Flood zones

Is my house in a flood zone? How to check, and why Zone X can still flood

A flood zone takes a few minutes to look up, and it decides whether your lender requires flood insurance. What it doesn’t tell you is whether the house will stay dry. Here is how to read it, and what to check beyond it.

By the Terreflow team · Published · Updated

Short answer

Enter the address in our free flood zone lookup or on FEMA’s Flood Map Service Center. Zones that start with A or V are high-risk areas, where a federally backed mortgage requires flood insurance. Zone X (B or C on older maps) covers what FEMA calls areas of moderate or minimal flood hazard, which FloodSmart says are “not entirely without risk”: in the 20 flood-prone counties we analyzed, one in three flood insurance claims came from these zones. Nationally, FEMA puts it at 29%.

How to look up a flood zone

  1. FEMA’s Flood Map Service Center. Type the address at msc.fema.gov to see the official Flood Insurance Rate Map (FIRM) or download a printable FIRMette; the NFHL viewer shows the same zones on a zoomable map, handy near a zone boundary. Note the map’s effective date (some are decades old), and check any preliminary maps: they don’t set requirements yet, but show where the zone is likely headed.
  2. Terreflow. The lookup above gives the zone; the Surface Drainage tab on our 3D map shows how the terrain around the address moves water, which the flood zone doesn’t cover.

Your lender will order its own flood zone determination; look it up yourself before you make an offer.

What each flood zone means

Zones beginning with A or V make up the Special Flood Hazard Area (SFHA): land expected to be flooded by the 1%-annual-chance flood, often called the “100-year flood.”

ZoneFEMA’s definition
A“Areas with a 1% annual chance of flooding and a 26% chance of flooding over the life of a 30-year mortgage.” No Base Flood Elevation is shown.
AE“The base floodplain where base flood elevations are provided.” The Base Flood Elevation (BFE) is the height the 1%-annual-chance flood is expected to reach.
AH“Areas with a 1% annual chance of shallow flooding, usually in the form of a pond, with an average depth ranging from 1 to 3 feet.”
AO“River or stream flood hazard areas, and areas with a 1% or greater chance of shallow flooding each year, usually in the form of sheet flow, with an average depth ranging from 1 to 3 feet.”
AR“Areas with a temporarily increased flood risk due to the building or restoration of a flood control system…”
A99“Areas with a 1% annual chance of flooding that will be protected by a Federal flood control system where construction has reached specified legal requirements.”
V, VE“Coastal areas with a 1% or greater chance of flooding and an additional hazard associated with storm waves.” VE shows a Base Flood Elevation; V does not.
X (shaded), B“Moderate flood hazard areas … between the limits of the base flood and the 0.2-percent-annual-chance (or 500-year) flood.” Areas protected by levees from the 1% flood are also shown this way.
X (unshaded), C“The areas of minimal flood hazard, which are the areas outside the SFHA and higher than the elevation of the 0.2-percent-annual-chance flood.” FloodSmart adds that they are “not entirely without risk.”
D“Areas where flood risk has not been determined and no flood hazard analysis has been conducted.” FloodSmart: “‘undetermined risk’ doesn’t mean ‘no risk.’”

Definitions quoted from FEMA’s glossary and Flood Zones page, and from FloodSmart’s What is my flood zone?

Inside the SFHA, in communities that take part in the National Flood Insurance Program, flood insurance is mandatory for a building with a mortgage from a federally regulated or insured lender, from a federal agency (FHA, VA or USDA loans), or sold to Fannie Mae or Freddie Mac. Outside it, flood insurance is optional, and most owners don’t buy it.

What “1% annual chance” really means

A “100-year flood” is not a flood that comes once a century. It is a flood with a 1% chance of happening in any given year, every year. Over a 30-year mortgage, that adds up to about a 26% chance of at least one such flood, slightly more than one in four. FEMA uses the same figure.

Why Zone X can still flood

Cross-section of a river valley. Zone AE covers the floodplain below the 1%-annual-chance flood level, shaded Zone X lies between the 1% and 0.2% levels, and unshaded Zone X covers the higher ground, where a house sits in a low spot that collects rain running off the slopes around it.
Flood zones follow the 1% and 0.2%-annual-chance flood levels of the river or coast. A house on higher ground can be in unshaded Zone X and still sit where local runoff collects.

What the claims data shows

Using FEMA’s public claims records (OpenFEMA, every NFIP claim since 1978), we counted how many claims in the 20 counties of our county drainage maps came from buildings rated in Zones X, B and C.

Across all 20 counties, 234,657 of 714,120 claims (33%) came from those zones. Around Houston, it was more than half. That is in line with FEMA’s national figure: from 2014 to 2024, nearly a third of NFIP claims (29%) came from outside current high-risk areas.

Bar chart of the share of NFIP flood insurance claims rated in Zone X, B or C in 20 counties, from 72% in Fort Bend County, Texas, and 55% in Harris County to 9% in Miami-Dade County; 33% across all 20 counties.
Around Houston, more than half of all flood insurance claims came from outside FEMA’s high-risk zones.
See the numbers for each county
CountyNFIP claims with a rated zoneIn Zone X, B or CShare
Fort Bend County, TX7,1585,18172%
Harris County, TX167,35791,76455%
Montgomery County, TX11,4186,18454%
Palm Beach County, FL10,4405,21350%
Brazoria County, TX20,3169,14545%
Maricopa County, AZ2,17296244%
St. Louis city, MO64622034%
Cook County, IL14,5564,53931%
Galveston County, TX60,54818,55431%
Jefferson Parish, LA134,77239,42629%
King County, WA3,30490727%
Orleans Parish, LA125,27031,81125%
Hamilton County, OH1,48137525%
Buncombe County, NC1,17029525%
Sacramento County, CA3,82093024%
Horry County, SC12,4292,84323%
East Baton Rouge Parish, LA22,0944,66721%
Charleston County, SC19,2382,30712%
Broward County, FL34,4753,90611%
Miami-Dade County, FL61,4565,4289%

How to read this. NFIP claims since 1978 with a rated zone (OpenFEMA, retrieved October 8, 2026). The rated zone prices the policy and can come from an older map; areas behind levees, common around New Orleans, are often mapped as Zone X. Few owners outside high-risk zones carry flood insurance, so these counts likely understate how often those zones flood.

What to do next

If the house is in Zone X

If the house is in a high-risk zone (A or V)

See the flood zone and the terrain together

Terreflow shows an address’s FEMA flood zone next to a terrain drainage map, so you can see whether a Zone X lot sits in the path of runoff.

Analyze an address →

Sources

More guides

Has this address flooded?What anyone can look up, what only the seller can get, and what the land itself tells you about past flooding. Terrain checklist before buying landTen checks for drainage, low spots, grading, slope, soils and wetlands, with free tools for each. How to read a drainage mapContour lines, high and low ground, and how to read Terreflow’s Surface Drainage view, including what it can’t tell you. Flood claims in the 50 largest citiesAll 50 ranked by FEMA flood insurance claims per 1,000 homes, with each city’s biggest flood year. How much sun does my property get?Average sunlight for any U.S. address, how a south or north slope changes it, and what the map can’t see. What soil type is my property?Sand or clay, what that means for drainage, and how to look up the soil under any U.S. address. What a FEMA risk rating meansHow the National Risk Index rates an area, why the same place gets different answers, and the limits of any single risk score. City & county flood mapsTerrain drainage, FEMA flood zones and flood history for flood-prone U.S. cities and counties.