Nobody tells you about the co-op math trap until you're already in it.
I'm selling my house in Connecticut and buying a co-op in the Bronx at the same time. Great plan. Then I sat down to model it.
The problem
Co-op boards require you to hold a reserve in liquid assets after closing. Not at closing. After. We're talking 6 to 24 months of combined mortgage and maintenance sitting in an account, untouched, to prove you can cover the payments. The catch: that reserve comes out of your sale proceeds, which shrinks your down payment, which grows your loan, which changes your monthly mortgage, which is exactly what the reserve is calculated from.
So the reserve depends on the mortgage. The mortgage depends on the down payment. The down payment depends on the reserve.
Pick a number. Any number.
The math
I could have guessed and iterated until it converged. Instead I worked out the closed-form algebraic solution. Let A be your available proceeds after selling costs and debt payoff, P the purchase price, m the monthly maintenance, N the required reserve months, and k the mortgage payment factor for your rate and term:
One equation. Exact answer. No guessing required.
The tool
I added an interactive calculator to the home sale dashboard I was already running. Sale price, purchase price, maintenance, rate, term, reserve months. Everything recalculates on every keystroke.
The monthly total goes green if you're comfortable, gold if you're stretching, red if it's too much. There's a save function that snapshots the inputs and outputs to a local file, and a CSV export for anything you want to revisit. The whole thing runs locally. No data leaves the machine.
What it produced
Running the numbers on the first building I viewed made the answer pretty clear. Views to die for. Units needed real work. Board required 24 months of reserves.
Hard pass.
Now I have a firm target range: a purchase price ceiling, a maintenance ceiling, and a monthly total I'm comfortable with. Every new listing gets measured against those numbers before I schedule a showing. The tool didn't change what I could afford. It just made the decision obvious instead of approximate.