What Is the National Flood Hazard Layer (NFHL)?

The National Flood Hazard Layer (NFHL) is FEMA's official digital flood map database for the entire United States, covering more than 90% of the population. It is the source behind every flood zone lookup, including FludZone.

The NFHL compiles all effective Flood Insurance Rate Map (FIRM) data into one searchable system, served through ArcGIS REST services. Government agencies, private tools, and the public can all access it freely.

The NFHL is not a single map but a geospatial database that contains multiple data layers. Together, these layers provide the complete picture of flood hazard information that FEMA has mapped for each community.

• Flood zone boundaries: The polygons that define each flood zone (including A, AE, AH, AO, AR, A99, V, VE, X, and D) for every mapped community. This is the core data layer that determines your flood zone designation.
• Base Flood Elevations (BFEs): Lines showing the predicted flood elevation during a 1% annual chance event. Available in zones where detailed studies have been conducted (primarily AE and VE zones).
• Floodway boundaries: The channel and adjacent areas that must remain open to convey floodwater without increasing flood heights. Mapped for studied riverine areas.
• FIRM panel boundaries: The grid of map panels that organizes the flood data into manageable sections, each with a unique panel number, effective date, and community reference.
• Letters of Map Change (LOMCs): Records of LOMAs, LOMRs, and LOMR-Fs that have modified the effective FIRM for individual properties or areas. These amendments are incorporated into the NFHL as they are issued.
• Community and political boundaries: Boundaries of NFIP-participating communities, counties, and other jurisdictions that provide geographic context for the flood data.

The NFHL is updated on a rolling basis as FEMA completes new Flood Insurance Studies and processes Letters of Map Change. There is no single annual release. Instead, the database reflects the most current effective data for each community as soon as FEMA publishes it.

This means different parts of the NFHL may reflect data from different study dates. A community that was restudied in 2024 will have newer data than a neighboring community whose maps date from 2005. The effective date for each area is recorded in the FIRM panel metadata.

When you look up a property on FludZone, you are querying the current effective data in the NFHL. The results reflect the most recent effective FIRM for that location, including any Letters of Map Change that have been processed.

Every flood map in the NFHL is backed by a Flood Insurance Study (FIS). The FIS is the technical report that documents how FEMA determined the flood zone boundaries, Base Flood Elevations, and floodway limits for a community. While the NFHL shows you the results, the FIS explains how those results were calculated.

A typical FIS includes the following:

• Community description: Geography, drainage patterns, and history of flooding in the study area.
• Engineering methods: The hydrologic and hydraulic models used to calculate flood flows and elevations.
• Flood profiles: Charts showing how flood elevation changes along each studied stream or coastline. These profiles contain more precise BFE data than the FIRM, which rounds values to the nearest foot.
• Floodway data tables: For each studied stream cross-section, the table shows the floodway width, water surface elevation, and the surcharge (increase in flood height) caused by encroachment.
• Vertical datum information: The survey reference used for all elevations in the study (typically NAVD 1988 for newer studies or NGVD 1929 for older ones).

FIS reports are available for free through FEMA's Map Service Center. They are primarily used by engineers, floodplain managers, and surveyors, but they can be useful to anyone who wants to understand the detailed basis behind their flood zone designation.

The NFHL is served through FEMA's ArcGIS REST services. Each layer in the database stores a different type of flood hazard information. Understanding the key layers helps you know what data is available and how tools like FludZone retrieve it.

These layers are publicly accessible at no cost. The ArcGIS REST endpoint supports spatial queries (point, polygon, envelope) and attribute queries, making it possible to build lookup tools that query multiple layers simultaneously for a single location.

The NFHL is the authoritative source for regulatory flood zone data, but it is not the only flood data available. Understanding how it relates to other sources helps you evaluate your complete flood risk picture.

Only the NFHL determines whether flood insurance is required by a federally backed mortgage lender. Other data sources can provide valuable context about flood risk but do not carry regulatory authority.

While the NFHL covers most of the US population, there are gaps. Areas designated as Zone D have been mapped but not studied, meaning FEMA has not performed a flood hazard analysis for that area. Zone D does not mean the area is safe from flooding. It means the risk is undetermined.

Some areas have no NFHL coverage at all. This is most common in remote, tribal, or recently incorporated areas where FEMA has not yet performed mapping. If a property returns no results from the NFHL, it does not mean there is no flood risk. It means FEMA has not mapped it.

FEMA prioritizes mapping updates based on several factors: population density, development activity, age of existing maps, frequency of flooding, and availability of updated topographic data (particularly LiDAR). Communities can also request new or updated studies through FEMA's Cooperating Technical Partners (CTP) program, which shares mapping costs between FEMA and local or state agencies.

Creating a FEMA flood map is a multi-year engineering process. Each map is backed by a Flood Insurance Study that follows a structured sequence of data collection, modeling, and community review. Here is how the process works from start to finish.

While the NFHL is the most authoritative source of flood hazard data in the United States, it has limitations that are important to understand.

• Not all areas are studied: Areas designated as Zone D have not had a flood hazard analysis performed. Some rural, tribal, and unincorporated areas may have limited or no coverage in the NFHL.
• Data age varies: Some communities have maps based on studies conducted decades ago. Conditions on the ground may have changed significantly since the original study. FEMA is continually working to update the oldest maps, but the process takes time.
• Boundary precision: NFHL boundaries are based on the best data available at the time of the study, but they are not survey-grade. Properties near zone boundaries may need an Elevation Certificate or site survey to confirm their actual status.
• Does not include all flood sources: The NFHL primarily reflects riverine and coastal flood hazards. It may not fully account for localized urban flooding, stormwater drainage issues, or groundwater flooding in all areas.
• Dam failure flooding is separate: FEMA flood maps do not show areas that could be inundated by a dam failure. Dam inundation maps are maintained by dam owners and state dam safety programs, not the NFHL. If you live downstream of a dam, check with your state dam safety office for information about potential inundation zones.
• Not a real-time system: The NFHL shows mapped flood hazard areas based on statistical analysis, not current conditions. It does not tell you whether flooding is happening now or predict specific storm events.

FludZone queries the NFHL in real time through FEMA's public ArcGIS REST API. When you enter an address, FludZone geocodes the location and queries the relevant NFHL data layers to retrieve your flood zone, SFHA status, Base Flood Elevation, FIRM panel number, and any applicable Letters of Map Change.

Because FludZone pulls directly from the same data source that FEMA and lenders use, the flood zone designation you see on FludZone is identical to what you would find on the FEMA Map Service Center or a lender's flood determination. The difference is in how the information is presented: FludZone translates technical FEMA codes into plain language and shows the practical implications for insurance and property ownership.