Line-item elevation compliance against the base flood elevation plus required freeboard — finished floors, equipment pads, electrical gear, wet well rims. AE (BFE-based) and AO (depth-based) modes. Runs entirely offline; inputs persist in this browser via localStorage.
Use limits — and the datum trap: Every elevation in this check must be on the same vertical datum. FIRMs published over the decades mix NGVD29 and NAVD88, which differ by roughly 2–2.5 ft in Southern California — comfortably more than most freeboard requirements, meaning a datum mixup can flip a pass to a fail (or worse, a fail to a paper pass). Check the FIRM panel's datum note against the survey's. The flood zone, BFE, and required freeboard come from the effective FIRM/FIS and the local floodplain ordinance (which often exceeds NFIP minimums); this tool just does the arithmetic.
Summary
Required minimum elevation
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Items passing
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Lowest margin
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How the Numbers Are Derived
The arithmetic (and where each number comes from)
AE/numbered A zones: required = BFE + freeboard. AO zones (shallow sheet flow, no BFE): required = highest adjacent grade + depth number + freeboard. Margin = proposed − required. The arithmetic is trivial by design — the whole game is the inputs: the effective FIRM panel and FIS profile for BFE (interpolate between cross-sections on the profile where the panel shows a range), the community's floodplain ordinance for freeboard, and a surveyed elevation on the right datum for the proposed value.
Zone cheat sheet
AE / A1–A30: 1% annual chance with BFE published. The workhorse zone for this check.
A (unnumbered): 1% annual chance, no BFE — the community or a study establishes one; don't guess.
AO: shallow flooding 1–3 ft, published as a depth number instead of an elevation.
AH: shallow ponding with a BFE — treat like AE.
VE: coastal wave action — freeboard measured to the lowest horizontal structural member, plus construction requirements far beyond this tool.
X (shaded/unshaded): outside the 1% floodplain — but critical facilities are often held to the 0.2% (500-yr) event by policy.
Why utility equipment keeps failing this check
Buildings get designed to the flood ordinance; the electrical gear, generators, and control panels get placed by whoever had space on the site plan. NFIP and most ordinances require utilities and service equipment elevated (or dry-floodproofed) to the same standard as the lowest floor — and a lift station whose motor control center goes underwater at the design flood is a lift station that fails exactly when the wet weather flows peak. Elevation certificates (FEMA form) document the as-built answer; this tool is for catching the problem while it's still a markup.
References
Effective FIRM panel and Flood Insurance Study — BFE/depth, zone, and datum. https://msc.fema.gov/