How to Convert Coordinates to State Plane Online for Free

If you work with survey data, civil engineering plans, or local government GIS records in the United States, you've run into State Plane coordinates — and you've probably needed to convert them to or from ordinary latitude/longitude at some point without wanting to open QGIS just to do it. This guide covers what State Plane is, why it exists, and how to convert coordinates for free using the GeoDataTools State Plane & EPSG converter.

What Is the State Plane Coordinate System?

The State Plane Coordinate System (SPCS) is a set of more than 120 coordinate systems covering the United States, with most states divided into two or more zones. Unlike UTM, which uses wide 6-degree-longitude zones and can introduce meaningful distortion at their edges, State Plane zones are drawn specifically so that distortion within any single zone stays under roughly 1 part in 10,000 — accurate enough for legal property surveys and engineering work where UTM or Web Mercator simply isn't precise enough.

This is why State Plane is the standard for county assessor parcel data, DOT survey control, and most municipal GIS — it's the coordinate system built for exactly the accuracy those use cases require.

Why State Plane Has So Many Zones

Large states are split into multiple zones because a single projection can't stay within that accuracy tolerance across a large area. California, for example, has six zones running roughly north to south; Texas has five. Each zone uses whichever projection minimizes distortion for its shape — states that are wider east-west typically use a Lambert Conformal Conic projection (two standard parallels), while states that are taller north-south, like Florida's east and west zones, use Transverse Mercator instead (a central meridian). Which projection a given zone uses isn't something you need to know to use it — it's baked into the zone's official parameters — but it's why the projection formulas differ from zone to zone even within the same system.

Converting Coordinates Online

The State Plane & EPSG converter covers this in two ways:

• Built-in zones: Select a common zone — California, Texas, New York Long Island, Florida East, Colorado Central, or Washington North — from the dropdown. Choose a direction (Lat/Lon → Projected, or Projected → Lat/Lon), enter your coordinates, and the conversion happens instantly.
• Custom EPSG code: For any other zone, or any EPSG-defined coordinate system at all, switch to Custom EPSG / proj4 and paste a proj4 definition string. You can get this for any EPSG code from epsg.io/<code>.proj4 — for example epsg.io/2229.proj4 returns the exact definition for California Zone 5 in US survey feet.

All math runs through proj4js, the same JavaScript projection library used across the GIS web ecosystem — nothing here is approximated or hand-rolled.

A Note on Units: Meters vs. US Survey Feet

State Plane systems were originally defined in feet, and most states still publish official coordinates in US survey feet. EPSG maintains both a feet-based code and a meters-based code for most zones — the built-in zones in this tool use the meters definitions. If you paste a custom proj4 string for a feet-based EPSG code instead, the tool doesn't silently convert units: the X/Y output will be in whatever unit that specific projection defines, so check which EPSG code (meters or ftUS) matches what you're working with before comparing numbers against another source.

Finding Your Zone or EPSG Code

If you're not sure which State Plane zone covers your location, search "[your county] State Plane zone" or check your state's Department of Transportation GIS page — most publish a zone map. Once you know the zone name, searching "NAD83 [zone name] EPSG" on epsg.io will surface the exact code, and appending .proj4 to that code's URL gives you the definition string to paste into this tool.

Your Coordinates Stay on Your Device

Like the rest of GeoDataTools, this conversion runs entirely in your browser — no coordinate, survey point, or property location is ever sent to a server. If you need to reproject an entire GeoJSON, KML, or Shapefile dataset rather than a single point, the GeoDataTools app's CRS reprojection feature handles that the same way, locally and privately.