Consilium Guides / FEMA flood zones
FEMA flood zones explained
FEMA flood zones are letters on a Flood Insurance Rate Map. Zones that start with A or V are high-risk Special Flood Hazard Areas. Zone X is moderate or minimal mapped risk. Zone X and Zone AE each have their own page. Check an address at the Flood Map Service Center, or see how a ZIP splits with the flood zone lookup.
How does FEMA determine flood zones?
FEMA studies how water moves through a watershed. Those flood studies use hydrologic and hydraulic modeling, topographic data, and historical flooding. The published result is a Flood Insurance Rate Map, and the digital National Flood Hazard Layer. A Letter of Map Amendment or Letter of Map Revision can change a structure or a panel later. Until that letter is effective, the published map controls. FEMA's flood map pages describe the process.
What is a floodplain?
A floodplain is land that floodwater can cover. The 1% annual-chance floodplain is the Special Flood Hazard Area on current maps. That phrase does not mean a flood happens once a century. It means the mapped annual chance is about 1 in 100. FloodSmart describes the chance over a 30-year mortgage as about 1 in 4 for homes in those high-risk areas.
Zone codes
| Zone | Mapped risk | High-risk SFHA | Insurance required with a federally backed mortgage | Meaning |
|---|---|---|---|---|
| A | High | Yes | Yes | 1% annual-chance flood, no base flood elevation published. |
| AE | High | Yes | Yes | 1% annual-chance flood with a base flood elevation. |
| AH / AO | High | Yes | Yes | Shallow flooding. AH has a ponding depth. AO has a sheet-flow depth. |
| A99 | High | Yes | Yes | SFHA protected by a federal flood-control system under construction. No base flood elevations. Mandatory purchase applies. |
| AR | High | Yes | Yes | A flood-control system that is decertified or being restored. Treated as high risk while that status applies. |
| V / VE | High, coastal | Yes | Yes | Coastal high hazard. VE includes wave heights of 3 feet or more and a base flood elevation. |
| X shaded | Moderate | No | No | 0.2% annual-chance flood, or shallow 1% flooding, or an area protected by a levee. |
| X unshaded | Minimal | No | No | Minimal mapped flood hazard. Flooding is still possible. |
| B / C | Moderate / minimal | No | No | Older-map equivalents of shaded Zone X (B) and unshaded Zone X (C). |
| D | Undetermined | No | No | Possible but undetermined hazard. Not a Special Flood Hazard Area. NFIP coverage can still be available. |
Sources: FEMA flood maps and FloodSmart zone descriptions. "Insurance required" means the federal mandatory purchase rule for a federally backed or federally regulated mortgage in an SFHA. A lender can require a policy in other zones too.
Zone A
Zone A is a high-risk area with a 1% annual chance of flooding where FEMA has not published a base flood elevation. The mandatory purchase requirement applies for federally backed or regulated mortgages.
Zone A99
Zone A99 is a Special Flood Hazard Area that will be protected by a federal flood-control system that is still under construction. Base flood elevations are not published. Because it is an SFHA, the mandatory purchase requirement applies. Details are in the mortgage guide.
Zones AH and AO
AH and AO are shallow-flooding SFHAs. AH is usually ponding. AO is usually sheet flow with an average depth. Both are high-risk for the insurance requirement.
Zones VE and V
V zones are coastal high-hazard areas. Zone VE adds a base flood elevation and is defined with wave heights of 3 feet or more. The mandatory purchase requirement applies.
Zone D
Zone D means the flood hazard is possible but undetermined. It is not an SFHA, so the federal mandatory purchase requirement does not apply. NFIP coverage may still be sold. A lender can require it.
Zone X and Zone AE
Zone X covers moderate and minimal mapped risk. Zone AE is the common high-risk zone with a base flood elevation. A Consilium Compare report shows the share of a ZIP in these groups. It is not a parcel determination. To check flood risk before you buy, start with the ZIP, then confirm the address on FEMA's map.
FAQ
How does FEMA determine flood zones?
FEMA uses flood studies, including hydrologic and hydraulic modeling, to map where water is expected to go. The result is a Flood Insurance Rate Map. Letters of Map Amendment and Letters of Map Revision record later changes. The current map is the one that applies.
What is a floodplain?
A floodplain is land that can be inundated by floodwater. On FEMA maps, the Special Flood Hazard Area is the land subject to the 1% annual-chance flood. Moderate and minimal zones are mapped separately.