Comparison
Emergency Buffer Runway Across Income Coverage: How Far a Buffer Stretches, and Where It Stops Depleting
As income during a disruption covers more of the essential outgoings it has to meet, how much longer does a buffer of a given size last — and at what point does the reserve stop being drawn down at all?
The short answer
Runway is the buffer divided by the share of outgoings income does not cover. A buffer of 3× monthly essential outgoings lasts 3.0 months with no income, 6.0 months at 50% coverage, and 30.0 months at 90%. The response is not proportional: the last 10% of coverage adds more than the first 50% does.
At 100% coverage the arithmetic stops producing a duration at all. Income meets outgoings, the monthly shortfall is zero, and there is nothing to divide the buffer by — so the model reports a state rather than a number: The buffer remains level under this constant monthly scenario. That is a statement about the assumptions, not about the future, and it is not the same thing as a runway that is very long. These are outputs of a model, not forecasts.
- Who it applies to
- A fixed reference set of buffer multiples under stated income-coverage ratios, with no immediate one-time cost and no currency anywhere. It compares ratios against each other, not households, incomes or products.
- What this does not tell you
- It does not describe how long a disruption would last, how likely any income coverage is, or whether a buffer is large or small. Those are judgments and forecasts; these are ratios.
Model Emergency Fund Runway v1.0, from the same calculation the Emergency Fund Runway Calculator runs.
What is being compared
One question is asked of every buffer multiple in the same way, so the multiples can be read against each other rather than one at a time. Nothing varies between rows except the size of the buffer, and nothing varies between columns except how much of the essential outgoings the continuing income covers.
| Buffer multiples compared | 1× · 2× · 3× · 6× · 12× monthly essential outgoings |
|---|---|
| Income coverage compared | 0% · 25% · 50% · 75% · 80% · 90% · 100% of essential outgoings |
| What the buffer contains | Whatever you would count as reachable and would actually use — the calculator’s usable resources, after any immediate one-time cost |
| What income coverage means | Income assumed to continue throughout the disruption, as a share of the essential outgoings it has to meet |
| Immediate one-time cost | None. Every scenario starts after any such cost has already been met |
| Runway unit | Monthly spending cycles, never calendar dates |
| Units | Ratios of monthly essential outgoings throughout. No currency, return, interest, inflation or tax is modelled |
The buffer multiples are illustrative: numbers spaced so the arithmetic is legible across the table, not typical holdings, recommended holdings or claims about what anybody keeps. UBWHY publishes no recommended number of months anywhere, and this table is not one. No multiple here is described or implied to be normal, prudent, adequate or safe, and the calculator these figures come from deliberately prefills nothing and suggests nothing.
The relationship, written once
The calculator divides the resources that survive the immediate cost by the monthly shortfall. Every figure on this page is that same division with the currency divided out of it:
runway in months = buffer multiple ÷ (1 − income coverage)
Buffer multiple is the usable liquid buffer expressed in months of essential outgoings: the money you could actually reach and would actually use, divided by what one month of essentials costs. Income coverage is the income assumed to continue during the disruption, divided by the same monthly figure. Both sides of the calculator's division are measured against one quantity, so that quantity cancels.
Because both inputs are ratios, the size of the household drops out. Someone whose essentials cost ten times as much, with ten times the buffer and ten times the income, sits on exactly the same cell. That is what makes a table of thirty-five cells a reference rather than thirty-five worked examples.
The denominator is the reason the relationship bends. Income coverage does not add months to the runway — it shrinks the monthly shortfall the buffer is spent against, and a shrinking denominator raises the quotient faster and faster. That is a rational relationship, not an exponential one, and the two behave differently: an exponential curve grows without bound as its input grows without bound, and this one rises steeply toward a single point that the input can actually reach.
What income coverage does to any buffer
The buffer multiple is a factor on the outside of the division, so it never changes the shape of the relationship — only its scale. One coverage ratio therefore stretches every buffer by the same factor, whatever the buffer is.
| Income coverage | Runway multiplier | Coverage not met by income |
|---|---|---|
| 0% | 1.00× | 100% |
| 25% | 1.33× | 75% |
| 50% | 2.00× | 50% |
| 75% | 4.00× | 25% |
| 80% | 5.00× | 20% |
| 90% | 10.00× | 10% |
| 100% | Not applicable — the buffer is not being depleted | |
Read the third column as the reason for the second. The multiplier is one divided by the share income leaves uncovered, so halving that share doubles the runway — and the share is halved by moving from 50% coverage to 75%, then again by moving from 75% to 90%. The coverage steps get smaller while the effect gets larger, which is the whole of the nonlinearity.
At 50% coverage a buffer lasts 2.00× as long as it would with no income; at 90% it lasts 10.00× as long. Neither figure depends on the buffer, and the table below is the same two facts applied to five buffers at once.
The runway each buffer produces
Read a row to see what income coverage does to one buffer, and a column to see what a larger buffer does at one coverage ratio. Both readings are the point of putting the two dimensions in one table — and they behave differently, which is the finding rather than an artefact of the layout.
| Buffer | 0% covered | 25% covered | 50% covered | 75% covered | 80% covered | 90% covered | 100% covered |
|---|---|---|---|---|---|---|---|
| 1× | 1.0 month | 1.3 months | 2.0 months | 4.0 months | 5.0 months | 10.0 months | Not applicable — the buffer is not being depleted |
| 2× | 2.0 months | 2.7 months | 4.0 months | 8.0 months | 10.0 months | 20.0 months | Not applicable — the buffer is not being depleted |
| 3× | 3.0 months | 4.0 months | 6.0 months | 12.0 months | 15.0 months | 30.0 months | Not applicable — the buffer is not being depleted |
| 6× | 6.0 months | 8.0 months | 12.0 months | 24.0 months | 30.0 months | 60.0 months | Not applicable — the buffer is not being depleted |
| 12× | 12.0 months | 16.0 months | 24.0 months | 48.0 months | 60.0 months | 120.0 months | Not applicable — the buffer is not being depleted |
Down a column the figures are proportional: doubling the buffer doubles the runway, at every coverage ratio. Across a row they are not, and the gap widens toward the right — which is why the two axes of this table cannot be read the same way.
Where the reserve stops depleting
The last column behaves differently from every other column, and the difference is structural rather than a matter of degree. It is not a very long runway. It is not a runway at all.
- Finite runway — coverage below 100%
- Income covers part of the essentials, a monthly shortfall remains, and the buffer is spent against it at a constant rate. There is a month count because there is something to count: at 3× and 90% coverage the model returns 30.0 months, and that is a duration the arithmetic actually produces.
- Reserve not depleting — coverage at 100%
- Income meets the essentials exactly, so the monthly shortfall is zero and the division has no divisor. The model returns no month count and a named state instead: The buffer remains level under this constant monthly scenario. The buffer is neither shrinking nor growing while the scenario holds. At 2× the published cell reads Not applicable — the buffer is not being depleted, and it reads the same at every other buffer in the table.
- Reserve growing — coverage above 100%
- Income exceeds the essentials, which the model treats as a separate state again: The buffer increases each month under this scenario. At 110% coverage the reference returnsNot applicable — the buffer is not being depleted, for the same reason as the column beside it: there is no depletion to measure the length of.
Neither of the last two states is permanence, independence, solvency or safety. Each is a statement about one set of assumptions held constant: that the income continues, and that the essential outgoings stay level. The model tests neither. An income that covers the essentials this month and stops next month puts the same buffer straight back into the first column, at whatever multiple it has left.
This is also why no figure appears in those cells. A very large number would be read as a duration, and there is no duration: the quantity is not large, it is absent. The calculator refuses the same substitution in the same words, and this table refuses it with the same words.
One buffer, read across the coverage ratios
Take the 3× row and read it left to right. The buffer does not move; only the coverage does, and every figure below comes out of the same model run the tables above are built from.
- 0% covered. 3.0 months of runway, against a monthly shortfall of 100% of essential outgoings.
- 25% covered. 4.0 months of runway, against a monthly shortfall of 75% of essential outgoings.
- 50% covered. 6.0 months of runway, against a monthly shortfall of 50% of essential outgoings.
- 75% covered. 12.0 months of runway, against a monthly shortfall of 25% of essential outgoings.
- 80% covered. 15.0 months of runway, against a monthly shortfall of 20% of essential outgoings.
- 90% covered. 30.0 months of runway, against a monthly shortfall of 10% of essential outgoings.
- 100% covered. The buffer remains level under this constant monthly scenario. There is no monthly shortfall, so there is no month count to state.
Moving from no income to 50% coverage adds 3.0 months — fifty points of coverage for one extra buffer's worth of time. Moving from 80% to 90% adds 15.0 months — ten points, for five times as much. And the ten points after that add nothing at all, because they end the depletion rather than slowing it. Three coverage steps, one unchanged buffer, and the last of them is a different kind of answer from the first two.
Income coverage is an assumption, not a probability
Every column on this page is a scenario input: a figure someone chooses in order to ask what follows from it. None of them is a likelihood, a forecast, an expected value or a rate at which anything happens. The table says what the arithmetic does at 90% coverage; it says nothing whatever about whether anybody's income would hold at 90% of their essentials during a disruption, and the model contains no machinery that could.
That distinction matters most in the right-hand columns, because those are the ones with the largest figures and the smallest margin for error. A scenario that assumes income covers 90% of essentials produces 30.0 months at 3×; if the real coverage turned out to be 50%, the same buffer produces 6.0 months. Being wrong about the assumption costs more the closer the assumption sits to parity, which is the practical consequence of the curve bending.
Why the buffer itself is a judgement rather than a valuation — what counts as reachable, what an asset would actually fetch, and why net worth answers a different question entirely — is the explainer's subject: Why Liquidity Is Not the Same as Net Worth. This page measures how runway behaves once a buffer has been settled on, rather than re-teaching what belongs in one.
A longer runway is not automatically a better position
Two households on the same cell of this table have the same ratio and may have almost nothing else in common. The cell is a statement about two numbers. What actually happens depends on what the disruption is, how long it lasts, whether the assumed income really continues, what the essentials could be cut to, what could be sold and at what price, and what else the money in the buffer was needed for.
Money held as a buffer is money not used for anything else, and this page does not calculate what that costs. UBWHY publishes no threshold at which a buffer becomes sufficient, and none of these rows is labelled adequate, prudent, comfortable or thin. A larger multiple is not a better one in any sense this model computes; it is a larger multiple. Nothing here is a reason to hold more cash, hold less, sell anything, buy anything or change anything.
This page belongs to a wider subject. Explore the Liquidity topic to see which UBWHY asset answers which question.
What this reference does not determine
The figures on this page are outputs of a UBWHY calculation model applied to the assumptions stated above. They are not data about anybody, an expected duration, a probability, or a forecast. The reference cannot determine:
- how long any real disruption would last, which nothing here predicts
- how likely any income coverage is, for anybody, in any circumstance
- what share of an asset you could actually reach in time, at that value, or without a penalty
- whether a buffer is large or small, because that is a judgment and this is a ratio
- any recommended number of months, which UBWHY publishes nowhere and does not publish here
- interest, investment return, inflation and compounding, all excluded by the model
- what holding a buffer costs you elsewhere, which is a real price this page does not compute
- tax of any kind, and no wrapper, allowance, jurisdiction or rate is modelled
- credit, insurance, benefits, severance or support from anyone else, none of which the model counts
- whether anything above is appropriate for any particular person
The model is a constant monthly scenario. It holds the income and the outgoings level for as long as the scenario runs, places no cost in any particular month, earns nothing on the buffer and charges nothing for holding it — so a disruption that matches one of these cells on average can still be one where the money runs out sooner than the cell says.
None of this is financial, legal or tax advice, and no product, provider or account is recommended, ranked or named anywhere on this page.
Where these figures come from
Every number above is produced by Emergency Fund Runway v1.0, the same calculation model behind the Emergency Fund Runway Calculator, run at build time over the reference set at monthly essential outgoings of one unit. Nothing on this page is typed by hand, and no second formula was written to produce it.
The boundary cell is additionally reproduced by the model's own verified test cases — EFR-2 is a 2× buffer at 100% coverage and returns the same non-depleting state with no month count, and EFR-1 reproduces a finite runway through the same dimensionless form at a ratio pair this table does not publish. Both are recalculated independently on every build. The remaining figures are pinned by exact regression tests against the same model, including the two properties the normalisation rests on: that the runway depends on the ratios rather than on the amounts, and that the multiplier table and the matrix agree at every cell.