Map Scale Calculator
Understanding OS Map Scales
Ordnance Survey produces maps at several scales for different purposes. Understanding scale is essential for navigation, trip planning, and geography coursework.
How Map Scale Works
A map scale is written as a ratio, such as 1:25,000. This means that 1 unit on the map equals 25,000 of the same unit in real life. The unit can be anything — centimetres, millimetres, or inches — as long as both sides use the same unit.
For practical navigation with OS Explorer maps (1:25,000):
- 1 cm on map = 250 m in real life
- 4 cm on map = 1 km in real life
- 1 mm on map = 25 m in real life
Ordnance Survey Landranger 1:50,000
The Landranger series covers the whole of Great Britain and is popular for cycling and driving route planning. At 1:50,000:
- 1 cm on map = 500 m in real life
- 2 cm on map = 1 km in real life
- 1 mm on map = 50 m in real life
Worked example: A path measures 4.5 cm on a 1:50,000 map.
4.5 × 50,000 = 225,000 cm = 2,250 m = 2.25 km
OS Explorer 1:25,000 — Best for Hillwalking
The Explorer series is the choice for walkers and hikers. Greater detail means footpaths, field boundaries, and rights of way are clearly shown. Grid squares on the map are 4 cm × 4 cm, each representing a 1 km × 1 km area on the ground.
Blue Grid Squares on OS Maps
OS maps use the National Grid system. The blue kilometre grid lines divide the map into 1 km × 1 km squares. Thick blue lines mark 10 km intervals, creating 10 × 10 km squares. Grid references use the letters and numbers printed along the edges.
Contour Lines and Intervals
Both 1:25,000 and 1:50,000 OS maps show contour lines at 10 metre vertical intervals. Every fifth contour is an index contour, drawn thicker and labelled with the elevation. Closely spaced contours indicate steep terrain; widely spaced contours indicate gentle slopes.
Area Calculations from Map Measurements
To calculate real area from a map measurement, the scale factor must be applied to both dimensions, so the area is multiplied by the scale squared:
- 1:25,000: 1 cm² on map = 0.0625 km² (6.25 hectares)
- 1:50,000: 1 cm² on map = 0.25 km² (25 hectares)
- 1:10,000: 1 cm² on map = 0.01 km² (1 hectare)
Example: A lake covers 3.2 cm² on a 1:25,000 map.
Area = 3.2 × 0.0625 = 0.2 km² = 20 hectares
Bar Scale vs Ratio Scale
Maps show scale in two ways. The ratio scale (e.g. 1:50,000) is a mathematical relationship. The bar scale is a printed ruler on the map that remains accurate even if the map is photocopied at a different size. Always use the bar scale on photocopied or printed maps, as the ratio scale will be wrong if the paper size has changed.
Common UK Map Scale Reference
| Scale | 1 cm = ? | 4 cm = ? | Common Use |
|---|---|---|---|
| 1:10,000 | 100 m | 400 m | Town/street plans |
| 1:25,000 | 250 m | 1 km | OS Explorer — walking |
| 1:50,000 | 500 m | 2 km | OS Landranger — cycling |
| 1:100,000 | 1 km | 4 km | Regional overview |
| 1:250,000 | 2.5 km | 10 km | Road atlas |
| 1:1,000,000 | 10 km | 40 km | National overview |
Frequently Asked Questions
What does a 1:25,000 map scale mean?
A 1:25,000 scale means every 1 unit on the map represents 25,000 of the same units in real life. So 1 cm on the map equals 25,000 cm (250 metres) in the real world. This is the scale used by Ordnance Survey Explorer maps, which are the standard choice for walking and hiking in the UK.
How many centimetres to 1 km on a 1:50,000 map?
On a 1:50,000 Landranger map, 2 centimetres on the map equals 1 kilometre in real life. To calculate: 1 km = 100,000 cm, divided by 50,000 = 2 cm. You can verify this by measuring the distance between two 1 km grid lines on the map — they should be exactly 2 cm apart.
How do I calculate area from a map?
Measure the area in cm² on the map, then multiply by the scale squared. On a 1:25,000 map, 1 cm² = 25,000² cm² = 625,000,000 cm² = 0.0625 km². On a 1:50,000 map, 1 cm² = 50,000² cm² = 2,500,000,000 cm² = 0.25 km². So a field measuring 2 cm × 1.5 cm on a 1:25,000 map has a real area of 3 cm² × 0.0625 = 0.1875 km² = 18.75 hectares.
What is the difference between 1:25,000 and 1:50,000 OS maps?
OS Explorer maps at 1:25,000 show more detail — footpaths, field boundaries, and rights of way are clearer, with 4 cm per km. They are ideal for walking and hiking. OS Landranger maps at 1:50,000 show 2 cm per km and cover a larger geographical area per sheet, making them better for cycling, overview planning, and road navigation.
What are the blue grid squares on an OS map?
The blue grid lines on OS maps form the National Grid. The small squares are 1 km × 1 km. On a 1:50,000 Landranger map these squares measure 2 cm × 2 cm; on a 1:25,000 Explorer map they measure 4 cm × 4 cm. Thicker lines at 10 km intervals create larger 10 × 10 km squares, marked with the grid letters. Grid references use these squares to pinpoint locations to within 100 m or 10 m accuracy.
What contour interval is used on OS maps?
OS Explorer maps at 1:25,000 use a 10 metre contour interval — each brown contour line represents a rise or fall of 10 metres in elevation. OS Landranger maps at 1:50,000 also use 10 metre intervals. Every fifth contour (50 m intervals) is thicker and labelled. In some high mountain areas, supplementary 5 m contours may appear as dashed lines.
How do I convert a real distance to a map distance?
Divide the real distance by the scale ratio. For example, to find how long 3.5 km appears on a 1:25,000 map: 3.5 km = 350,000 cm ÷ 25,000 = 14 cm on the map. Or on a 1:50,000 map: 350,000 ÷ 50,000 = 7 cm. You can then use a ruler or string to mark this distance on a physical map.
Quick Reference
| Scale | 1 mm = | 1 cm = |
|---|---|---|
| 1:10,000 | 10 m | 100 m |
| 1:25,000 | 25 m | 250 m |
| 1:50,000 | 50 m | 500 m |
| 1:100,000 | 100 m | 1 km |
Area Quick Reference
| Scale | 1 cm² on map |
|---|---|
| 1:10,000 | 0.01 km² (1 ha) |
| 1:25,000 | 0.0625 km² (6.25 ha) |
| 1:50,000 | 0.25 km² (25 ha) |
| 1:100,000 | 1 km² (100 ha) |