How Whatif forecasts a YNAB budget

Whatif takes the budget you already keep in YNAB and walks it forward one day at a time, starting from today’s balances. Every day adds what lands that day and takes off what leaves, so the forecast can tell you the lowest your cash will get, the day it gets there, and whether it ever goes below zero.

YNAB is built around the money you have today. Whatif answers the other question: where that money will be next month, and the month after.

What it reads

Whatif connects to YNAB with read-only access. It cannot change a budget, a transaction, a category or a target, because the access it is granted does not permit it.

From your budget it reads:

• your accounts and their balances, which are where the forecast starts;
• your scheduled transactions, which are what lands and leaves on known days;
• your categories and their targets, which are what you expect to spend.

Those become your plan: income, spending by category, debts and assets. The plan is yours to correct. If a target is out of date, you state the real figure, and the forecast uses yours from then on.

How each day is worked out

The forecast is arithmetic. For each day ahead it starts from the day before, adds what you are paid that day, and takes off what is due: bills on their due dates, spending by category, and debt payments on the days you make them.

Question	What the forecast answers
Will this hold until payday?	The lowest your cash gets before then, and the day it does
Will cash go below zero?	The first day it would, or that it does not
Where does the year leave me?	Cash and net worth at the end of the forecast
What if I cut one thing?	The same forecast with and without the change

The figures come from rows you can see and correct, so a number you do not believe can be checked. Why your YNAB balance is lowest the day before payday works through one budget’s months this way, a day at a time.

This is the forecast for the sample budget Whatif opens, a day at a time for a year:

Lowest point

$276.98

14 Feb 2027

Below zero

Never

Not on any day shown

What it leaves out until you say

A forecast that guesses quietly is worse than one that asks. So Whatif leaves some things out of the cash line until you answer:

• A category with no target and no scheduled transaction. It is left out rather than averaged from past months, and you can count it with the figure you expect.
• A credit card nobody has said how they pay. Its balance is shown as a question at the top of the forecast, never placed on a date Whatif made up.

What it will not do

There is no investment return, inflation figure or tax model anywhere in it, and it gives no advice. It reports the arithmetic on your own budget, and the conclusions are yours.

It also does not copy how YNAB assigns money to categories. It forecasts cash leaving your accounts, which is the thing that can run out.

Changing one thing

The forecast is where you start. What if is where you change one thing — cut a category, add a one-off, move your income — and see it as a second line against the forecast, before anything is saved.

You can look around a sample budget first, without connecting your own.