Emergency Fund Runway Calculator

The model is deliberately small. It adds up what you could actually use, takes off a cost you would pay immediately, works out what a month of the disruption costs you net of any income that continues, and divides one by the other. There is no return, no interest, no inflation and no compounding anywhere in it.

What counts as usable

Cash you can reach now, plus whatever share of another asset you say you would genuinely use. The share is applied to the amount you entered, and the remainder is reported separately:

usable other assets    U_A = A * u
excluded by your share  X_A = A * (1 - u)
usable resources        B   = C + U_A

C is cash, A the displayed amount of the other asset and uthe share you entered. X_A is shown beside the bridge and is never added back into any available-funds figure.

The usable share of other assets is your assumption. UBWHY does not verify their price, access time, tax cost or saleability.

The remainder of the other assets you entered is excluded from every figure above, at the share you chose. It is not counted as available, and it is not established as unavailable.

The immediate cost comes first

A cost you would pay at once is taken out before any monthly runway begins, because that is when it happens. If it is larger than everything usable, there is no runway to report yet — the shortfall has to be closed first:

funding gap        G_0 = max(0, O - B)
resources left     R_0 = max(0, B - O)
and always         B - O = R_0 - G_0

The monthly shortfall

Expenses less the income you assume continues. Only one of the two lines below can be positive, which is why a scenario is either drawing the buffer down or it is not:

monthly shortfall  M = max(0, E - I)
monthly surplus    S = max(0, I - E)

The runway, and the three answers it can be

if G_0 > 0   no monthly runway has begun; the gap is G_0
if M   > 0   T = R_0 / M  monthly cycles
if M   = 0   the buffer is not being depleted

The third line is the one worth reading twice. A buffer that is not shrinking does not have unlimited runway. It has a scenario in which nothing is being drawn down, which is a statement about the figures you entered and not about the future. This tool never prints an infinite runway, and there is no state in which it could: the answer in that case is a sentence, not a number.

Where a finite runway exists it is also broken into whole cycles and the share of the next one. Those are monthly spending cycles from the start of the scenario, not calendar months, and the tool does not convert them into a depletion date. A date would need paydays, bill dates and a partial-month calendar, none of which this model has.

What would stop the depletion

Both reversal figures are the monthly shortfall, from two directions: the monthly improvement that brings the buffer level, and the extra usable funds that buy one more cycle at the current shortfall.

monthly improvement to stop depletion  = M
extra funds for one more cycle         = M

That improvement can come from income, from expenses, or from both. The tool does not prescribe which, and it does not claim either is achievable.

Your target, if you set one

UBWHY publishes no universal emergency-fund target. There is no recommended number of months here, and nothing is prefilled.

target requires    B_t = O + M * t_g
still needed       G_t = max(0, B_t - B)
above requirement  Q_t = max(0, B - B_t)

Where the monthly shortfall is zero the requirement collapses to the immediate cost, whatever target you chose. That is arithmetic about the scenario you entered, and it is not a finding that the target is met for good: it does not test what happens if the income you assumed continues later stops.

What this does not calculate

This is a constant monthly scenario, not a forecast. It does not model income ending later, changing expenses, investment returns, taxes, benefit eligibility, credit access or the time required to sell an asset.

• Changing expenses, income that ends after a stated month, irregular paydays or bill calendars. Every month in this model is the same month.
• Inflation, interest, investment return or tax. Money held here neither grows nor loses purchasing power.
• The time required to sell an asset, the price it would fetch, a penalty for reaching it early, or the tax on the sale.
• Insurance claims, benefit eligibility, severance, credit or overdraft capacity, or support from anyone else. None of it is counted, in either direction.
• How long a real disruption would last. The result is a property of the numbers entered, not a forecast of the event.
• A correct emergency-fund size. There is no recommended number of months here, and no judgement about the one you chose.

A longer runway is not automatically a better position. Money held as a buffer is money not used for anything else, and this tool does not calculate what that costs, does not set a right amount to hold, and cannot tell you how long a real disruption would last.

Why an asset can add to net worth and still be no help in an emergency, what a usable share really represents, and why liquidity has a price of its own: Why Liquidity Is Not the Same as Net Worth.