Offline Map Rectangle Viewer – Does Your Tile Set Cover This Area?

Offline map tiles are pre-rendered map image squares (typically 256×256 or 512×512 pixels) that are stored locally on a device, allowing map navigation without an internet connection. Tiles are organized in a pyramid structure: each zoom level divides the world into a grid of 2^zoom × 2^zoom tiles. At zoom 0, the entire world fits in one tile; at zoom 10, you have over 1 million tiles globally. Offline map applications like Organic Maps, OsmAnd, and custom solutions using MBTiles files rely on this tiling scheme to deliver seamless offline navigation.

To verify coverage, define your area of interest as a bounding box (rectangle defined by NW and SE corners). For each zoom level, convert the lat/lon corners to tile coordinates using the Web Mercator projection formula. The number of tiles needed is (maxTileX − minTileX + 1) × (maxTileY − minTileY + 1). Sum across all zoom levels to get the total. This tool automates that calculation—simply draw a rectangle on the map and see exactly which tiles are needed at each zoom level, along with storage estimates.

MBTiles is an open specification for storing map tiles in a single SQLite database file. It was developed by Mapbox and is widely supported by tools like QGIS, TileMill, and many mobile mapping SDKs. An MBTiles file contains a tiles table with columns for zoom_level, tile_column, tile_row, and tile_data (the image blob). This format is ideal for offline map distribution because it packages thousands of tiles into one portable file. Storage requirements depend directly on the geographic area, zoom range, and tile format (PNG, JPEG, WebP, or vector PBF).

Storage needs grow exponentially with zoom level. A single city (e.g., London) at zooms 0–14 might require 50–200 MB. A whole country at zooms 0–16 can require 2–10 GB. Key factors: Geographic extent (larger area = more tiles), Zoom range (each additional zoom level roughly quadruples tile count), Tile format (PNG ≈ 18 KB/tile, JPEG ≈ 12 KB, WebP ≈ 7 KB, vector PBF ≈ 5 KB), and Content density (urban areas produce larger tiles than oceans or deserts). Use this tool's storage estimator with your actual bounding box and zoom range for a realistic projection.

This is due to the quadtree pyramid structure of web maps. At zoom level z, the world is divided into 4^z tiles. Going from zoom z to z+1 quadruples the global tile count. For a fixed geographic area, the tile count typically increases by a factor of ~3–4× per zoom level (slightly less than 4× because edge effects matter more at lower zooms). For example, an area needing 20 tiles at zoom 8 might need ~70 at zoom 9, ~250 at zoom 10, and ~900 at zoom 11. This exponential growth is why most offline map downloads limit the maximum zoom to 14–16 for large regions.

Web Mercator (EPSG:3857) is the projection used by almost all web mapping services including Google Maps, OpenStreetMap, and Mapbox. It maps the ellipsoidal Earth onto a flat square, stretching polar regions significantly. The valid latitude range is approximately ±85.06°—beyond this, the projection diverges to infinity. This tool uses the standard Web Mercator tile coordinate formulas: tileX = floor((lon+180)/360 × 2z) and tileY = floor((1 − ln(tan(lat)+sec(lat))/π)/2 × 2z). Understanding this projection helps explain why tiles near the poles cover much smaller real-world areas than equatorial tiles.

Several strategies help minimize storage: Limit zoom range—only download the zoom levels you truly need (e.g., zooms 10–15 for city navigation). Use efficient formats—WebP tiles are 40–60% smaller than PNG with similar quality; vector tiles (PBF/MVT) are even more compact. Clip to your exact area—avoid downloading entire country packages if you only need a specific region. Consider raster compression—JPEG at 75–85% quality works well for satellite imagery. Use planetiler or similar tools to generate custom tile sets with optimized encoding. This tool helps you precisely calculate what each optimization saves by comparing tile counts and estimated sizes across formats.

Raster tiles (PNG/JPEG/WebP) are pre-rendered images—each tile is a 256×256 pixel picture of the map at that location. They're simple to serve and render but consume more storage and can't be re-styled. Vector tiles (PBF/MVT format) contain geographic data (points, lines, polygons) that the client renders on-the-fly. They're typically 50–70% smaller, support dynamic styling (dark mode, language switching), and scale beautifully across resolutions. However, they require a compatible renderer (MapLibre, Mapbox GL). Most modern offline map solutions (OsmAnd, Organic Maps) use vector tiles for efficiency.