneven result

You have €10,000 in savings and a shortfall of €3,000 a month.

Runway = €10,000 ÷ €3,000 ≈ 3.3 months

Month by month, the balance looks like this:

The formula says “3.3 months”; the monthly table says the money runs out during month 4. Both are correct — they describe the same situation at different levels of detail. In practice, bills don't arrive evenly, so treat a figure like 3.3 as “a little over three months”, not as a date.

Minimum reserves

Running your savings down to zero is not a realistic plan. You will probably want a buffer for tax, unexpected costs or simply peace of mind. Subtract that minimum reserve before calculating:

Runway to reserve = (available cash − minimum reserve) ÷ monthly shortfall

With €18,000 in savings, a €6,000 minimum reserve and a €1,500 shortfall:

Runway to reserve = (€18,000 − €6,000) ÷ €1,500 = 8 months

Eight months is a more honest planning horizon than twelve. It's the time you have to act before you start using money you meant to keep. If your minimum reserve is higher than your current savings, you are already below your own safety line — that's important to know too.

When cash flow is positive

If income exceeds expenses, dividing your savings by a negative shortfall produces a meaningless negative number. The correct conclusion is simply that your savings are not being depleted under these assumptions. Saying you have “infinite runway” is equally misleading: income can change at any time. A better question is what happens if a specific source of income disappears?

For example, earning €5,000 against €4,000 of expenses gives you +€1,000 a month. If a client paying €3,000 left, cash flow would become −€2,000 a month, and with €18,000 in savings and a €6,000 reserve you'd reach that reserve after six months.

Why a month-by-month projection is better

The single-division formula assumes nothing changes. Real plans have dates:

• A client gives notice now but stops paying in two months.
• You cut expenses, but some contracts take a month to cancel.
• A new client starts, but their first payment arrives in month four.

A month-by-month projection handles this naturally: start with today's balance, then for each month add that month's income and subtract that month's expenses. Here is the earlier example (−€2,000 a month after a client leaves) with a Plan B: cut €500 of expenses immediately, and win €2,000 of new monthly income from month four.

The balance bottoms out at €13,500 and starts rising again — it never gets near the €6,000 reserve. The simple formula couldn't show you that.

Realistic limitations

Any runway figure is an estimate. Keep these limitations in mind:

• Income is uncertain. Clients pay late, projects slip, and new work takes longer to find than hoped.
• Expenses are lumpy. Annual insurance, tax payments and equipment replacements don't arrive as neat monthly averages.
• Taxes lag income. Tax on past earnings may be due at a time when current income is low.
• Inflation and interest slowly change both costs and the value of savings.

Treat runway as a planning tool, not a promise. Recalculate whenever something changes, and leave yourself a margin.

Use the free calculator

The Client Loss & Income Runway Calculator does the month-by-month projection for you. Enter your savings, minimum reserve, expenses and clients, choose which income could disappear, and compare your current plan, the loss, and your Plan B on one chart. It reports both the month your balance first falls below your reserve and an approximate figure, so you can see the result at both levels of detail. The full set of formulas is documented on our methodology page.

This guide is general information, not financial, legal or tax advice. Your situation may differ; consider speaking to a qualified professional before making significant decisions.