How Much Runway Does a Services Business Actually Need?

Every runway calculator you can find asks for two inputs: cash in the bank and monthly burn. It divides one by the other and tells you how many months you have. That model was built for venture-funded product businesses, where the assumptions behind it are all true, revenue is small and predictable, costs are largely engineering, and the thing you are counting down to is the next funding round.

Almost none of that describes a services business, and the model fails in a specific and dangerous way when you apply it anyway. It tells you that you are fine.

Search the question and you will find plenty written about client concentration, but nearly all of it treats concentration as a valuation problem, how a buyer discounts your business at exit, what a lender thinks of your credit file. That is a real concern for a firm being sold. It is not the concern of a founder trying to work out how much cash to keep in the bank this year, and the two need completely different arithmetic.

The number a services business needs is not months of burn. It is the cash required to survive losing your largest client, for as long as it takes to replace them, while paying people you cannot legally or practically shed on that timeline. That is a different calculation with a different answer, and it is usually a larger one.

Why months-of-burn is the wrong model here

The burn model assumes costs are flexible and revenue is stable. In a services business both assumptions are inverted: costs are close to fixed and revenue can be cancelled with notice.

Three structural differences do the damage.

Your costs are people, and people are semi-fixed. A product business under pressure can cut cloud spend, pause marketing, and defer hiring, and its cost base genuinely moves within a month. Your cost base is salaries. Salaries do not fall when revenue does. They fall when you make people redundant, which involves notice periods, statutory dues, and a real cost to execute, and which permanently reduces your capacity to deliver the work you still have. Cutting costs in a services business is not a dial. It is a one-way door with a fee attached.

Your revenue is cancellable on notice, in blocks. A subscription business loses customers one at a time, in a curve you can forecast. You lose them in lumps, on thirty or sixty days’ notice, and one lump can be a quarter of the business. Nothing about the previous six months predicts it, because the decision is made in a meeting you are not in.

Recognised revenue is not cash. The burn model quietly assumes what you invoice is what you bank. In services the gap between the two is the whole problem, and it widens exactly when things get difficult, clients under pressure pay later, and a client on their way out pays latest of all. If that distinction is not already sharp for you, cash flow vs profit is the foundation this post is built on.

Put together, these mean a services business under stress has costs that are slow and expensive to reduce, revenue that can disappear in a single conversation, and collections that stretch at the worst moment. The burn model sees none of this. It sees a healthy business with eight months of cover.

The question that actually sets the number

Ask what happens if your largest client leaves tomorrow, and how long it takes to replace that revenue, not how long you can survive with no revenue at all.

The all-revenue-stops scenario is the one people model, and it is the wrong one. It is both too pessimistic (all your clients will not leave in the same month) and, more importantly, useless, it produces a number so large you dismiss it, which means you end up planning against nothing at all.

The realistic catastrophic scenario for a services business is narrow and specific: your largest client gives notice. Everything else keeps running. That is the event to size against, because it is the one that actually happens, and it has three parameters you can measure today.

One: your fixed cost floor. Not your current monthly spend, the floor you can realistically get to within one quarter without breaking your ability to serve the clients you still have. Salaries of people you will keep, rent, software, compliance, the minimum of everything else. Most founders have never separated their spend into “floor” and “above floor,” and the exercise is uncomfortable, because the honest floor is usually much closer to the current number than expected.

Two: replacement time. How long from a client giving notice to a replacement of similar size paying you, which is your sales cycle, plus contracting, plus onboarding, plus their first payment terms. That final leg is the one everybody forgets. A ninety-day sales cycle with sixty-day payment terms means the money arrives five months after you start looking, and the search does not start on the day of the notice; it starts a couple of weeks later once you have accepted that it is real.

Three: your concentration. What proportion of monthly revenue your largest client represents, and the same for your top three. Do this on gross revenue and again on margin contribution, because they are frequently different clients, the largest by fee is not always the largest by profit, which is the entire subject of which client is actually profitable when everyone works on everything. The margin figure is the one that matters here.

The arithmetic, worked through

Take a hypothetical services business, purely to show the mechanics. Monthly revenue of ₹20,00,000. Fixed monthly costs of ₹16,00,000, of which ₹13,00,000 is the floor, the team, rent and systems that have to survive for the remaining clients to be served. The largest client is ₹6,00,000 a month, or 30% of revenue. Notice period is sixty days. The sales cycle for an account of that size runs about ninety days, and new clients pay on thirty-day terms.

The burn model says: ₹4,00,000 a month of surplus, comfortably profitable, and with ₹20,00,000 in the bank you have “infinite runway.” No alarm anywhere.

Now run the actual event.

The cash trough runs roughly months 3 to 6, and the deficit accumulated across it is on the order of ₹4,00,000 to ₹5,00,000, modest, and easily covered by the ₹20,00,000 balance. This business is genuinely fine.

Change one input. Make the largest client ₹10,00,000 a month, 50% of revenue, which is not unusual for a firm that grew on the back of one relationship. Now revenue drops to ₹10,00,000 against a ₹13,00,000 floor, the monthly deficit through the trough is ₹3,00,000 rather than ₹2,00,000, the trough is longer because replacing a ₹10,00,000 account takes longer than replacing a ₹6,00,000 one, and, the part that does the real damage, you cannot cut to the floor, because the floor was calculated to serve a client who has left, and cutting further means you cannot win the replacement. The same balance now looks thin, and it got there by changing one variable that the burn model does not ask about.

That is the point of the exercise. Two businesses with identical revenue, identical costs, identical cash and identical burn have entirely different risk, and the only variable that separates them is concentration.

Concentration-adjusted runway, as a rule you can apply

Size your buffer as: months to replace your largest client, times the monthly deficit you would run after cutting to your floor, plus one quarter of the floor as a margin for the things that go wrong at the same time.

Written out:

Buffer = (replacement months × post-loss monthly deficit) + (0.25 × annual fixed floor ÷ 4)

The three components, and why each is there:

Replacement months. Sales cycle, plus contracting, plus onboarding, plus first payment terms, plus two weeks of denial. Use your actual longest recent cycle, not your average, you are sizing against a bad case, and averages are made of good cases too.

Post-loss monthly deficit. Revenue after the loss, minus your realistic floor. If that is positive, you do not have a runway problem for this client, and you can stop. Run the calculation again for your top three combined, because a downturn that takes one client often takes two.

The quarter of floor. This is not padding. It is there because the events correlate: the quarter you lose a large client is disproportionately the quarter another one stretches payment from thirty days to sixty, a receivable goes bad, and something breaks that needs money. If a payment problem is already live, the escalation sequence is the client has stopped paying, and the reason this term exists is that the two situations turn up together far more often than independence would suggest.

Two adjustments for how the business is actually structured:

• Project work needs more than retainer work. A retainer book gives you notice periods and a predictable pipeline. A project book can simply end, with everything finishing in the same quarter and no notice at all. If most of your revenue is project-based, treat your replacement time as longer and your notice period as zero.
• A senior-heavy team needs more than a junior-heavy one. Senior people are harder to re-hire, so cutting them is more costly to reverse, which means you will, correctly, resist cutting them, which means your realistic floor is higher than the one on paper. Be honest about this when you calculate the floor rather than discovering it in the moment.

Why “three to six months of expenses” is wrong in both directions

The standard advice is wrong for a low-concentration business and dangerously wrong for a high-concentration one, because it is indexed to expenses rather than to exposure.

For a firm with twenty clients and no single account above 8% of revenue, six months of expenses is a large amount of dead capital. No plausible single event takes out enough revenue to need it. That business can hold considerably less and put the difference into hiring, or capability, or simply take it out, the risk it is insuring against does not exist in the shape the rule assumes.

For a firm where one client is half the revenue, six months of expenses may not be enough, because the relevant clock is not “how long until I run out of money” but “how long until I have replaced that revenue,” and those are unrelated quantities. If your replacement cycle genuinely runs eight months, six months of cover means the business fails on month seven with a signed contract in hand and nothing to bridge to it. That is a distinctly ugly way to go, and it is what the expenses-based rule quietly permits.

The rule is popular because it is easy and because it works passably for personal finance, where the model does fit, an individual’s income really does stop entirely, and the expenses really are the number. That version is a genuinely good rule and is covered properly in how to build an emergency fund. Applying it unchanged to a business with lumpy, cancellable, concentrated revenue is where it breaks.

The Indian specifics that change the number

If you bill in India, several structural cash effects sit between “the client agreed to pay” and “you have the money,” and they all push the buffer up.

GST is paid on the invoice, not on the payment. You remit output GST for the month you invoiced, regardless of whether the client has paid. On a large invoice with long terms, that is a genuine outflow funded by you, in advance, on money you have not received. TDS is deducted before you see it. Your client withholds tax at source, so the cash landing in your account is below the invoice value, and you recover the difference much later against your own liability. Both of these are covered properly in GST and TDS both hit your cash before the client does, and the practical consequence for this calculation is that your cash revenue in any month is meaningfully below your invoiced revenue while your costs are at full value.

Statutory dues on exit are real money. If part of your response to losing a client is reducing headcount, notice pay, gratuity where applicable and leave encashment all land in the same quarter as the revenue loss. The cost of shrinking is front-loaded and the saving is back-loaded. Model the cut as an outflow first.

If you bill abroad, the currency moves against you at inconvenient times. Foreign receivables convert at whatever the rate is on the day, not the day you quoted, and that exposure sits on top of everything else, billing abroad, spending at home covers how to think about it.

If the number comes out uncomfortably large

Reduce the exposure rather than trying to save your way to the buffer, because the buffer is a symptom and the concentration is the disease.

Most founders who do this calculation for the first time get a number they cannot fund out of current profit in any reasonable period. That is useful information, not a failure. The buffer is one of four levers and usually the slowest:

Shorten replacement time. The largest single input in the formula. A pipeline that exists before you need it, a handful of live conversations you maintain even when full, cuts months off the worst case, and costs almost nothing but discipline. Most services businesses do zero business development while busy, which is precisely what makes the replacement clock long.

Reduce the concentration directly. Not by refusing good work, but by deliberately weighting new business toward accounts that reduce the ratio, and by being clear-eyed that a very large client is a commercial risk as well as a commercial win. Growing the denominator works as well as shrinking the numerator.

Improve the terms. Shorter payment terms, longer notice periods, and part-advance billing all reduce the depth of the trough without needing a rupee more in the bank. Notice period is the most under-negotiated term in most services contracts and it is worth real money in exactly this scenario, ninety days instead of thirty is a full month of deficit removed from the calculation.

Arrange credit before you need it. A facility agreed while the business looks strong is available on much better terms than one sought during the quarter you lost a quarter of your revenue. Lenders price on what the business looks like when you ask, and that is a reason to ask early rather than a reason to borrow.

The scope creep problem sits underneath all of this: the client whose fee has stayed flat while the work quietly grew is both less profitable and harder to replace than the number suggests, and how a profitable retainer quietly becomes an unprofitable one is the diagnostic for that. A book full of those is more concentrated in effort than it looks in revenue.

Read it quarterly, in fifteen minutes

Recalculate every quarter, because the inputs move without announcing themselves, and the most common way a business becomes over-concentrated is by growing.

The pattern is ordinary. Your best client is delighted and expands their scope. You are pleased. Revenue is up, the relationship is strong, and the concentration ratio has moved from 22% to 41% over four quarters without a single decision being taken to allow it. Nothing on the P&L flags this; it looks like an excellent year. Concentration risk is one of very few business risks that grows precisely when things are going well, which is why it needs a scheduled check rather than a triggered one.

Four numbers, one page, every quarter:

1. Largest client as a share of monthly revenue, and separately, as a share of margin.
2. Top three combined, the same two ways.
3. Realistic fixed floor, re-honest about who you would actually not let go.
4. Current replacement time, based on the last two deals you actually closed rather than on how long you would like it to take.

Then re-run the formula. It takes about fifteen minutes, and the value is not the answer in any single quarter but the direction across four. A concentration ratio moving steadily up while the cash balance stays flat is a business becoming quietly more fragile during what feels like its best year, and that pattern is legible a long way in advance, but only to someone who is looking at it. Which is also why the weekly cash view is one of the five things a founder should never fully delegate: the monthly P&L is where this hides, and it hides well. If reading that statement is not yet second nature, start with how to read a profit and loss statement and come back to this.

The one-sentence version: a services business does not need months of runway, it needs enough cash to replace its largest client, and if you have never worked out which of those two numbers is bigger, it is the second one.