If you’ve ever picked up an OS map and stared at the front wondering what “1:25,000” actually means, you’re not alone. It’s one of the first things that trips people up when they start learning to read a map scale, and honestly, it’s not as complicated as it sounds. Once it clicks, it clicks. And after that, you’ll use this knowledge on every single walk.
So let’s break it down.
What Does Map Scale Actually Mean?
The ratio explained
A map scale is just a ratio. It tells you the relationship between a distance on the map and the same distance in the real world.
When you see 1:25,000 printed on an OS Explorer map, it means that 1 unit on the map equals 25,000 of the same units on the ground.
One centimetre on the map equals 25,000 centimetres in real life. And 25,000 centimetres is 250 metres. That’s the whole thing.
Turning the numbers into real distances
Here’s a quick reference to make it stick.
- On a 1:25,000 map, 1 cm = 250 metres, and 4 cm = 1 kilometre.
- On a 1:50,000 map, 1 cm = 500 metres, and 2 cm = 1 kilometre.
So if you measure a route on a 1:25,000 map and it comes out at 8 cm, your walk is about 2 kilometres. Easy.
The bigger the second number in the ratio, the more the map is zoomed out, and the less detail it shows.
In the UK, Ordnance Survey produces two main map series that walkers use. Knowing the difference will save you a lot of confusion.
OS Explorer 1:25,000, the walker’s map
The OS Explorer is the one you want for most hill walking. It’s the detailed one. At 1:25,000 scale, you get:
- Field boundaries (walls, fences, hedges)
- Footpaths and bridleways clearly marked
- Contour lines at 5-metre intervals (10 metres in mountainous areas)
- Individual buildings, car parks, phone boxes
The level of detail is brilliant for navigating on foot. You can relate what you see on the map directly to what’s in front of you on the ground. The orange covers are the giveaway.
If contour lines still look like a confusing mess of squiggles, our guide on how to read contour lines breaks them right down.
OS Landranger 1:50,000, the bigger picture
The OS Landranger maps have pink covers and show twice the area of an Explorer on the same sheet. That makes them useful for planning longer routes across a bigger area, but the detail is reduced.
At 1:50,000, field boundaries disappear and the contour interval is 10 metres throughout. That matters when you’re trying to spot a specific ridge or work out the exact shape of the ground. For hill walking and serious navigation, the Explorer is nearly always the better choice.
How to Measure Distance Using Map Scale
Once you know the scale, measuring distances becomes straightforward. There are a few ways to do it.
The bar scale method
Every OS map has a bar scale printed on it, usually near the legend. It’s a ruler-like graphic that shows exactly how far a given distance on the map represents in the real world.
- Mark the start and end of the section you want to measure on a piece of paper or string.
- Hold it up to the bar scale on the map.
- Read off the real-world distance.
If your route isn’t a straight line, walk the paper along the route in short segments, marking each time you need to turn. Add them up at the end.
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The grid square method
OS maps have a blue grid printed on them. Those are the grid squares, and on both 1:25,000 and 1:50,000 maps, every grid square measures exactly 1 km by 1 km on the ground regardless of scale.
On a 1:25,000 map the squares are 4 cm wide on paper. On a 1:50,000 map they’re 2 cm wide. That’s a handy size reference when you’re measuring without a ruler.
If your route covers three full grid squares in a straight line, that’s 3 kilometres. No maths needed beyond counting.
The compass ruler shortcut
If you’ve got a baseplate compass in your hand, there’s an even faster method. Use the millimetre ruler along the edge of the compass plate.
On a 1:25,000 map, 1 mm = 25 metres. On a 1:50,000 map, 1 mm = 50 metres. Measure in millimetres, multiply, done.
Which Map Scale Should You Use for Walking?
For most day walks in the UK hills, the 1:25,000 OS Explorer is the right choice. The detail is there when you need it, especially for navigating off well-marked paths, identifying features in poor visibility, or following a precise route across open ground.
That said, in very featureless terrain like open moorland or snowfields, the extra detail on a 1:25,000 can actually clutter the picture. Some navigators prefer a 1:50,000 Landranger in those conditions because the contours are easier to read quickly. Many experienced walkers carry both scales on longer trips and switch between them.
You may also come across Harvey Maps, which use a 1:40,000 scale and are popular with mountain marathons and fell runners. They’re excellent maps, but for most beginners starting out with OS, stick with the Explorer.
If you’re not sure which sheet you need for your area, our guide to OS map numbers will sort you out.
If you’re navigating on foot in the UK and you’re not sure which to buy, get the Explorer.
Getting Confident With Your Map
Understanding map scale is the foundation. Once you can see the relationship between the paper in your hand and the ground beneath your feet, everything else starts to make sense, from measuring distances to reading contours to plotting routes. If you want the full picture, our guide on how to read an OS map ties it all together. (LINK “how to read an OS map” → /how-to-read-an-os-map)
The natural next step is the rest of the prep that gets you safely out the door. My free Digital Prep & Planning Course walks you through planning a walk from start to finish, including kit, weather, and route choice, and it’s the easiest place to carry on from here.
And when you’re ready to go from “I can read the scale” to “I can navigate anywhere in the UK in any weather”, the full Mountain Navigation Course covers it all, step by step.
What does 1:25,000 mean on a map?
It means that 1 unit on the map equals 25,000 of the same units on the ground. In practical terms, 1 centimetre on the map equals 250 metres in real life. So if you measure a route on a 1:25,000 OS Explorer map and it comes out at 8 cm, your walk is about 2 kilometres. Once that clicks, reading distances off a map becomes second nature.
Is a 1:25,000 or 1:50,000 map better for walking?
For most hill walking in the UK, 1:25,000 is the better choice. The OS Explorer at this scale shows field boundaries, detailed footpaths, and contour lines at 5-metre intervals, which gives you far more to work with on the ground. The 1:50,000 Landranger covers twice the area but loses that detail. Many experienced navigators carry both and switch depending on the task.
How do I measure distance on an OS map?
Two easy methods. First, use the bar scale printed on the map. Mark your route on a piece of paper and hold it against the bar scale to read the distance. Second, count the blue grid squares your route passes through, as each one is exactly 1 kilometre on the ground regardless of which OS map you’re using. For straight sections you can also use the millimetre ruler on your compass. On a 1:25,000 map, 1 mm equals 25 metres.
What is the contour interval on an OS Explorer map?
On most OS Explorer maps (1:25,000) the contour interval is 5 metres, meaning each contour line represents a 5-metre rise or drop in height. In mountainous areas the interval increases to 10 metres. On a Landranger (1:50,000) it’s always 10 metres. Check the legend on your specific map to confirm, especially if you’re walking in hilly terrain.
Do I need to buy a new map or can I use an old one?
For the actual scale and grid information, an older map is fine. Map scale and the grid system never change. What does go out of date is local detail, like footpath diversions, new buildings, and changes to forest cover. For most walks that won’t affect you, but if you’re relying on specific paths, it’s worth checking against a newer edition. Ordnance Survey updates maps on a continuous revision cycle.
