MainMath
spec-build underwriting · by Rich Main
Free tool · no account · by Rich Main

Know if a lot is worth building on before you spend a dollar.

Type four numbers. You get the profit, the cash you need, and — the part most calculators skip — the most you can pay for the dirt and still hit your margin.

Runs in your browser. Nothing to install.

How to use it

Five boxes. Thirty seconds. Built for a gut check, not a bank package.

1
Lot priceWhat they are asking for the dirt. House + lot if it is a tear-down.
2
Home sizeThe square feet you would actually build.
3
Build $/sqftYour all-in hard cost. Use a real bid, not a wish.
4
Sale $/sqftWhat finished homes sell for nearby, from sold comps.
5
Read the gold boxMax lot price is your ceiling. Offer below it, never above.
+
Full UnderwriteOpen it to change soft costs, contingency, permits, loan terms, partner split.
+
SaveName the deal and save it on your device. Print it or export JSON.
!
Check itEvery number is an assumption you control. Verify costs and comps before you commit.
What the numbers mean ▾

Net margin — profit divided by sale price. Under 20% on a spec build is thin.
Cash you need — the most money that has to leave your pocket at any point, interest included.
Max lot price — the highest offer that still hits your target margin. This is the negotiation number.
All-in $/sf — every cost divided by finished square feet. Compare it to the sale $/sf. That spread is the deal.

Net margin
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Deal score
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1 · The deal

Numbers you know from the listing. Everything else is an assumption you can change.

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$
$
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mo
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$
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Financing & partnership ▾
Hard money loan
Off = all cash, no points or interest
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$
Draw-based interest
Off = charge interest on the whole loan from day one (their method)
Partner on the deal
Off = you keep 100% of profit and cash
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Their tool showed your return on the whole profit even when you only keep 20% of it. This splits both sides, so the return you see is actually yours.
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2 · The numbers

Quick look — market averages behind the scenes.

Net profit
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after financing + selling
Cash you need
—
peak equity, incl. interest
Annualized return
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on your cash
All-in cost
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—
Max lot price at target margin
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Underwriting breakdown
My profit
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My cash-on-cash
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Sensitivity — margin % by build cost × sale price ▾
Monthly cash schedule ▾
Estimates only. Not lending, tax, legal, or investment advice. Every number here is an input you control — verify costs with bids, comps with sold data, and loan terms with a lender before you commit money.

Get the calculator

This tool is free. Put your name and email in and it opens. I'll send deal notes and new tools — no spam, and I never sell your details.

Add your name and a real email.

Your details stay with Rich Main. Nothing is sold or shared.
Prefer email? rich@themainbuild.com