What Is a Maidenhead Grid Locator (QTH)? Grid Square System Explained

📘 Definition

The Maidenhead Locator System (also called QTH locator or grid square) divides the Earth's surface into a hierarchy of progressively smaller rectangles, each encoded as an alphanumeric string. The first two characters (field) identify a 10° × 20° area, the next two digits (square) subdivide it into 1° × 2° cells, and the optional final two characters (subsquare) refine to approximately 5 × 10 km. For example, IO91wm places you in central London. The system was adopted by the International Amateur Radio Union (IARU) in 1980 and replaced the older QRA locator. It is universally used for logging satellite contacts, contest exchanges, and coverage analysis — including Orbital Radar's propagation heatmap, which shows which grid squares fall within a satellite's coverage footprint.

10° × 20° (1,100 × 2,200 km)

Field (2 chars)

1° × 2° (110 × 220 km)

Square (4 chars)

2.5' × 5' (5 × 10 km)

Subsquare (6 chars)

IO91wm

London Example

Understanding Maidenhead Grid

How the Grid Encodes Position

The encoding starts at 90°S, 180°W (the anti-meridian/South Pole corner). The first character (A–R) encodes 20° of longitude, the second (A–R) encodes 10° of latitude. The next digit (0–9) subdivides longitude by 2°, the fourth digit subdivides latitude by 1°. The final optional pair (a–x) provides the subsquare at 5' × 2.5' resolution. This gives 18 × 18 = 324 fields, each containing 100 squares, each containing 576 subsquares — totalling over 18.6 million unique 6-character locators worldwide.

Grid Squares in Satellite Work

During satellite contacts, operators exchange 4-character grid squares as part of the standard QSO (contact). This is shorter than exchanging full coordinates and provides enough precision to confirm both stations shared the same satellite pass. In satellite contests (AMSAT Field Day, ARRL VHF+), each unique grid square worked counts as a multiplier, making the propagation heatmap invaluable for planning which passes cover the most valuable grids.

Grid Square Examples

Location	4-Char Grid	6-Char Grid
London, UK	IO91	IO91wm
New York, USA	FN30	FN30as
Tokyo, Japan	PM95	PM95ss
Sydney, Australia	QF56	QF56od
São Paulo, Brazil	GG66	GG66qk

Frequently Asked Questions

How do I find my Maidenhead grid square? ▼

Enter your city, postcode, or coordinates in the amateur radio pass predictor — it computes your 6-character grid locator automatically and displays it in the QTH field. You can also enter a grid square directly and the tool converts it to coordinates.

Why do hams use grid squares instead of coordinates? ▼

Grid squares are shorter to transmit (6 characters vs 15+ for full coordinates), easy to speak phonetically over radio, and provide just enough precision for satellite work. During a 5-minute pass shared with many operators, brevity matters — every second counts.

What is a "grid chase" or "grid expedition"? ▼

Some operators travel to rare or hard-to-reach grid squares to activate them for other operators. Working a rare grid via satellite is particularly prized because the pass window is short and the coverage overlap must be precisely timed. The propagation heatmap helps identify which passes cover these rare grids.